Using an elevation image service as the DEM input

When orthorectifying imagery, the digital elevation model (DEM) can come from two sources: an .flt (ASCII floating point) file or an image service definition serving elevation data. It is recommended that you create an image service definition to serve the elevation data that you will use as input to orthorectify imagery.

By using an image service definition as the DEM input, you have the ability to update the DEM without affecting the orthorectified image service definition. You can also use any of the elevation raster types as input.

To use an image service definition serving elevation data as the DEM input, you need to do the following:

1. Create an image service definition using the Elevation service type.
   • Generate service overviews.
   • You cannot use the Visualization Elevation process since you will not be visualizing this data. Also, you cannot use an elevation service that has a Visualization Elevation process applied because the cell values output from the image service definition are altered depending on the visualization method used.
2. Compile the elevation image service definition.
3. Publish the elevation image service definition.
4. Create an image service definition reference file (.ISRef) for the elevation image service definition.
   • You can do this by adding the image service definition in ArcMap and saving it as an .ISRef file, or you can start ArcCatalog and create a new image service reference.

The .ISRef file is then used as the DEM input.

Converting to ellipsoidal heights

Orientation parameters for satellite or aerial imagery are relative to ellipsoidal or orthometric heights on a specific height datum. It is important to know which type you are dealing with and ensure that the elevation model used is the same. For example, SRTM elevation data cannot be used directly to orthorectify QuickBird satellite imagery. You must apply the equation below to the SRTM data to convert it to ellipsoidal heights.

To convert to approximate ellipsoidal heights for a local area, you can apply a scale and shift using the terrain offset and scale parameters on the Orthorectification tab when adding the orthorectified raster type. Alternatively, you can use the Convert Pixel Type process or Spectral Matrix process to convert the orthometric height using a geoid within the elevation image service. To do this conversion, you will need to apply the following equation:

h = H + N 
where,
h = ellipsoidal height
H = orthometric (geoid) height
N = geoidal separation

If using the Spectral Matrix process to apply an offset to an elevation image service definition, the number of output bands will be 1, the bands gains will be 1, and the offset to convert from geoid height to ellipsoidal height will be entered in the Band offset text box.