How To: Maintain bit depth of raster data after a projection


Instructions provided describe how to maintain bit depth of projected raster data. Projecting a raster dataset between coordinate systems can rotate the image. This will add NoData cells to the outside of the image to fill in the rotated areas and cause the data range to increase. This is normal behavior for raster data.

For example, if a raster dataset has values 0 to 255 (8-bit) before the projection, these same values will be preserved in the output. However, the addition of the NoData values will increase the data range to 16-bit.


Follow the steps below to change the NoData values to a value that exists in the previous data range using Spatial Analyst. For example, convert the NoData values to something between 0 and 255 to maintain an 8-bit range. A value of 0 is used in the following procedures.

  • ArcGIS 9

    1. Open the Single-Output Map Algebra (SOMA) tool.
    2. Set the Input Raster as the newly projected raster dataset.
    3. Set the Output Raster.
    4. Enter the following into the Map Algebra Expression Box:


    Where [proj_ras] is the name of the Input Raster.

  • Pre-ArcGIS 9

    1. Open up the Spatial Analyst > Options.
    2. Set the Working Directory.
    3. Set the Extent and Cell Size to the input projected raster dataset.
    4. Open the Raster Calculator and enter the following expression:


    Where [proj_ras] is the name of the projected raster dataset.
    5. Right-click the layer.
    6. Select Make Permanent to save the resulting Calculation.

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