drawmap - draw customized maps, using raw USGS data files

drawmap [-l latitude1,longitude1,latitude2,longitude2] [-L]

[-o output_file.sun] [-d dem_file1 [-d dem_file2 [...]]]

[-c contour_interval_in_meters]

[-C contour_interval_in_meters]

[-g gnis_file] [-a attribute_file] [-x x_size] [-y y_size]

[-w] [-n color_table_number] [-r relief_factor] [-z]

[-i] [-h] [-t] [dlg_file1 [dlg_file2 [...]]]

This is the manual page for version 2.5 of drawmap.

The U.S. Geological Survey, and other sources, support sites on the Internet with many gigabytes of raw geographic data, mostly for the USA. Drawmap draws maps, using a subset of the available data. The relevant subset includes:

250K Digital Elevation Model (DEM) files

Each file covers a block, one-degree square, with a 1201 by 1201 grid of elevations (in meters). The extra sample in each direction is due to overlap of the DEM files at their edges. (Files for Alaska use smaller grids, with only 401 or 601 samples in the east-west direction.) For Hawaii and the "lower 48," the one-degree square is covered by elevation samples spaced 3 arc-seconds apart; and you will often hear these files called 3-arc-second files. In terms of distance along the ground, the sample spacing varies with latitude. It is generally less than 100 meters. (The "250K" means that the data were digitized from a map at the scale of 1:250,000.) Files of this type are currently available for free download from the USGS.

24K Digital Elevation Model (DEM) files (in the 'classic' format or the SDTS format)

These files usually result from digitizing a "quad" map sheet. (Each 1-degree block of latitude and longitude, covered by a 250K DEM file, is further subdivided into an 8-by-8 grid to produce smaller blocks called quads.) The number of samples in each direction varies, from quad to quad, but there is roughly one sample per second of arc. These files differ from the 250K DEM files in that the samples are spaced a fixed number of meters apart rather than a fixed number of arc-seconds. Because each quad represents an area that is 7.5 minutes of latitude by 7.5 minutes of longitude, you will also hear these files called 7.5-minute DEMs. The USGS provides 24K DEM data for free download, in the Spatial Data Transfer System (SDTS) format. Some files in the older format are available from other sources, and may be available for purchase from the USGS.

100K Digital Line Graph (DLG) files (in the 'optional' format or the SDTS format)

These files come in collections, each of which covers a quarter of the one-degree square covered by a 250K DEM file. The files contain information that allows segmented linear and polygonal features to be drawn on maps, including boundary lines, hydrographic features (streams, lakes, and so on), transportation features (roads, rail lines, pipelines, and so on), public land survey data, and hypsographic lines (the familiar contour lines of a topographic map). The different general classes of data come in separate files. Files of this type are currently available for free download from the USGS, in either the 'optional' format or the SDTS format.

24K Digital Line Graph (DLG) files (in the 'optional' format or the SDTS format)

Like the 24K DEM files, each of these files covers a single quad. Except for their inherently greater detail, these files are essentially the same as the 100K DLG files. As with the 24K DEM files, these files are freely available in SDTS format, but are harder to come by in the 'optional' format.

GTOPO30 files

GTOPO30 files are similar to 250K DEM files, but with samples spaced 30 arc-seconds apart, and with a quite different format. (For the purposes of this manual page, you can consider GTOPO30 files as just another variety of DEM files.) While these files have relatively low resolution, they have the virtue of providing full coverage for the entire planet. Furthermore, they are currently available for free download.

Geographic Names Information System (GNIS) files

These files are basically lists of place names, with the addition of latitude/longitude and other information. They are available for free download.

If a DEM or DLG file is one of the above types, and is from the USGS, there is a good chance that drawmap can use it. However, given the range of available files, this is not a certainty. For example, the USGS used to distribute 100K DLG files in 'standard' format, with the characters 'std' as part of the file name. Neither the USGS nor drawmap support these files anymore, but you may still be able to obtain them. There is also a certain amount of variability in the format of USGS SDTS files, and drawmap may not be able to handle some of the variants, especially if I have never obtained a relevant sample file to test against. Files from non-USGS sources may also be usable. However, people don't always strictly follow the relevant standards when they create DEM or DLG files, and the standards can sometimes be difficult to interpret, and drawmap isn't infinitely adaptable, so not all files can be processed.

Using the data in the various files, drawmap can produce various kinds of customized maps, including shaded relief maps (with or without roads, streams, place names, and so on) and topographic maps (again, with or without additional features).

The output is an image, in SUN rasterfile format, which can be viewed with your favorite image viewer, or converted to other forms for display or hard copy output. (My preferred viewing/converting packages are the "ImageMagick" package and the "xv" package. However, I prefer them only in the sense that I have them on my system. I would imagine that other packages would be quite satisfactory as well.)

The only type of map projection currently supported is not technically a projection at all, but simply a grid of latitudes and longitudes. (The word "projection" is a mathematical term describing the "stretching" of the earth's roughly-spherical surface onto a flat map.) Over limited areas, this latitude/longitude grid approximates a family of projections called cylindrical projections. By default, the grid is square, in the sense that 1000 pixels in the latitude direction represent the same number of degrees as 1000 pixels in the longitude direction. You can make the grid non-square by playing with the "-x" and "-y" options.

