i.points is an imagery function that enables the user to mark points on a (raster) image to be rectified and then input the geographic coordinates of each point for calculation of a coordinate transformation matrix. i.points must be followed by use of the GRASS program i.rectify, which rectifies the image using the transformation matrix coefficients calculated by i.points.

Rectification is the mapping (transformation) of an image from one coordinate system to another. The geometry of an image extracted into a GRASS LOCATION having an x,y coordinate system is not planimetric. To create a planimetric image, that is, to convert the x,y coordinate system into a standard coordinate system (for example, the UTM coordinate system or the State Plane coordinate system), points from a map having the standard coordinates must be associated with the same points on the image to be rectified. i.points enables the user to mark points on an image and input the standard coordinates for that point. i.points then calculates a least squares regression using the two coordinate systems (x,y and standard) for the marked points. A matrix containing transformation coefficients is the output file for i.points.

During the process of marking points and entering map coordinates, the user can compute the RMS (root mean square) error for each point entered. i.points does this by calculating the transformation equation (the same one that is calculated in the GRASS program i.rectify), and then plugging these results into an equation for RMS error.

i.points offers a zoom option to locate precisely the point to be marked on an image. This program also offers the user the option of acquiring standard coordinates for a marked point from a map layer in the target data base.

i.target must be run before running i.points to enable the PLOT RASTER option to be used and to identify a target data base LOCATION_NAME and MAPSET for the rectified image. To run i.points, a graphics monitor is required.

The procedure for marking points, entering coordinates, and calculating RMS error is described below.

The first prompt in the program asks the user for the imagery group to be registered. Note that if i.target is not run before i.points, the i.points program will display the following error message:

ERROR: Target information for group [spot] missing

Please run i.target for group [spot]

After entering the group to be registered the terminal screen displays the message:

Use mouse now...

The graphics monitor displays the following screen:

______________________________________________________________

| imagery filename (mag) | target filename (mag) |

|_________________________________|___________________________|

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|_________________________________|___________________________|

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|_________________________________|___________________________|

| QUIT ZOOM PLOT RASTER ANALYZE| |

|_________________________________|___________________________|

A pop-down menu like that shown below will be superimposed on the left half of the screen:

__________________________________

| Double click on raster map layer|

| to be plotted |

| Double click here to cancel |

|_________________________________|

____________________

| Mapset demo |

|___________________|

| spotclass| spot.1|

|__________|________|

| composite| spot.2|

|__________|________|

| spot.3 | |

|__________|________|

Any single raster map layer in the imagery group may be used on which to mark points, and the user can mark points on more than one raster map layer in the imagery group to accumulate the suggested minimum number of 12 points. Any raster map layer in the imagery group can be rectified (using i.rectify) based on the transformation matrix computed from these points.

The imagery file chosen by the user is displayed in the upper left quadrant of the screen.

To magnify the displayed file, the user must place the mouse cross hairs on the word ZOOM. The following menu will then be displayed at the bottom of the screen:

____________________________________________

| Cancel| Box| Point| Select type of ZOOM|

|_______|_____|_______|_____________________|

The user has the option of identifying the zoom region either by using the mouse to make a box, or by using the mouse to mark the two diagonal points of the desired region. The terminal screen will display a mouse button menu to guide the user in identifying the corner points of the region.

To mark the points on the image that correspond to the points on a standard coordinate system map, the user must place the mouse cross hairs on the corresponding location on the image to be marked and press the left hand button on the mouse. A diamond shaped symbol will be marked on the image. The user's terminal will display the following menu:

___________________________________________________________

| Point 1 marked on the image at| |

| East: 1023.77 | |

| North: -164.41 | |

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|__________________________________________________________|

| Enter coordinates as east north: |

|__________________________________________________________|

The user then enters the easting and northing (separated by a space) for the point marked on the image. If the user wishes not to enter a coordinate, he or she may simply hit RETURN to return control to the mouse; the marked point then disappears.

