Assigning a Projection to a Raster File

1. gather any information about the raster file, such as the extents of the image, the projection used, etc…
2. create a world file (sidecar file) for the image
3. you may now use the image with the world file if you want to input the projection every time, or you can use gdalwarp to create a new file with the projection information included

• use the cs2cs command to convert known extend coordinates

For a given raster image, we knew the limits of the latitude and longitude, and that the file was using Mercator projection

• we were given:

      Location: 15 to 60 degrees North latitude, 50 to 130 degrees West longitude
      Orientation: North toward the top, Mercator projection
      Image Data: shaded and colored SRTM elevation model
      Original Data Resolution: SRTM 1 arcsecond (about 30 meters or 98 feet)
      Date Acquired: February 2000 

• command used to determine mercator projection data:(numbers are x y as in long lat, unless option flags are used)

  cs2cs +proj=latlong +ellps=WGS84 +to +proj=merc +ellps=WGS84 +lat_ts=37.5 +lon_0=-90 <<EOF
  > -130 15
  > -50 60
  > EOF
  
  Output:
  -3537017.77     1333018.49 0.00
  3537017.77      6642819.94 0.00

• with this output and determining the resolution of the raster file at 9600 x 7240, we knew how to create an appropriate world file (coordinates are based on top left of image):

  scale:
  (3537017.77 x 2) / 9600 = 736.8787021
  
  world file contents:
  736.8787021
  0
  0
  -736.8787021
  -3537017.77
  6642819.94

• we also knew how to adjust the projection in mapserver:

  ...
  	EXTENT -3537017.77 1333018.49 3537017.77 6642819.94
  ...
  	PROJECTION
  		"proj=merc"
  		"ellps=WGS84"
  		"lat_ts=37.5"
  		"lon_0=-90"
  	END
  ...