There are several reasons for using this approach. First, the 250K DEM data contain sample points that are evenly spaced in degrees of latitude and longitude, making a simple grid the natural choice. (Drawmap was originally written solely with 250K DEM data in mind.) Second, this is an intuitive projection for small-area maps. Over small areas, it approximates the Transverse Mercator projection, a cylindrical projection that is often used for topographic maps. Third, the USGS DLG data and 24K DEM data are specified in Universal Transverse Mercator (UTM) coordinates, which are based on a rectangular grid of x-y distances. Over small areas, degrees of arc make a reasonable substitute for x-y distances, as long as the horizontal and vertical directions are appropriately scaled. And, finally, the latitude/longitude grid is also acceptable for large-scale maps. Since users of drawmap can produce maps covering any amount of territory, the latitude/longitude grid is a lowest-common-denominator that can handle any request.

In order to find your way around the data, it is useful to know something about the UTM system, which is an international military standard that divides the world into 60 zones (like panels on a beach ball), each of which is 6 degrees of longitude in width, and runs from 80 S to 84 N. A UTM projection, of a given zone, has a central meridian bisecting the map from top to bottom, which serves as a reference from which the locations of other features are derived. Zone 1 runs from 180W to 174W, with its central meridian at 177W. Successive zones run to the east, with zone 2 beginning at 174W.

In the UTM system, the location of a feature is specified by its distance to the north of the equator, in meters, and its distance eastward from the central meridian, in meters plus 500,000. In the southern hemisphere, 10,000,000 is added to the distance north from the equator. (The purpose of the 500,000 and 10,000,000 offsets is to avoid having any negative distances.) Drawmap internally converts UTM distances into latitude/longitude coordinates before plotting features on a map.

Included with drawmap are two programs, utm2ll and ll2utm, that you can use to convert back and forth between UTM coordinates and latitude/longitude coordinates. You don't really need these to use drawmap, but they are useful in their own right. (Be sure to read the associated manual pages to get information on conversion accuracy.)

The result of the cylindrical projection is to map each one-degree by one-degree latitude/longitude patch (on the curved surface of the Earth) into a rectangular area (on the map projection). In the process, of course, the projection distorts shapes and areas as it stretches the beach-ball panels into flat rectangles; and these deviations get larger as the distances from the central meridian and equator increase.

Distortion may also occur due to the way the latitude lines are projected. In the classical Mercator projection, for example, the latitude lines are spaced farther and farther apart as they near the poles (reaching infinity at the poles themselves). This gives the map some useful directional properties, but grossly distorts shapes and areas near the poles. (You can approximate this kind of stretching with drawmap by using the "-x" and "-y" options to vary the number of pixels per longitudinal or latitudinal degree; but remember that, within a drawmap map, latitude and longitude always vary linearly.)

It is a fact of life that mapping a sphere onto a flat piece of paper is going to produce distorted results. Various types of map projections are chosen for the ways they preserve one or more valuable features of a globe-shaped map (features like shape, area, distance, and direction). In the Transverse Mercator projection, the distortions are reasonable for points that are within several degrees of the central meridian, and for maps that aren't too near the poles. In fact, a cylindrical projection has the property that it is "conformal" in the mathematical sense, meaning that it preserves angles (and hence shapes and areas) within small areas of the resulting map. The classical Mercator projection is also conformal in the cartographic sense, meaning that it preserves angles everywhere on the map. (In other words, if the great-circle route from Newark to Peoria is X degrees east-of-north on the globe, then a straight line from Newark to Peoria on a classical Mercator map is also at an angle of X degrees east-of-north.) Of course, over large areas, the classical Mercator projection can grossly distort shapes and areas.

Since, with drawmap, you can define your own image boundaries, the output map may span any portion of one or more UTM zones, and zero or more central meridians may appear at arbitrary positions within the map boundaries. Over small areas, stretching latitude/longitude angles into a square grid (which is what drawmap does) produces roughly the same map image as a square grid of UTM coordinates would. Try to keep the map area smaller than a UTM zone, and center the map on a central meridian, if you want to use the map as a UTM surrogate. UTM coordinates are usually used for areas much smaller than a UTM zone, such as a 7.5-minute USGS quad. For such small areas, the geometrical difference between latitude/longitude angles and surface distances is small.

At the time this manual page was updated (August, 2001), various DEM, DLG, and GNIS files were available for free download by following appropriate links from http://mapping.usgs.gov/. For some files, convenient graphical interfaces were available to let you locate desired files by clicking on a map. Some DEM, DLG, and GNIS files, and the GTOPO30 files too, were available via FTP from edcftp.cr.usgs.gov, in the pub/data directory. (In the case of GNIS files, there was simply a pointer to another download site.) Access to the various files changes over time, so you may have to do some searching to find what you want.

Ordinary DEM and DLG files (that is, non-SDTS and non-GTOPO30 DEM and DLG files) are in (gzip-compressed) ASCII text format, and are human readable (when uncompressed) except that they generally don't contain linefeeds to structure them into easily-editable lines of text. (Some newer DLG files do have linefeeds; and I have come across some DEM files with linefeeds also.) The web site provides information on how to add linefeeds and view the file contents, but drawmap is able to read and use the files in their native state (in gzip format, with a ".gz" suffix on the file name). Drawmap can also process the files in uncompressed form. It is okay to have linefeeds in ordinary DLG files, as long as no line is longer than 80 bytes (including the linefeed); and it is okay to have linefeeds in ordinary DEM files, as long as no line is longer than 1024 bytes (including the linefeed). The drawmap distribution contains the block_dlg and block_dem programs to add appropriate linefeeds to DLG and DEM files but, beyond that, you are on your own if you want to muck around inside the files.