In addition to acquiring reference points from a standard map, the user has the option of acquiring the reference points from a raster map layer in the target data base LOCATION_NAME. The data base raster map layer is displayed by placing the mouse cross hairs on the words PLOT RASTER. The following line is then displayed at the bottom of the graphics monitor:

________________________________________________

| Cancel| Indicate which side should be plotted|

|_______|_______________________________________|

Which side of the graphics monitor is to be plotted is indicated by placing the mouse cross hairs on the half of the graphics monitor screen that the user would like to use, and pressing the left mouse button. The following pop-down menu will be superimposed on the half of the screen that was chosen:

_________________________________________

| Double click on raster (cell) map layer|

| to be plotted |

| Double click here to cancel |

|________________________________________|

______________________________

| Mapset demo |

|_____________________________|

| tm.rectified | |

|______________|______________|

| tm.classified| |

|_____________________________|

| Mapset PERMANENT |

|_____________________________|

| elevation | geology |

|______________|______________|

| slope | soils |

|______________|______________|

| aspect | |

|______________|______________|

| roads | |

|______________|______________|

After the raster map layer is displayed the following message appears at the bottom of the graphics monitor:

______________________________________

| input method | keyboard| screen|

|_________________|__________|________|

If the user wishes to use the plotted raster map layer only as a comparative reference, then the keyboard can be chosen as the means to input coordinates corresponding to the marked points on the image. This is done by placing the mouse cross hairs on the word KEYBOARD and pressing the left button on the mouse.

If the user selects the SCREEN option, then points marked on the image will automatically be associated with the coordinates from the corresponding points on the target data base map layer. In this option, when the user marks a point on the image, the following menu is displayed at the terminal:

___________________________________________________________

| Point 5 marked on the image at| |

| East: 1023.77 | |

| North: -164.41 | |

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| Point located at | |

| East: 679132.57 | |

| North: 4351080.67 | |

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|__________________________________________________________|

| use mouse now... |

|__________________________________________________________|

The user then uses the mouse to mark a corresponding point on the displayed image from the target data base. The coordinates for the target data base map layer are automatically saved as the coordinates corresponding to the marked point on the image.

After a number of points have been marked (4 to 7), the user can check the RMS error of the points marked on the image. This is done by placing the mouse cross hairs on the word ANALYZE at the bottom of the graphics monitor. An error report resembling that shown below is superimposed on the graphics monitor:

______________________________________________________________________

| error image target |

|# row col target east north east north |

|____________________________________________________________________|

|1 0.0 -0.9 1.0 1048.5 -144.8 679132.5 4351080.6|

|2 0.4 1.0 1.3 2153.1 -567.2 684314.7 4399001.4|

|3 -1.2 -0.5 .6 1452.8 -476.5 567841.4 3457682.8|

|4 1.1 0.5 1.3 1034.0 -109.2 677573.8 4352626.4|

|5 -2.7 14.0 14.2 1048.6 -144.9 679132.6 4351080.7|

| |

|____________________________________________________________________|

| overall rms error: 4.46 |

|____________________________________________________________________|

The following menu then appears at the bottom of the graphics monitor:

____________________________________________________________________

| DONE| PRINT FILE| Double click on point to be included/excluded|

|_____|____________|________________________________________________|

The RMS error for the image is given under the column TITLEd "error" and subTITLEd "row" and "col". In the above report, point number 1 is 0.0 rows and -0.9 columns from the predicted location calculated from the transformation equation. The RMS error for the target raster map layer is listed under the heading "target". This is the RMS error for the east and north coordinates of the target map layer, but it is presented in the table using one general value. The overall RMS error is displayed at the bottom of the screen in meters. Points that create high RMS error are displayed in red on the graphics monitor (represented here in italics).

The location of the point marked on the imagery group file is given under the heading "image" and the subheadings "east" and north". The location of the point in the target data base is given under the heading "target" and the subheadings east" and "north". If the user would like to exclude or include a point, this can be accomplished by placing the mouse cross hairs on the point number to be included (if the point is absent) or excluded (if the point is displayed) and pressing the left button on the mouse twice. When a point is excluded, it is not afterwards included in the calculation of the RMS error, or included in the final transformation matrix. However, it can be retrieved within i.points at any time by double clicking with the mouse as described above.

To end the i.points program place the mouse cross hairs on the word QUIT; the marked points (including coordinates) will be saved.

A good rule of thumb is to mark at least 12 to 15 points which are evenly distributed over the entire imagery group file in order to obtain an accurate transformation equation for the rectification process. The RMS error may increase with more points added, but the transformation equation will be more accurate.

An RMS error of less than or equal to approximately one resolution unit (pixel or cell) for the image being rectified is generally considered acceptable.

In order to use a digitizer with i.points, at least one digitizer driver besides "none" (the on-screen digitizer) must be available in the digitcap file.

This program is interactive and requires no command line arguments.

GRASS Tutorial: Image Processing

g.mapsets

i.group

i.rectify

i.target

i.vpoints

Michael Shapiro, U.S.Army Construction Engineering Research Laboratory

Last changed: $Date: 2005/01/13 16:28:58 $

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