I am using QGIS 2.18.19 to produce viewsheds of a proposed development to show where the development would in theory be visible from. I am using Ordnance Survey Terrain 5 as my height data (*.asc) which I have loaded and saved out as a GeoTIFF so that it is correctly geo-referenced to the coordinate reference system I am using (OSGB36).
At present I have run a viewshed of the development using the DTM which gives the theoretical visibility as if the earth was bare. I have a separate vector dataset of woodland which I have converted to a raster.
Firstly, how can I add give the woodland raster a notional height so that each block of woodland has a height of 10m?
Secondly, how do I add the raster of the woodland (with notional woodland height) to the DTM so that I have a raster of the DTM with woodland heights?