Assign Geocoding Information to a Data File - ERDAS Imagine Tutorial
Assign Geocoding Information to a Data File - ERDAS Imagine Tutorial

Software requirements: IMAGINE Essentials

You may often have to assign geocoding information to an image or data file that was created without geocoding, or which was imported with the generic binary importer. In this example, we will use the Image Info feature in ERDAS IMAGINE to assign geocoding to an NOAA HRPT of the U.S. east coast and Atlantic Ocean. The original image came with upper left and lower right geographic coordinates and nothing else. In many cases, your image may have coordinates, pixel spacing, units of measurement, projection system, spheroid and datum.

In this example, we know that the NOAA HRPT image has these upper-left and lower-right coordinates:

• UL: 44.626N, -77.590W
• LR: 34.389N, -64.398W

1. Open the image in a Viewer. Using the ImageInfo icon, , open the ImageInfo dialog.

Because the image has neither map model nor projection assigned, the Map Info section of the ImageInfo dialog displays the file coordinates. From the Layer Info section, we can see that this image is 2047 x 1231 pixels in size.

2. Using arithmetic, we can determine the image's size in decimal degrees.

• 44.626N - 34.389N = 10.237 degrees N latitude
• 77.590W - 64.398W = 13.192 degrees W longitude

3. Calculate the pixel size: Using arithmetic, divide the image's height and width by the number of pixels.

• 10.237N / 1231 pixels = .00831600 degrees N per pixel.
• 13.192W / 2047 pixels = .00644140 degrees W per pixel.

4. Determine if your image's upper left file pixel corresponds to the upper left image pixel. Some images are skewed or rotated, and the upper left image pixel may not correspond exactly to the upper left file pixel, as in this image:

If your image is not rotated, proceed to Step 5. If your image is rotated, use the inquire cursor and zoom functions to determine the exact number of pixels of offset. Align the inquire cursor with the upper left pixel of imagery. Look at the file location of the pixel. (i.e., pixel and line location). Calculate the number of pixels from the upper left corner of imagery to the upper left corner of the file (0,0). Use the calculated pixel size from Step 3 to calculate the upper left corner coordinates for the file.

5. In the ImageInfo dialog, select Edit | Change Map Model.... The Change Map Info dialog will open.

6. In the Change Map Info dialog, manually enter the information for upper left X and Y, and pixel size X and Y. In the Projection: line, use the drop-down list to select Geographic (Lat/Lon). The units will automatically change to degrees. Select OK, then select Yes in the Attention dialog. You will see the information in the Map Info section of the ImageInfo dialog change.

7. Clear the image from the Viewer. Changes to the projection may not be made while the image file is in use by the Viewer.

8. In the ImageInfo dialog, select Edit | Add/Change Projection. The Projection Chooser dialog will open. Under the Custom tab, use the available pull-down menus to refine the parameters of the projection system. In this example, we will keep the Geographic projection type and use the Clarke 1866 spheroid. Select OK. You will see the projection information change in the ImageInfo dialog.

9. Close the ImageInfo dialog. Re-display the image in the Viewer.

Source: http://www.erdas.com