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We have got a quick bird DEM of our site. After getting actual on ground elevation values with a differential GPS device, we have found that there is a difference of average 12 feet between DEM and Actual ground Points. nw i want to alter elevations of that DEM based on actual values.

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    Please specify what software is available and how exactly you wish to alter the DEM values. Raster calculations are of course possible on different GIS but it's hard to answer properly without more information. For instance, do you wish to simply add or subtract 12 feet to all pixels, or something more elaborate? – Nir Aug 28 '15 at 5:43
  • Thank you for your Response. Available software are Arcgis and global mapper . What i wish is that DEM adjusts its elevation values based on elevation values of Bench mark points that have accurate elevations from site survey. – Ibsham Aug 29 '15 at 7:38
  • Sample your points using DEM. In points table calculate difference between elevations. Interpolate this difference using any method, e.g. IDW. Sum original DEM with 'correction' raster – FelixIP Aug 30 '15 at 4:23
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Two ways to go about this using ArcGIS:

Interpolate - makes more sense: Use the Raster to Point tool on your DEM and set the Field option to Value. You will get a layer of points for each center of pixel, with the elevation value of the pixel stored for each point. Then use the Create TIN tool and use both layers as inputs - the layer of elevation values from GPS and the new layer from Raster to Point tool. after that, use TIN to Raster. You may instead copy the GPS elevation points to the new DEM points layer, and perform an interpolation such as IDW or Natural Neighbor. This or the TIN may also be done after deleting DEM points adjacent to the GPS points.

Replace specific pixels - somewhat tricky: Use the tool Point to Raster on your GPS elevation measuerments, set the cell size the same as your DEM pixel size and in the tool Environments, set Processing Extent the same as your original DEM. You will get a raster with many NoData values and some elevation values, according to how many of these you had. Now open tool Field Calculator, use an expression that replaces pixels in your DEM with pixels in the GPS raster that are not NoDatas:

Con(IsNull("GPS_Raster") == False, "GPS_Raster","DEM")

I'll add that I use the extent of the original DEM and then ignore the NoDatas because otherwise, operating on two rasters, it seems that the field calculator uses their intersection, and not extent of the larger one.

And perhaps it would make more sense to use the Raster Calculator to add or subtract a constant value of deviation (you mentioned 12 feet) from your raster.

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