i.vpoints

This program enables the user to identify coordinate pairs of points from a vector map or keyboard entry and corresponding points in an image to be rectified. The map coordinate values of each point are used to calculate a transformation matrix. The operator may then use the i.rectify program to rectify the image using the transformation matrix coefficients calculated from the control point file created in i.vpoints. The i.rectify program performs a first, second or third order transformation of the image.

The first step is to display the unrectified image and corresponding vector map data. The operator would then mark corresponding control point locations on the image and map. To identify the precise location of a point to be marked, i.vpoints has a zoom option. In addition to marking control points on an image to be rectified and inputting their world coordinate values using the keyboard, i.vpoints has the option to simultaneously display vector map data available in the targeted database, and identify on the vector map the location of the corresponding marked points. When this option is chosen, the coordinate values are input automatically. Any GRASS map layer or vector map in the targeted database LOCATION can be displayed using i.vpoints. The i.vpoints program also has the capability of overlaying (i.e., warping) the vector data onto the raster image to visually check the accuracy of the registration based on the current set of active control 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. The i.vpoints program does this by calculating a transformation equation (the same one that is calculated in the GRASS program i.rectify 2). Coefficients are computed for the equation. The coefficients are then used in the equation along with the x,y coordinates of the marked points. The results are plugged into an equation for RMS error. The interpretation of RMS error is described in the ANALYZE subsection.

The procedures for marking control points (registration points), displaying vector map layers, overlaying vector maps onto the raster image, and calculating RMS error are described in the following sections .

To enter the program (the i.vpoints program requires the use of a graphics monitor) type i.vpoints: i.vpoints

The first prompt in the program asks for the imagery group to be registered . . .

Enter imagery group to be registered

Enter 'list' for a list of existing imagery groups

Enter 'list -f' for a verbose listing

Hit RETURN to cancel request

>

For example, >list

might produce the following response:

<list>

Available groups

- - - - - - - - - - - - - - - - - - - - - - - - - -

test

- - - - - - - - - - - - - - - - - - - - - - - - - -

hit RETURN to continue

whereas, >list -f

might produce the following response

Available groups

- - - - - - - - - - - - - - - - - - - - - - - - - -

test

photo in PERMANENT

res2 in tifftest

- - - - - - - - - - - - - - - - - - - - - - - - - -

hit RETURN to continue

The imagery group entered above should contain the files that you wish to rectify. After entering the group to be registered, the terminal screen displays the message:

>test

<test>

Use mouse now . . .

And the color graphics monitor displays the following screen:

Any single file in the imagery group may be used to mark points, and points can be marked on more than one file in the imagery group to accumulate the suggested minimum number of points (3 for a 1st order transformation, 6 for a 2nd order, and 10 for a 3rd order). Any file in the imagery group can be subsequently rectified (using i.rectify) based on the transformation matrix computed from these points.

The chosen file is displayed in the upper left quadrant of the monitor at a default magnification based on the extent of the current active window.

RASTER IMAGE

The raster image option on the menu at the bottom of the window allows the user to display any single file in the imagery group in the upper right quadrant of the window screen. The option provides the same file selection pick list as is presented when you first enter the i.vpoints program. When you select this option, the program will erase the data contained in all of the four quadrant windows and will reinitialize all program values.

VECTOR MAPS

The vector maps option on the menu at the bottom of the screen allows the user to display vector map data in the upper right quadrant of the screen. After selecting the vector map layer to display, a menu selection bar appears along the bottom on the screen. This pick list is used to select the line color (blue, gray, green, red, white, or yellow) for the selected vector data layer.

Refresh

The refresh option on the main menu allows the user to refresh" or re-draw the displayed vector data. This function will erase all outlines showing the limits of previously zoomed areas. A "yes/no" roompt will appear:

Refresh Map ? NO YES

Zoom

To enlarge a raster or vector image, place the mouse cross hairs on the word zoom on the main menu and press the left button. The following menu will be displayed at the bottom. of the screen:

CANCEL BOX POINT Select type of zoom

You have the option to identify the map extent of the zoom window using either the mouse to define a box, or the mouse to mark a center point from which to enlarge the image. The box option first prompts you to identify a starting corner for the zoom region and then allows you to define the area to be zoomed using a rubber band box. The prompts appear as follows...