In general, you can add or remove records to or from a DLG or GNIS file, as long as you don't violate the record structure. For example, I have added linefeeds to a DLG file (using the block_dlg program), deleted a record, added a record, and then used drawmap to process the file. If you want to do this sort of thing, then you may also want to get copies of the various guides and standards for the different kinds of files. These documents are available through the web sites.

Using SDTS files is a bit more complicated. SDTS data generally come in the form of tar archives, compressed with gzip. Each such archive should be unpacked into a separate directory. This is true even if there are several archives in a single directory on the download site. (Transportation archives, for example, normally come in triples --- one each for roads, railroads, and other transportation features. These triple archives should be unpacked into three different directories to avoid files from one archive overwriting files from another.)

When you provide SDTS files as input to drawmap, you don't have to include all of the unpacked files on the command line. For DEM files, each archive should contain one or more files with names like ????CEL@.DDF, where the '?' symbol stands for any single character, and the '@' symbol stands for any single digit. Use one or more of these file names (each preceded by "-d") just as you would an old-style DEM file name, and drawmap will figure out the names of the other files in the archive.

For DLG files, each archive should contain one or more files with names like ????LE@@.DDF. Use one or more of these file names just as you would an optional-format DLG file name. There is also a Master Data Dictionary available for each kind of DLG file. At present, drawmap makes no use of these.

Once you have unpacked the archives, you can compress the individual files with gzip if you wish. If you do compress them, compress every file that has a ".DDF" extension. You can also change the file names to all lower case, but don't mix and match upper and lower case files. Other than changing upper to lower case, DO NOT change the file names. Drawmap uses the file names to deduce what to do.

The GTOPO30 files also come in archives, and must be unpacked before use. (You don't need to unpack each archive into a separate directory, but it isn't a bad idea.) Once they are unpacked, you can compress the individual files if you wish, as long as you compress both the ".HDR" file and the ".DEM" file, which are the only files that drawmap uses. (The same guidelines apply as for SDTS files: try to be consistent with upper/lower case, compression, and the like.)

There is one GTOPO30 archive that contains a Polar Stereographic projection of Antarctica. Drawmap can't handle that one. On the FTP site, there is also a gtopo30hydro directory. The files in this directory are derived from GTOPO30 data, but use a Lambert Azimuthal Equal Area projection. Drawmap does not currently handle these either.

To use GTOPO30 files, simply invoke the "-d" option, and provide as a parameter the file whose name ends in ".HDR" (or ".HDR.gz" if you compressed the individual files). Use caution with GTOPO30 data. Each data set spans a large area, and the memory needed to read it all in can be enormous. You can limit the amount of memory required by using the "-l" option to restrict the range of the image to a subset of the available GTOPO30 data.

Be careful with downloads. Some download software will uncompress gzip files during a download but still store the files with a ".gz" suffix. Other download software will leave the data compressed, but remove the ".gz" suffix. Drawmap will become confused when this happens. It relies on the suffix to determine the file type.

If you provide all three types of data (DEM, DLG, and GNIS) as input, then drawmap will first produce a shaded relief map (or, when "-c" or "-C" is specified, a contour map), and then overlay it with data from the DLG files (with the data from each DLG file, in succession, being overlaid on all previous data), and then overlay everything with place names from the GNIS file. If you omit the DEM data, then the shaded relief (or contouring) is replaced by a simple white background.

Drawmap will take whatever information you provide and assemble a map containing just that information. If you provide information that falls outside of your specified map boundaries, it is simply ignored. If you supply any DEM data, and if you don't specify a contour map (via the "-c" or "-C" option), and if there is room, a color key will be placed at the bottom of the map to help you interpret the shaded relief. If you specify the "-c" or "-C" option, then a message about the contour interval will appear at the bottom of the map, if there is room.

Also, if there is room, a title will be placed at the top, containing the lowest and highest values of longitude and latitude for this map, and containing the latitude, longitude, and elevation of the points on the map of lowest and highest elevation. (Actually, of course, there may be multiple points on the map that attain the lowest or highest elevation, but drawmap shows only the first ones that it finds. Furthermore, for low-resolution output images, that have small x and y pixel dimensions relative to the granularity of the available DEM data, drawmap may be a little sloppy about the exact latitude and longitude, and about the exact maximum and minimum elevations.) If only one DEM file is supplied, the location name from the DEM file header will be included in the title. (Sometimes, it is hard to figure out exactly what the correct name is, so don't be surprised if the title looks a bit strange.)

Latitude and longitude tick marks will be placed around the map boundaries, with one tick every tenth of a degree. Tick marks at full degrees and half degrees will be larger and (if there is room) will have text next to them that specifies the latitude/longitude. Tick marks can be turned off with the "-t" option.

North is always at the top of the map, and east is always at the right.

-l latitude_low,longitude_low,latitude_high,longitude_high

You usually must provide latitude and longitude coordinates that define two diagonal corners of the image. They must be separated by a comma or other non-space character (as in: -l 34.3,-109,35.9,-109.713), and they must be in decimal degrees. Note that east longitude is positive and west longitude is negative. Similarly, north latitude is positive and south latitude is negative. If you only provide one "-d dem_file" option, then you can omit the "-l", and the corners of the single DEM file will be used to define the map boundaries. This is useful when you are simply trying to figure out what area a given DEM file covers.

-L

Print out the program license information and exit.

-o output_file.sun

You may provide an output file name. It can be any name that you choose. By convention, SUN rasterfile images have a ".sun" file name extension, but you can omit it if you wish. If you provide no name, then "drawmap.sun" is used. (If you use the "-h" option, and provide no name, then "drawmap.pgm" is used.)

