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Contrast stretch for DEM symbolization

January 18 2012 | 0 comments
Categories: Map Data, Symbology

I am struggling with the standard deviation stretch type when manually selecting and symbolizing break points in my color ramp. A majority of the area, coastal Gulf of Mexico, is very low relief, with some mountainous areas in Mexico. The higher portions of the DEM are not symbolizing correctly. Using the stretch type min-max, I get correct symbolization of all areas but not with the color break points I have defined. The color stretch also appears more like classified symbology as opposed to algorhithmic color ramps. Attached is an example. Could you please help explain what I am overlooking. I have tried many different ways to work around this. Thanks.

Mapping Center Answer:

The best way to get specific colors to be assigned to specific values is to use a colormap file rather than a color ramp. Using this approach, the bathymetry (or topographic) values will be assigned a specific RGB color value. The only glitch right now is that with ArcGIS 10, all values in a colormap file must be positive (this is fixed in the next version of the software). So you have to convert your bathy data to positive values (add the lowest value to all cell values using the Plus tool in Spatial Analyst) and then create a colormap file with those values in it along with the RGB values you want to use.

Learn more about colormap files in the online help: Key concepts of raster dataset color maps.

You can also check out two blog entries in which we describe working with color map files and bathy/topo data (thus dealing with the issue of requiring all positive values: Using ETOPO1 data and Symbolizing ETOPO1 data.

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