Flags:

-r

Reverse transformation; 3D vector features to 2D

-t

Do not copy attribute table

--overwrite

Allow output files to overwrite existing files

--help

Print usage summary

--verbose

Verbose module output

--quiet

Quiet module output

--ui

Force launching GUI dialog

Parameters:

input=name [required]

Name of input vector map
Or data source for direct OGR access

layer=string

Layer number or name (’-1’ for all layers)
A single vector map can be connected to multiple database tables. This number determines which table to use. When used with direct OGR access this is the layer name.
Default: 1

type=string[,string,...]

Input feature type
Options: point, line, boundary, centroid
Default: point,line,boundary,centroid

output=name [required]

Name for output vector map

column=name

Name of attribute column used for height
Can be used for reverse transformation, to store height of points

height=float

Fixed height for 3D vector features

Transform 2D vector features to 3D

# 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

Transform 3D vector features to 2D

v.to.3d -rt input=elev_lid792_bepts output=elev_lid_2d