-d dem_file

You can provide as many DEM files as you want. (There is a hard-coded limit of 1000 files in the source code, but it is easily changed.) Since each file covers a limited area, it can take quite a few to cover the image if you specify a large latitude/longitude range for the image boundaries.

You don't, of course, have to provide enough files to cover the whole map area. Areas not covered by a DEM file will simply have a white background. If you have selected the "-c" or "-C" option, there will be anomalous contour lines along the edges of these white areas. If you are using 24K DEM data, there may also be anomalous contour lines around the outer boundaries of the map.

The DEM files will be processed into multicolored shaded relief (or contour lines), serving as a background for any other features you add to the map. If you are trying to draw a contour map using hypsographic data from DLG files (as opposed to drawing a contour map using the "-c" or "-C" option, along with data from DEM files), then you probably don't want to provide any DEM files. The DEM data would mix with the DLG contour lines to produce a confusing morass.

Note that files are processed in the order given. Thus, if you want to provide a 250K DEM file, and overlay parts of it with one or more 24K DEM files, then you want to have the 250K DEM file first in the argument list. This overlays the high-resolution data over top of the low-resolution data. Furthermore, the decision of whether or not to smooth the final image is made based on the last DEM file processed. It is usually desirable to base this decision on the highest-resolution data present.

-c contour_interval_in_meters

This option has no effect unless you provide one or more DEM files. The DEM files are normally processed into multicolored shaded relief. If you include the "-c" option, then the shaded relief is replaced by a set of contour lines (orange lines on a white background) that represent elevations separated by the given contour interval (in meters). Note that it is also possible to generate contour lines by using data in hypsographic DLG files, making the "-c" option seem somewhat redundant. However, at the present time, the area covered by the available DEM files is a superset of the area covered by hypsographic DLG files. Furthermore, the "-c" option allows finer control over the spacing of contour lines than is available with hypsographic DLG data. On the other hand, the DLG data is likely to be more precise about the locations of contours.

-C contour_interval_in_meters

This option is exactly the same as the "-c" option, except that it doesn't use a white background. Instead, it fills in the areas between the orange contour lines using a rotating set of solid colors. These distinct colors make it easier to follow elevation contours as they swirl around the map. (The colors come from the same set used to generate shaded relief, except that white is excluded because it tends to stand out too much from the other colors.)

-g gnis_file

Only one GNIS file is allowed. This is not really a restriction since you can edit these files with an ordinary text editor, making them contain whatever place names you want to include. In fact, it is normally necessary to winnow out much of the available GNIS data; otherwise the map would be plastered nearly solid with place names.

The GNIS data generally come in separate files, one for each US state. Files can be in one of two different formats: a fixed-field-width format in which fields are padded out with white space, and a tokenized format in which the fields are separated by the delimiter "','". You can mix together records from both formats in your customized GNIS file.

WARNING: The format of both kinds of GNIS files has changed; and drawmap will not properly process the older files. If place names don't show up on your maps, then you may need to download newer GNIS files. The newer files have records that begin with a postal code, like NJ, NY, or WY.

The llsearch program (included in the drawmap package) allows you to extract all place names within a certain range of latitudes and longitudes. You can manually edit the resulting extracted data and make further reductions. Each GNIS entry has a field that denotes its type, such as "ppl" for a populated place and "summit" for a mountain top. These fields can help you to narrow down your choices.

The place names are added to the image on top of any other features that you choose to include. A small "+" sign denotes the actual location of the feature.

-a attribute_file

There are three high-level types of objects in a DLG file: Nodes (points where lines join), Areas, and Lines. These objects often have attribute codes associated with them. Each attribute code consists of a major code and a minor code. The major code denotes a particular general type of feature, such as 50 for hydrographic features. The minor code denotes a subtype, such as 412 for a stream, or 421 for a lake or pond.

You can provide an attribute file to control what DLG information is included in the image. Each line in the file consists of a letter 'N', 'A', or 'L' (for Node, Area, or Line), followed by a pair of numbers to denote the major and minor codes, followed by any comments you choose to add. The fields should be separated by white space. Lines that begin with '#', or white space, are ignored.

A negative number, for either the major or minor code, matches anything. Thus, an attribute specification of "L -1 -1" will draw all lines in the DLG files, whether they have associated attribute codes or not. (Omitting the attribute file, or providing the "L -1 -1" attribute specification, ensures that every possible line is drawn, except for the "neatlines" that form a rectangle around the boundaries of the data from each DLG file.) If only the minor code is negative, then all lines of a given major type are drawn. (For example, an attribute specification of "L 050 -1" will match all hydrographic features.)

At present, drawmap makes no use of Node data from the DLG file(s). Thus, there is no need for any "N" entries in the attribute file.

If no attribute file is given, drawmap will ignore the Area data from the DLG file. If Area attributes are specified in an attribute file, then drawmap will attempt to fill the specified types of areas with the same color as the boundary lines that surround them.

The chief use for this is to fill in lakes, reservoirs, and the like. However, because the area-filling algorithm is currently not very robust, and because the Area data in the DLG file can be somewhat ambiguous, it is possible for the outside of an area to be filled in instead of the inside. (I have had this happen often in practice, especially when stretching a map in one direction by specifying unusual map dimensions with the "-x" and "-y" options.) This potential problem is the reason why areas are not filled in unless you make an explicit request in an attribute file.

Another common problem is that sometimes lakes or rivers will be only partially filled in. The reasons for this are beyond the scope of this manual page, but are discussed in comments in the drawmap source code. One solution to both of these problems is to not have drawmap fill any areas. Instead, fill in the areas yourself using an image editor.

