The flag -r enables to perform reverse transformation, i.e., transform 3D vector to 2D by omitting z-coordinate. The height of input 3D features can be optionally stored in column.
When transforming 2D vector features to 3D based on attribute, all NULL values are silently converted to height 0.0.
The reverse transformation, 2D to 3D, is possible for points and lines. In the case of lines, the reverse transformation should be used only when all vertices of a line have the same z-coordinate (for example contours).
# convert z-values from string to double v.db.addcolumn map=geodetic_pts columns="Z_VALUE_D double precision" v.db.update map=geodetic_pts column=Z_VALUE_D qcolumn=Z_VALUE v.db.select map=geodetic_pts columns=cat,Z_VALUE,Z_VALUE_D # convert 2D vector point map to 3D based on attribute v.to.3d input=geodetic_pts out=geodetic_pts_3d column=Z_VALUE_D
v.to.3d -rt input=elev_lid792_bepts output=elev_lid_2d