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How is the NDVI Difference Image Service created?
This service is created by taking the difference (subtracting) of the NDVI values between two years of comparison, adding 255 to that number and multiplying the result by 0.5.
The NDVI function creates a raster with values that range from -1 to 1, which is a 32-bit raster/service. 32-bit services can be very slow, thus a scaling factor is applied to get values to be less than 255. This results in an 8-bit service.
The difference image service range is manually defined as 27-227. Choosing the DRA option from the Image Analysis Window will give the actual values.
Below are the steps to understand the creation process:
1. Add NDVI difference Image Service and turn on dynamic range adjustment (DRA) with no stretch. This gives the actual values instead of the manually defined range of 27-227.
2. Add the Multispectral 1990/2005 Image Services and add the Image Analysis Window (IAW) NDVI function to each service.
3. Remove the colormap function from both layers through the Layer Properties > Functions tab and apply DRA from the IAW with no stretch to see the actual NDVI values. Keep in mind that this value is scaled from the -1 to 1 range by multiplying by 100 and then adding 100.
4. Identify the pixel values and subtract 2005 from 1990 (140 - 119 = 21).
5. Add the calculated difference value to 255 and multiply by 0.5 (21 + 255 = 276 and then 276 * 0.5 = 138). The NDVI difference pixel equals the same value.
ArcGIS Desktop URL to imagery services (use this link in ArcMap): http://imagery.arcgisonline.com/ArcGIS/services