The distribution for drawmap includes a file, called "attrib_codes," which is pulled from a USGS guide, and describes various major and minor codes. The distribution also contains a sample attribute file, called "attributes." The sample attribute file contains Area attribute specifications that will cause lakes, ponds, streams, and reservoirs to be filled in. (Both of these files are probably somewhat dated. More current information can be obtained by downloading the appropriate standards documents from the USGS.)

Precious little error checking is done on the data in the attribute file, so be careful.

There is a debugging feature associated with the attribute file. If you specify a major code of 10000, and a minor code of your choosing, then the minor code is taken to be a specific node, area, or line identifier. (Within each node, area, or line record in a DLG file, the first integer in the record is an identifier for the node, area, or line. In general the nodes, areas, and lines are numbered sequentially, starting at 1.)

By specifying Area or Line attributes with major codes set to 10000, you can draw individual areas or lines from a DLG file. This can be useful when you are trying to fine-tune a map or find the source of some problem. When using this feature, it is probably not a good idea to include more than one DLG file in the input arguments. This is because the Node, Area, and Line identifiers are unique within individual files but are re-used from file to file. Thus, if you specify multiple DLG files, you may have a hard time figuring out which file is the source of each area or line on the output map.

Roads and trails show up in red, pipelines and railroads in black, hydrographic features in blue, hypsographic data in orange, boundaries in gray, vegetative features in green, and other data in black.

-x x_size and -y y_size

The horizontal and vertical dimensions of the map, in picture elements (pixels), can be specified via the x and y options. You can supply either or both of them. If you don't provide them, they will be selected so that 250K DEM data can be displayed at one half of full resolution.

As a special case, if you give only a single DEM file, and don't use the "-l", "-x", or "-y" options, drawmap will automatically produce a complete map at full resolution.

For most 250K DEM files, full resolution is 1200 pixels per degree of longitude or latitude, but it is 400 or 600 pixels per degree of longitude for Alaskan files. The full resolution of GTOPO30 files is 120 pixels per degree. For 24K DEM files, resolution is more complicated. The data in these files are sampled on a uniform UTM grid instead of on a latitude/longitude grid, and the elevations may be sampled at spacings of 10 or 30 meters, and the number of samples varies with latitude. Thus, the resolution (in terms of latitude and longitude) can vary considerably from file to file. I use 3600 pixels per degree as a very rough rule of thumb. It is a reasonable approximation for files (with 30-meter spacing) in equatorial regions, but becomes considerably less accurate as one moves north or south. I like this particular number because it is exactly equal to the number of arc-seconds in a degree.

It is generally desirable to specify small x and y values, when you are first trying to fine tune your map, because (at full resolution) even a single one-degree block covers a 1200 by 1200 image, which is larger than many display screens.

Note that the x and y values define the boundaries of the actual map area, but do not define the size of the output image. Drawmap also adds a white border around the image, which makes the output image a bit larger than the x and y values would otherwise imply.

Note also, that it is best to choose x and y values that are some integer multiple or sub-multiple of the resolution of the DEM data. For 250K DEM data, the resolution "1200 times the width and height of the image in degrees of longitude and latitude." For example, if the image is to cover an area that is 0.1 degree square, then the automatically-chosen value for x and y is 60, and full resolution would require x and y to be set to 120. If you want to specify your own dimensions with "-x" and "-y", then it is best to choose an integer multiple or sub-multiple of the full resolution of 120. Choices, in this case, might include 30, 120, 240, and so on. If you choose strange values for x and y, then the program may produce shaded relief that contains odd-looking linear artifacts. If you aren't providing DEM data, then you don't need to worry about this constraint.

Similar comments apply to DEM files for Alaska, except that full resolution is 400 or 600 pixels per degree of longitude. GTOPO30 files are also similar to 250K DEM files, but their full resolution is ten times smaller.

The situation for 24K DEM files is more complicated, since they aren't perfectly rectangular. You may have to try a few different "-x" and "-y" values until you get good results. One starting point is to provide a single 24K DEM file, without using the "-l", "-x", or "-y" options. Drawmap will display the image in full resolution, and will tell you what x and y values it picks. (Alternatively, you can use the "-i" option to print out some information about the DEM file, including its extent in both the x and y directions.) You can use this information to derive approximate x and y values for maps that contain multiple 24K DEM files. However, because of the odd shapes of the 24K quads, you may still have to "twiddle" your derived values for the best results.

Be careful about choosing x and y values that are near, but not equal to, the full resolution of the data. Under these conditions, drawmap has a hard time transferring the data to the image without creating some image blemishes. As an example, if the DEM data has a resolution of 1200 elevations per degree, then an x or y value of 1190 or 1210 would not be the best choice. These values for x or y would be likely to result in a checkerboard effect in areas where the elevation changes slowly.

Note that, when the resolution of the source data doesn't match the resolution of the desired image, drawmap may silently apply a filter to the source data, or to the output image, to blur things out somewhat. This can improve the appearance of the completed map. When the resolutions are about the same, no filtering is done, because I prefer isolated image blemishes to non-localized blurring of the map.

-w

The DLG files describe bodies of water within land areas. However, they don't generally provide polygonal areas to define sea-level water in coastal areas. When you use the "-w" option, drawmap will attempt to make ocean areas bright blue, just like the inland waterways. This feature is provided as an option, rather than as the default, because it sometimes produces odd results. For example, some DEM data in the Sacramento, CA, area give elevations below sea level. With the "-w" option, the map ends up with anomalous-looking sub-sea areas surrounded by water. (This representation may, in fact, be correct. The areas may be polders, pumped out for farming purposes. I don't know. But they look odd.)

