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I am trying to invert a raster in ArcGIS (10.5), and have used the following expression (from esri support page) in the raster calculator:

invert = (("raster_file" - maximum) * -1) + minimum

However, the high and low values of the new raster are slightly different from the original raster:

original low:0.1551740020513535 original high:0.8855689764022827

new low:0.1852769702673 new high:0.84637397527695

Why is this happening, and what can be done?

  • I just copied the values directly from the Source tab in Raster properties into the raster calculator... – Morani May 29 '18 at 10:48
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    Could it be that your min and max value are not exact value but only estimate, you could try to calculate statistics to get real minmax and try again – J.R May 29 '18 at 11:25
  • Thanks @J.R, would the figures in the raster properties source tab under Statistics be estimates? As figures are present there already, if I do a raster calculation will it replace these figures? – Morani May 29 '18 at 11:29
  • You should check the statistic creation parameter, skip row and column should be at 1 and skip value at 0 to get exact value – J.R May 29 '18 at 11:47
  • I checked said paramaters and both x and y skip factors were already on 1, and I set the statistics ignore value to 0 (not sure if this is what you meant by skip value), and doing so produced the same output as before with different min/max values... – Morani May 30 '18 at 8:47
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The reason I was getting different maximum and minimum values after using raster calculator to invert the values was because I was using different extents. My original raster covered the whole of the British Isles (including Ireland), however, when I was using raster calculator to invert the values, I had set the extent to just the island of Ireland, thus was getting slightly lower maximum and slightly higher minimum values (obviously not capturing the more extreme values from the larger Great Britain).

When I checked this by using the above equation and setting the extent to the whole of the British Isles, the output values for min/max were identical to the original raster.

Lesson learnt: ALWAYS CHECK THE EXTENT!

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