DESCRIPTION

v.transform performs an affine transformation (translate and rotate) of a vector map. An affine transform includes one or several linear transformations (scaling, rotation) and translation (shifting). Several linear transformations can be combined in a single operation. The command can be used to georeference unreferenced vector maps or to modify existing geocoded maps.

NOTES

When using an ASCII table containing source and target coordinate pairs, in each row four coordinate values separated by white space have to be specified. Comments are permitted and have to be indicated by a '#' character.

Example for a points file of a linear transformation from XY to UTM coordinates (L: left, R: right, U: upper, L: lower, N, S, W, E): 

# Linear transformation from XY to UTM coordinates:
# 4 maps corners defined
# UL NW
# UR NE
# LR SW
# LL SE
-584  585  598000 4920770
 580  585  598020 4920770
 580 -600  598020 4920750
-584 -600  598000 4920750

The ground control points may be also (ir)regularly distributed and can be more than four points.

Transformation parameters (i.e. xshift, yshift, etc.) can be fetched from attribute table connected to the vector map. In this case vector objects can be transformed with different parameters based on their category number. If the parameter cannot be fetched from the table, default value is used instead.

Affine Transformation Matrix

The affine transfomation matrix can optionally be printed with the '-m' flag. The format of the matrix is: 
| x_offset a b |
| y_offset d e |
This format can be used in the Affine() function of PostGIS [Affine(geom, a, b, d, e, xoff, yoff)], or directly compared to the output of a similar operation performed in R.

EXAMPLE

DXF/DWG drawings

Most DXF/DWG drawings are done within XY coordinate space. To transform them to a national grid, we can use 'v.transform' with a 4 point transformation. 

v.transform -t in=watertowerXY out=watertowerUTM points=wt.points zscale=0.04 zshift=1320

Extrude 2D vector points to 3D based on attribute column values

Spearfish example with manual table editing for vertical shift: 
# create table containing transformation parameters:
echo "create table archsites_t (cat int, zs double)" | db.execute
# insert transformation parameters for category 1:
echo "insert into archsites_t values (1, 1000)" | db.execute
# insert transformation parameters for category 2 (and so forth):
echo "insert into archsites_t values (2, 2000)" | db.execute

# perform z transformation:
v.transform -t input=archsites output=myarchsites3d column="zshift:zs" table="archsites_t"
# drop table containing transformation parameters:
echo "drop table archsites_t" | db.execute
The resulting map is a 3D vector map.

Extrude 2D vector points to 3D based on attribute column values

Spearfish example with automated elevation extraction for vertical shift: 
# work on own map copy:
g.copy vect=archsites@PERMANENT,myarchsites
# add new 'zs' column to later store height of each site:
v.db.addcol myarchsites col="zs double precision"

# set region to elevation map and fetch individual heights:
g.region rast=elevation.10m -p
v.what.rast myarchsites rast=elevation.10m col=zs
# verify:
v.db.select myarchsites

# perform transformation to 3D
v.transform -t myarchsites output=myarchsites3d column="zshift:zs" table=myarchsites
# drop table containing transformation parameters
v.db.dropcol myarchsites3d col=zs
The resulting map is a 3D vector map.

SEE ALSO

v.in.ogr

AUTHOR

Radim Blazek, ITC-irst, Trento, Italy,
Column support added by Martin Landa, FBK-irst (formerly ITC-irst), Trento, Italy (2007/09)

Last changed: $Date$