-n color_table_number

Drawmap provides a choice of four color schemes for shaded relief. The default is color table 2, which provides a natural-looking color scheme. Using the "-n" option, you can instead choose color table 1, a very-neutral non-obtrusive scheme; color table 3, a natural-looking but garish scheme reminiscent of maps in school textbooks; or color table 4, a very garish scheme designed to provide good perception of variations in elevation. From 1 to 4, the tables are ranked in order of increasing obtrusiveness.

Note that the natural scheme isn't perfect. What looks natural for seacoast plains and mountains may not look as good for highland plains and mountains. The selected color scheme is a compromise. If you are adventurous, you can modify the software to provide additional color tables of your own devising. The software is specifically designed to make such modifications reasonably painless, as long as you are familiar with the 'C' programming language.

For elevations below sea level, drawmap simply re-uses one of the colors used above sea level. A grayish or blueish color is selected, if possible. The reason for the re-use is that sub-sea-level areas are rare, and color table space is a scarce commodity.

-r relief_factor

Normally, when drawing shaded relief, drawmap manipulates the colors in the color table so as to provide maximum sharpness in the relief. In other words, the shading varies from full brightness all the way to black.

You can use the "-r" option to change this behavior. A relief_factor of 1.0 duplicates the default behavior, and is the same as providing no "-r" option at all. A relief_factor of 0.5 causes the colors to shade from full brightness down to half brightness. A relief_factor of 0.0 yields full-brightness color bands, with no shaded relief at all. (The "-r" option has no effect when you are requesting contours with the "-c" or "-C" option.)

You can provide any relief_factor you want, as long as it falls in the range from 0.0 through 1.0. However, keep in mind that the color tables are not infinitely adjustable. As you vary the relief_factor from 0 through 1, the color scheme will change, at most, eighteen times. Thus, it is pointless to provide lots of digits of precision in the relief_factor.

One use of this feature is to provide faint shaded relief, as a background for data you consider more important (such as roads on a road map). For this application, you might choose color table 1 or 2, with a relief_factor of 0.1.

-m relief_magnification

Some regions of the world are relatively flat, with only minor relief. In such regions, it may be desirable to make the relief stand out more sharply. The "-m" option allows you to supply a magnification factor that enhances elevation differences. The factor must be greater than or equal to 1.0, and the default value is 1.0. (The "-m" option has no effect when you are requesting contours with the "-c" or "-C" option.)

In order to use this feature, it is useful to know a little about how shaded relief is generated. We begin by assuming that the sun is shining from the northwest, so that slopes facing to the north or west will be more brightly lit than slopes facing south or east. At any given point on the map, we first note the exact elevation of the point. This information is used to select the overall color at that point, such as green, yellow, red, brown, and so on. We then find the difference in elevation between the given point and a couple of nearby points. These results are used to make a rough estimate of the direction the land is sloping at the given point. This estimate is then used to modulate the light/dark shading of the point to reflect the degree to which the point is in sun or in shade. The actual degree of light/dark shading is the result of a hand-tuned algorithm, developed largely through trial and error.

When you provide a magnification factor, the height differences (between the given point and its neighbors) are multiplied by the given factor. Thus, if a given height difference is Z meters, and the magnification factor is 2, the shading is done as if the height difference were 2Z meters. This may result in a somewhat brighter highlight, or a somewhat deeper shadow, or no noticeable change, depending on the direction that the land is sloping. Note that the actual elevation of the point is not modified. Thus, if the elevation calls for the point to be green, it will remain green no matter how large a magnification factor you provide. The only impact of the "-m" option is to modify the light/dark shading at each point.

Don't expect amazing results. The shading calculations are not linearly related to height differences, so the magnification factor has only a limited effect. To maximize perception of height differences, you might want to try the "-z" option, with or without the "-m" option. Remember too that drawmap's shading is only a crude simulation of the light and shadow of real relief. If you want more realistic shading, you might want to use the "-h" option to generate a file of elevation data, which can be imported into a ray-tracing program to produce a more realistic three-dimensional appearance.

-z

When the given DEM data span a small range of elevations, shaded relief uses only a small portion of the color table. In fact, if the range of elevations is small enough, the entire map may end up using only a single color, with whatever light/dark shading is called for by the limited roughness of the terrain. This results in a pretty boring map.

For these situations, drawmap provides the "-z" option, which specifies that the entire range of available colors be used to represent the given terrain. For example, assume that the data only contain elevations between 4567 feet and 5799 feet. Normally (depending on the chosen color table), the color "green" might represent elevations from 0 feet to 1000 feet, and thus no green would appear in the map. With the "-z" option, however, green will instead represent elevations from 4567 feet to 4655 feet, and will show up in the low-lying areas of the map. All of the other available colors will also show up, each representing its proportion of the elevation range. (The "-z" option has no effect when you are requesting contours with the "-c" or "-C" option.)

-i

When you provide this option, drawmap doesn't produce a map. Instead it prints out some useful information about all of the DEM and DLG files that you specify on the command line.

For a DEM file, the information includes: the file name, the DEM name, the latitude/longitude of the southeast and northwest corners, the minimum and maximum elevation, the number of samples in the x and y directions, and an indication of whether or not the file contains linefeeds.

For a DLG file, the information includes: the file name, the DLG name, the postal codes touched by the file (e.g. MT, TX, RI), the type of data present in the file, the latitude/longitude of the southeast and northwest corners, and an indication of whether or not the file contains linefeeds.

