v.rectify(1grass) GRASS GIS User's Manual v.rectify(1grass)
NAME
v.rectify - Rectifies a vector by computing a coordinate transforma-
tion for each object in the vector based on the control points.
KEYWORDS
vector, rectify, level1, geometry
SYNOPSIS
v.rectify
v.rectify --help
v.rectify [-3orb] input=name output=name [group=name] [points=name]
[rmsfile=name] [order=integer] [separator=character] [--over-
write] [--help] [--verbose] [--quiet] [--ui]
Flags:
-3
Perform 3D transformation
-o
Perform orthogonal 3D transformation
-r
Print RMS errors
Print RMS errors and exit without rectifying the input map
-b
Do not build topology
Advantageous when handling a large number of points
--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
output=name [required]
Name for output vector map
group=name
Name of input imagery group
points=name
Name of input file with control points
rmsfile=name
Name of output file with RMS errors (if omitted or ’-’ output to
stdout
order=integer
Rectification polynomial order (1-3)
Options: 1-3
Default: 1
separator=character
Field separator for RMS report
Special characters: pipe, comma, space, tab, newline
Default: pipe
DESCRIPTION
v.rectify uses control points to calculate a 2D or 3D transformation
matrix based on a first, second, or third order polynomial and then
converts x,y(, z) coordinates to standard map coordinates for each ob-
ject in the vector map. The result is a vector map with a transformed
coordinate system (i.e., a different coordinate system than before it
was rectified).
The -o flag enforces orthogonal rotation (currently for 3D only) where
the axes remain orthogonal to each other, e.g. a cube with right angles
remains a cube with right angles after transformation. This is not
guaranteed even with affine (1st order) 3D transformation.
Great care should be taken with the placement of Ground Control Points.
For 2D transformation, the control points must not lie on a line, in-
stead 3 of the control points must form a triangle. For 3D transforma-
tion, the control points must not lie on a plane, instead 4 of the con-
trol points must form a triangular pyramid. It is recommended to inves-
tigate RMS errors and deviations of the Ground Control Points prior to
transformation.
2D Ground Control Points can be identified in g.gui.gcp.
3D Ground Control Points must be provided in a text file with the
points option. The 3D format is equivalent to the format for 2D ground
control points with an additional third coordinate:
x y z east north height status
where x, y, z are source coordinates, east, north, height are target
coordinates and status (0 or 1) indicates whether a given point should
be used. Numbers must be separated by space and must use a point (.) as
decimal separator.
If no group is given, the rectified vector will be written to the cur-
rent mapset. If a group is given and a target has been set for this
group with i.target, the rectified vector will be written to the target
location and mapset.
Coordinate transformation and RMSE
The desired order of transformation (1, 2, or 3) is selected with the
order option. v.rectify will calculate the RMSE if the -r flag is
given and print out statistcs in tabular format. The last row gives a
summary with the first column holding the number of active points, fol-
lowed by average deviations for each dimension and both forward and
backward transformation and finally forward and backward overall RMSE.
2D linear affine transformation (1st order transformation)
x’ = a1 + b1 * x + c1 * y
y’ = a2 + b2 * x + c2 * y
3D linear affine transformation (1st order transformation)
x’ = a1 + b1 * x + c1 * y + d1 * z
y’ = a2 + b2 * x + c2 * y + d2 * z
z’ = a3 + b3 * x + c3 * y + d3 * z The a,b,c,d coefficients are deter-
mined by least squares regression based on the control points entered.
This transformation applies scaling, translation and rotation. It is
NOT a general purpose rubber-sheeting, nor is it ortho-photo rectifica-
tion using a DEM, not second order polynomial, etc. It can be used if
(1) you have geometrically correct data, and (2) the terrain or camera
distortion effect can be ignored.
Polynomial Transformation Matrix (2nd, 3d order transformation)
v.rectify uses a first, second, or third order transformation matrix to
calculate the registration coefficients. The minimum number of control
points required for a 2D transformation of the selected order (repre-
sented by n) is
((n + 1) * (n + 2) / 2) or 3, 6, and 10 respectively. For a 3D trans-
formation of first, second, or third order, the minimum number of re-
quired control points is 4, 10, and 20, respectively. It is strongly
recommended that more than the minimum number of points be identified
to allow for an overly-determined transformation calculation which will
generate the Root Mean Square (RMS) error values for each included
point. The polynomial equations are determined using a modified Gauss-
ian elimination method.
SEE ALSO
The GRASS 4 Image Processing manual
g.gui.gcp, i.group, i.rectify, i.target, m.transform, r.proj, v.proj,
v.transform,
Manage Ground Control Points
AUTHOR
Markus Metz
based on i.rectify
SOURCE CODE
Available at: v.rectify source code (history)
Accessed: unknown
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GRASS 7.8.7 v.rectify(1grass)
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