I have a clipped surface using Washington State Plane North (feet) added to a QGIS project using the same CRS. I am trying to calculate the area under this surface (a dirt pile) using SAGA Raster Volume. The volume returned by SAGA is crazy big. I have tried different versions of QGIS and get the same results. The dirt pile has the correct elevation values (in feet above NAVD88). The surface draws in the correct location. Why does this tool return such a high number? Does SAGA Raster Volume not return the volume in map units?
Unfortunately, SAGA does not know NAVD88. When the data is given in CRS unknown, SAGA tends to assume it is geographical (latlong) coordinates.
Your cell size (0.54 ft) is mistaken as 0.54 degrees (~ 30 miles or 160,000 feet) so the cell volume would be around 160^2 * 10^6 * height (cu-ft). Then SAGA sums up all these cell volumes.
Alternative options would be:
- If you are using QGIS 3.6 - Raster surface volume tool in the
Processing Toolbox > Raster analysis.
- If you are using QGIS 3.0~3.4 - Wait for SAGA 7.3 and use it under Processing Saga NextGen Provider Plugin.
- If you are using QGIS 2 (e.g. 2.18) - Use Raster Calculator to subtract Base Level. Then use Raster layer statistics to calculate SUM.