Drawmap may not always be able to find the postal codes in a DLG file, so don't be upset if the field is blank. In DEM files, the DEM name may contain some postal-code information, but not always. SDTS files aren't human-readable, so their linefeed information is omitted.

When the "-i" option appears, all other options are ignored except the "-d" option.

-h

When you provide this option, drawmap doesn't produce a map. Instead it takes the DEM information and produces a height-field file, in Portable Graymap (PGM) format. The file is readable ASCII, beginning with the line "P2", which identifies the file as a PGM file. The next line contains the x and y dimensions, and the maximum elevation in the file, separated by white space. Then the file includes all of the elevation samples, one per line, beginning with the top west-to-east row, and followed by all of the other rows in sequence. Finally, there are some commentary lines containing information about the data, including the latitude/longitude of the southeast and northwest corners, and the minimum and maximum elevations.

This file is suitable for use by ray tracing programs (such as the readily-available POV-Ray(tm) program) to produce 3-dimensional renderings of terrain. (It is also viewable by some image viewers, such as the "xv" viewer, and can be used as input to custom-built programs that process elevation data.)

Unless you select a file name, with the "-o" option, the file will be called "drawmap.pgm".

Any elevations less than zero are bumped up to zero, and any areas of the image that contain no DEM data have their elevations set to zero. (In the latter case, the points are not included when determining the minimum elevation in the file. In the former case, the minimum elevation will be zero.)

Drawmap will also generate a file called "drawmap.pov". This file is a rough attempt at a POV-Ray (version 3) file which, together with the PGM file, can be used to produce a rendering of the 3-dimensional terrain. The file will probably require manual editing to get things the way you want them, but it is at least a start. There are some minimal instructions, embedded in the file as comments, but you are assumed to be familiar with POV-Ray before you use the "-h" option.

-t

Normally, drawmap will put tick marks and latitude/longitude numbers around the borders of the map. However, for maps that span large regions of the earth, these tick marks and numbers can overlap and interfere with one another. Drawmap makes a limited attempt (with emphasis on the word "limited") to reduce the density of the markings as the map area increases. Rather than try to adapt to any situation, though, I chose to provide the "-t" option, which totally shuts off production of tick marks and latitude/longitude legends. It is for use in situations where the border markings become cumbersome.

dlg_file

Any argument that doesn't match any of the above options is assumed to be a DLG file. You can add as many as you like. Note that files are processed in the order given, and each file is overlaid by the ones that come after it. Thus, you generally want to put "transportation" files after "hydrography" files, so that roads will be shown as crossing over streams instead of the other way around.

To generate a simple shaded relief map for a portion of the southern California coast, with the size of the map set to a reduced resolution of 300x300 pixels (full resolution would be 1200x1200):

drawmap -d santa_ana-w.gz -l 33,-117,34,-118 -x 300 -y 300

To extract the upper right quadrant of the above map, and display it at full resolution:

drawmap -d santa_ana-w.gz -l 33.5,-117,34,-117.5 -x 600 -y 600

To add in some place names from a GNIS file (that you have prepared in advance, using llsearch):

drawmap -g gnis_santa_ana_west -d santa_ana-w.gz -l 33.5,-117,34,-117.5 -x 600 -y 600

To add in some DLG files for hydrography:

drawmap -g gnis_santa_ana_west -d santa_ana-w.gz -l 33.5,-117,34,-117.5 -x 600 -y 600 santa_ana-e_CA/hydrography/522274.HY.opt.gz santa_ana-e_CA/hydrography/522275.HY.opt.gz santa_ana-e_CA/hydrography/522276.HY.opt.gz santa_ana-e_CA/hydrography/522279.HY.opt.gz

To draw a map of western Europe, using GTOPO30 data, first download the w020n90.tar.gz and w020n40.tar.gz archives, and unpack them by typing:

gunzip -c w020n90.tar.gz | tar xf -

gunzip -c w020n40.tar.gz | tar xf -

Then draw a map by typing:

drawmap -t -x900 -y1200 -w -l30,20,70,-10 -d W020N90.HDR -d W020N40.HDR

As distributed, drawmap is limited to 1000 DEM files, one GNIS file, and one attribute file. The DEM limit is easily changed in the code. As explained above, the GNIS limitation is not really a limitation, since you can concatenate as many GNIS records as you want into a single file. I'm not sure how to implement multiple attribute files, or even what they would be used for. The number of DLG files is only limited by your system's limits on command-line length.

Another limitation arises from the fact that drawmap must be able to read all of the input data into memory. If you want to produce large maps, then you must have large memory.

When dealing with 24K DEM data, there will often be visible seams between the data from different files. There are several reasons for this. First, there can be marked differences in data quality between files. Lower quality data can have a lot of anomalous "fuzz", which forms discontinuities with adjacent data of higher quality. Even if one ignores other sources of discontinuity between data blocks, the visual difference between the two quality levels can be quite obvious.

Second, the calculations used to map raw data into the image may produce discontinuities, because numeric rounding may pull a data point one way, at the edge of one block of data, and may push an adjacent data point the other way, at the edge of the adjacent block of data.

Third, it goes without saying that there may be residual bugs in the code that handles the DEM files.

Finally, there may be flaws in the data itself. For example, some 24K SDTS DEM files, produced before January 2001, are known to have small positional errors. (The non-SDTS DEM files don't suffer from this problem.)

Similar seams may appear between blocks of GTOPO30 data, but aren't usually as obtrusive.