CANCEL Mark the first corner of region

CANCEL Define the region

After marking the first corner of the region to be enlarged, hold down the left button and move the mouse to change the size and shape of the rubber band box. After defining the area to be enlarged, press the right button to accept it.

The point method for enlarging an image will display a mouse menu to guide you in selecting the appropriate enlargement. To enlarge or reduce the magnification factor, place the cursor on the "+" or -" box and press the left button on the mouse.

You may zoom either the raster or the vector display.

Upon accepting the new region limits, the raster or vector data are redisplayed in either the lower left (raster) or lower right (vector) windows.

The extent of the zoomed area is outlined on the unzoomed image in the main window area.

While the main menu is displayed, you can mark corrosponding control points on the raster and vector images or enter map coordinates from the keyboard. If you are using coordinates taken from a reference map, circle these points and then use whatever means you have available to identify as precisely as possible the coordinate values for these points. Digitizing software is recommended, especially GRASS v.digit. Once you have determined the standard coordinates (for example, UTM's) of each circled point, you are ready to mark the points on the displayed image.

To mark the points on the image, that correspond to the points on the standard coordinate map, place the mouse cross hairs on the point on the image to be marked (you will probably have to ZOOM to find the exact spot) and press the left hand button on the mouse. A diamond shaped symbol will be marked on the image. The text monitor will display the following screen:

Point 1 marked on the image at

East: 1023.77

North: -164.41

Enter coordinates as east north:

Analyze

After a number of points have been marked (a minimum of 4 for a 1st order transformation, 7 for a 2nd order, and 11 for a 3rd order), the RMS error of the points marked on the image can be checked. This is done by placing the cross hairs on the word ANALYZE on the main menu at the bottom of the monitor. The following error report is superimposed on the monitor:

error image target

# col row target east north east north



1 -0.9 0.0 1.0 1048.5 -144.8 679132.5 4351080.6

2 1.0 0.4 1.3 2153.1 -567.2 684314.7 4399001.4

.

.

.

Overall rms error 76.85

The RMS error for the image being rectified is recorded under the column "error" and subTITLEd "row' and "col". In the above report, the marked point number 1 is 0.0 rows and -0.9 columns from the predicted location calculated by the transformation equation. The RMS error for the target database map is recorded under the heading "error" and the subheading "target". This is the RMS error for the east and the north coordinate values of the target map, but it is represented in the table using one general value. The overall RMS error for the image is displayed at the bottom of the screen in meters. Points that generate a high RMS error are displayed in red on the monitor. The x,y coordinate values of the point marked on the image being rectified are recorded under the heading "image" and the subheadings "east" and "north". The standard coordinate values of the point in the target database are recorded 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 then pressing the left button on the mouse twice. When a point is excluded, it is not included in the calculation of the RMS error, or included in the final transformation matrix. However, it can be retrieved within i.vpoints at any time by double clicking with the mouse as described above.

The following menu appears at the bottom of the monitor:

DONE PRINT FILE OVERLAY DELETE ON Transformation -

-> 1st ORDER Double click on point to be DELETED

Selecting DELETE ON will toggle the option to DELETE OFF, the toggle option is used to allow the user to physically remove a control point from the POINTS file instead of just flagging it as an non-active reference point.

Overlay

Overlay allows the user to overlay the vector map(s) onto the raster image. Overlay can be used to warp (register) and display the selected vector file data on top of the raster image contained in the upper left window of the color screen. An inverse coordinate transformation is performed using the currently active order of transformation (i.e., first, second, or third).

Overlay vectors on raster image NO YES

By selecting the 1st ORDER option, the user may select the order of transformation desired:

Select order of transformation 1st Order 2nd Order 3rd Order

The program will immediately recalculate the RMSE and the number of points required.

Quit

To exit the i.vpoints program, place the mouse cross hairs on the word QUIT at the bottom of the monitor and all of the marked points (including coordinates) will be saved.

GRASS Tutorial: Image Processing

g.mapsets

i.group

i.points

i.rectify

i.target

William R. Enslin, Michigan State University Center for Remote Sensing

Radim Blazek (update to GRASS 5.7)

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

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