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--- docs:gis:projections:assigning_a_projection_to_a_raster_file [2007/04/19 17:40] – created billh
+++ docs:gis:projections:assigning_a_projection_to_a_raster_file [2008/08/03 00:25] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
+====== Assigning a Projection to a Raster File ======
+  - gather any information about the raster file, such as the extents of the image, the projection used, etc...
+  - create a [[:docs:gis:world_files|world file]] (sidecar file) for the image
+  - 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
+===== Tips =====
+  * use the cs2cs command to convert known extend coordinates
+===== Example =====
+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:<code>
+    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
+</code>
+  * command used to determine mercator projection data:(numbers are x y as in long lat, unless option flags are used)<code>
+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
+</code>
+  * 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):<code>
+scale:
+(3537017.77 x 2) / 9600 = 736.8787021
+world file contents:
+.8787021
+-736.8787021
+-3537017.77
+6642819.94
+</code>
+  * we also knew how to adjust the projection in mapserver:<code>
+...
+	EXTENT -3537017.77 1333018.49 3537017.77 6642819.94
+...
+	PROJECTION
+		"proj=merc"
+		"ellps=WGS84"
+		"lat_ts=37.5"
+		"lon_0=-90"
+	END
+...
+</code>