Calculate Volume over sloped ground in GRASS GIS

Question

I am trying to do some volume calculations on a DEM with GRASS GIS.

However, I believe r.volume measures everything from 0.

The ground underneath the pile I'm measuring is sloped.

I would like to calculate the volume to reflect this, but I don't know the best way to go about it.

Answer 1

Assuming you start with a DEM called terrain, I would try to:

I. Digitize a border around the pile. Either:

1. Prepare a text file with the polygon vertices. The format is called 'standard' and is described in https://grass.osgeo.org/grass64/manuals/v.in.ascii.html. It will look something like:

B N
  x1 y1
  x2 y2
  ...
  xN yN
  x1 y1

where N is the number of vertices in one boundary. The structure repeats for more than one boundary. Then, import the text file: v.in.ascii -n format=standard input=textFile.txt output=pile_boundary

2. If you're using grass6, you might just be able to digitize the boundary using: v.digit -n map=pile_boundary bgcmd="d.rast map=terrain_surface"
3. For grass7, v.edit may help although I haven't used it.

II. Make a new terrain surface by punching out the pile and estimating the ground under the pile

# convert the digitized vector boundary to raster area
v.to.rast type=area use=cat input=pile_boundary output=pile_area
# punch out the pile and reinterpolate underneath it to estimate the terrain surface bellow the pile. 
# FYI, grass 6 and 7 r.mapcalc syntax is a little different.
r.mapcalc expression='terrain_wo_pile=if(isnull(pile_area),terrain,null())'
r.fillnulls input=terrain_wo_pile output=terrain_below_pile

III. Calculate the area of the pile itself

r.mapcalc expression='pile_height_above_terrain=terrain-terrain_below_pile'
r.volume data=pile_height_above_terrain clump=pile_area