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I'm trying to find a way to lower a GeoTIFF raster file by a fixed value. I will try to describe my process so far.

I start by downloading a GeoTIFF from the usgs webviewer, these are in CRS "EPSG:4269 - NAD83". I import that into QGIS and apply the " INVERSE(EPSG):3856, INVERSE(DERIVED_FROM(EPSG)):1737" transformation to transform to WGS84. After this step I clip the raster to my project limits by creating a new polygon shapefile layer and clipping the raster to that. I end up with a nice raster that is just where I need it to be however the elevation data is in navd88 orthometric heights and I need them to be in ellipsoid heights. The difference in my region is about 75 feet or 23 meters more or less. So is there a way to simply lower the elevation of a raster by 23 meters while retaining the horizontal placement?

I have tried to extract contours and then use the rastorize tool with a "fixed burn in value" of -23 but that levels the whole raster to -23 elevation. I have tried the rastorize tool with different settings and have gotten a mix of results with non being close to what I am looking to do. I have also tried changing the min/max values in the symbology tab of the properties and then exporting the raster layer through export>save as and selecting the "rendered image" output mode but the resulting file was actually higher and not lower.

I am not the most fluent in python coding or QGIS in general.

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Here are some instructions on using the raster calculator. All you would need to do is subtract 23 from your existing layer (assuming your elevation values are in meters) or 75if the units of measure are in feet.

Better still though would be do the vertical datum conversion using something like vDatum.

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