ULXMAP, ULYMAP, XDIM, and YDIM are the settings within the .HDR file that are used for determining a SPOT image's three control points. PIXEL_ROTATION_SIN and PIXEL_ROTATION_COS are optional rotation settings within the image's .REP file; this file is only present with Geospot products. If a .REP file is not present or the .REP file does not contain any PIXEL_ROTATION_SIN and PIXEL_ROTATION_COS settings, here are the values to use for these settings when calculating control points:
PIXEL_ROTATION_SIN = 0PIXEL_ROTATION_COS = 1The following table shows the pixel coordinates and corresponding equations for determining the three control points. The column on the right of the table shows the results of calculating the control points using the sample data from above. In this case, the image has no .REP file.
| Image X,Y | Equation | Example |
|---|---|---|
|
0,0 |
ULXMAP,ULYMAP |
522147,4086917 |
|
1,0 |
ULXMAP + PIXEL_ROTATION_COS * XDIM, ULYMAP + PIXEL_ROTATION_SIN * XDIM |
522157,4086917 |
|
0,1 |
ULXMAP + PIXEL_ROTATION_SIN * YDIM, ULYMAP - PIXEL_ROTATION_COS * YDIM |
522147,4086907 |
Notice that the control points are at the top left corner of the image (0,0), one pixel to the left (1,0), and one pixel below (0,1); no additional accuracy is gained by choosing control points that are any further apart. Here are the settings you would enter in the Register Raster Image dialog box when registering the sample SPOT image:
| Label | Map X | Map Y | Image X | Image Y |
|---|---|---|---|---|
|
Pt 1 |
522147 m |
4086917 m |
0 |
0 |
|
Pt 2 |
522157 m |
4086917 m |
1 |
0 |
|
Pt 3 |
522147 m |
4086907 m |
0 |
1 |
See Registering the Coordinates of a Raster Image if it is unclear to you where to enter this information.