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.
