Did an aerial drone survey and have generated a mosaic and .shp file with the elevation and contour data using Pix4D. However, some of the areas elevations are off because the elevations represent the top of the canary grass in those areas.
I would like to be able to subtract approx. 4 feet from the elevations in those areas. Is this possible. If it is, could you outline the steps as I am still learning QGIS.