There is another issue involving 24K DEM data. Each 24K quad represents a latitude/longitude square, with one eighth of a degree on each side. However, the native coordinate system for 24K DEM quads is a UTM grid. In UTM coordinates, the latitude/longitude square becomes an approximate quadrilateral, which often has no two sides of the same length. The four sides of the quad will usually be tilted at slight angles to the UTM axes. It is this odd-shaped quad that is stored in a 24K DEM file, as a set of elevation samples that are evenly-spaced in UTM coordinates. (The spacing is normally 10 meters or 30 meters.) Different columns of sample points may contain different numbers of samples, depending on where the columns intersect the slanting sides of the quad.

An evenly-spaced collection of UTM points does not map onto an evenly-spaced set of latitude/longitude points. In order to map the UTM data onto a latitude/longitude grid, drawmap must warp the points into new relative positions, turning the unevenly-shaped UTM quadrilateral into a latitude/longitude square. (You might picture this by imagining an odd-shaped quad, which is cut out of a rubber sheet and covered with a uniform grid of dots, and which is then stretched into a perfect square.) During this warping process, rounding quantization can produce some diagonal artifacts in the map image. (We could sidestep this issue by making drawmap produce maps using an optional UTM grid, rather than always using a latitude/longitude grid. However, drawmap is not presently so endowed.)

Rounding quantization may also sometimes produce anomalous vertical and horizontal linear features on the map, in the form of small discontinuities in the changing elevation. This can happen for data of any level of detail; and is a result of trying, for example, to stretch a 100-by-100 grid of data points to cover a 101-by-101 grid of image pixels. The data don't quite fit, and something, somewhere, has to give.

Whether artifacts are horizontal, vertical, or diagonal, their incidence can sometimes be reduced by adjusting the values of the "x" and "y" image dimensions. However, while this is usually straightforward for 250K DEM or GTOPO30 data, it can be a challenge for 24K DEM data. This is because, while drawmap can tell you the height and width, in UTM samples, for the raw 24K DEM data, it doesn't know how to figure out an optimal width and height after the data are warped onto a latitude/longitude grid. Furthermore, when you provide more than one 24K DEM file, there is no truly optimal width and height for the image, because the quadrangle covered by each file has a slightly different shape from the quadrangles adjoining it. What works well for one quad may not be the best for another. I don't have a general rule of thumb for adjusting the width and height. I usually just try a few minor tweaks, to the "x" and "y" values, and pick the one I like the best.

Faced with various possible image artifacts, drawmap tries to smooth things out, but faces a tradeoff between making the image look good (and blurring some of the good data), or leaving the good data unaltered (resulting in some esthetic imperfections). Drawmap tends to err on the side of leaving good data untouched at the expense of leaving some artifacts in isolated spots on the image. It tries hardest to preserve the pristine data when the map image dimensions are approximately the same as the resolution of the raw data. This is because, in such cases, there is approximately a one-to-one mapping between the raw data and pixels in the image. It seems useful to preserve this correspondence, and not to blur it with smoothing algorithms.

When you specify image dimensions that differ considerably from the resolution of the data, drawmap takes more liberties in its attempt to produce pleasing results. If the map resolution is greater than the resolution of the data, then drawmap must replicate data points in order to fill up the map. It does some smoothing on the finished image to reduce the resulting "checkerboard" effect. If the data have considerably greater resolution than the map image, then drawmap has more data than it needs, so it averages adjacent data points to determine each elevation in the map. Thus, one alternative for reducing image artifacts is to produce a map at, say, half resolution. If, for example, a 24K DEM file has a 300-by-400 sample grid, then you might try drawing a map with "x" set to 150 and "y" set to 200. The averaging operation will then smooth the data, which will usually reduce image artifacts.

If you are using the "-c" or "-C" option, and the given DEM files do not fully cover the image, there may be anomalous contour lines along the borders of the valid data. (This happens when you fail to completely tile the image with DEM files.) This problem is a result of the way the contours are produced. It may get fixed some day, but isn't a high priority since it is usually hard to mistake these anomalies for valid data.

The code that reads SDTS files is not a complete implementation of all of the relevant standards involved in SDTS. In particular, SDTS relies on the ISO 8211 standard, and it would not be at all difficult to construct a valid ISO 8211 file that drawmap would be unable to read. The code is intended to be smart enough to read SDTS files from the USGS, and hopefully from other sources, but it is not necessarily smart enough to read any file you might throw at it. If you find a USGS SDTS file that drawmap can't read, I would be interested in hearing about it. (I can't promise to fix the problem, because the range of possibilities is large, and I don't want to end up trying to support every dialect that happens to pop up.)

Most of the SDTS DEM files I have examined store elevations as binary two's-complement integer numbers. Some files, however, store them as binary floating-point numbers, in IEEE 754 format. When it encounters such a file, drawmap simply assumes that your computer uses IEEE 754 floating point as its native floating point format. If this assumption is not true, then the file won't be correctly parsed.

There may be some DLG attribute codes that are not properly handled. While I have downloaded and processed thousands of DLG files, in the various supported formats, I can't be sure that this subset of the available files spans the full range of possibilities. Also, it is not always clear, in the relevant specifications documents, exactly how attributes should be encoded, in either the old-style DLG files or the newer SDTS DLG files. I know of at least a few ambiguities that I am not sure how to handle. These are documented in the source code. Furthermore, there are numerous special cases, some of which appear to involve relatively small subsets of the USA. I put a lot of effort into trying to properly process the attributes, but it is difficult to test every possible situation, and my patience for dealing with finicky details is not infinite.

llsearch(1), utm2ll(1), ll2utm(1), block_dlg(1), block_dem(1), sdts2dem(1), sdts2dlg(1), pgm(1)