.TH v.rectify 1grass "" "GRASS 8.3.1" "GRASS GIS User's Manual"
.SH NAME
\fI\fBv.rectify\fR\fR  \- Rectifies a vector by computing a coordinate transformation for each object in the vector based on the control points.
.SH KEYWORDS
vector, rectify, level1, geometry
.SH SYNOPSIS
\fBv.rectify\fR
.br
\fBv.rectify \-\-help\fR
.br
\fBv.rectify\fR [\-\fB3orb\fR] \fBinput\fR=\fIname\fR \fBoutput\fR=\fIname\fR  [\fBgroup\fR=\fIname\fR]   [\fBpoints\fR=\fIname\fR]   [\fBrmsfile\fR=\fIname\fR]   [\fBorder\fR=\fIinteger\fR]   [\fBseparator\fR=\fIcharacter\fR]   [\-\-\fBoverwrite\fR]  [\-\-\fBhelp\fR]  [\-\-\fBverbose\fR]  [\-\-\fBquiet\fR]  [\-\-\fBui\fR]
.SS Flags:
.IP "\fB\-3\fR" 4m
.br
Perform 3D transformation
.IP "\fB\-o\fR" 4m
.br
Perform orthogonal 3D transformation
.IP "\fB\-r\fR" 4m
.br
Print RMS errors
.br
Print RMS errors and exit without rectifying the input map
.IP "\fB\-b\fR" 4m
.br
Do not build topology
.br
Advantageous when handling a large number of points
.IP "\fB\-\-overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB\-\-help\fR" 4m
.br
Print usage summary
.IP "\fB\-\-verbose\fR" 4m
.br
Verbose module output
.IP "\fB\-\-quiet\fR" 4m
.br
Quiet module output
.IP "\fB\-\-ui\fR" 4m
.br
Force launching GUI dialog
.SS Parameters:
.IP "\fBinput\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name of input vector map
.br
Or data source for direct OGR access
.IP "\fBoutput\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name for output vector map
.IP "\fBgroup\fR=\fIname\fR" 4m
.br
Name of input imagery group
.IP "\fBpoints\fR=\fIname\fR" 4m
.br
Name of input file with control points
.IP "\fBrmsfile\fR=\fIname\fR" 4m
.br
Name of output file with RMS errors (if omitted or \(cq\-\(cq output to stdout
.IP "\fBorder\fR=\fIinteger\fR" 4m
.br
Rectification polynomial order (1\-3)
.br
Options: \fI1\-3\fR
.br
Default: \fI1\fR
.IP "\fBseparator\fR=\fIcharacter\fR" 4m
.br
Field separator for RMS report
.br
Special characters: pipe, comma, space, tab, newline
.br
Default: \fIpipe\fR
.SH DESCRIPTION
\fIv.rectify\fR 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 object 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).
.PP
The \fI\-o\fR 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 (1\ust\d order) 3D transformation.
.PP
Great care should be taken with the placement of Ground Control Points.
For 2D transformation, the control points must not lie on a line, instead
3 of the control points must form a triangle. For 3D transformation, the
control points must not lie on a plane, instead 4 of the control points
must form a triangular pyramid. It is recommended to investigate RMS
errors and deviations of the Ground Control Points prior to transformation.
.PP
2D Ground Control Points can be identified in
\fIg.gui.gcp\fR.
.PP
3D Ground Control Points must be provided in a text file with the
\fBpoints\fR option. The 3D format is equivalent to the format for 2D
ground control points with an additional third coordinate:
.br
.nf
\fC
 x y z east north height status
\fR
.fi
where \fIx, y, z\fR are source coordinates, \fIeast, north, height\fR
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.
.PP
If no \fBgroup\fR is given, the rectified vector will be written to
the current mapset. If a \fBgroup\fR is given and a target has been
set for this group with \fIi.target\fR,
the rectified vector will be written to the target location and mapset.
.SS Coordinate transformation and RMSE
.PP
The desired order of transformation (1, 2, or 3) is selected with the
\fBorder\fR option.
\fIv.rectify\fR will calculate the RMSE if the \fB\-r\fR 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,
followed by average deviations for each dimension and both forward and
backward transformation and finally forward and backward overall RMSE.
.SS 2D linear affine transformation (1st order transformation)
.br
x\(cq = a1 + b1 * x + c1 * y
.br
y\(cq = a2 + b2 * x + c2 * y
.SS 3D linear affine transformation (1st order transformation)
.br
x\(cq = a1 + b1 * x + c1 * y + d1 * z
.br
y\(cq = a2 + b2 * x + c2 * y + d2 * z
.br
z\(cq = a3 + b3 * x + c3 * y + d3 * z
The a,b,c,d coefficients are determined 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
rectification 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.
.SS Polynomial Transformation Matrix (2nd, 3d order transformation)
\fIv.rectify\fR 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
(represented by n) is
.br
((n + 1) * (n + 2) / 2)
or 3, 6, and 10 respectively. For a 3D transformation of first, second,
or third order, the minimum number of required 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 Gaussian elimination method.
.SH SEE ALSO
The GRASS 4 \fI
Image
Processing manual\fR
.PP
\fI
g.gui.gcp,
i.group,
i.rectify,
i.target,
m.transform,
r.proj,
v.proj,
v.transform,
\fR
.br
\fI
Manage Ground Control Points
\fR
.SH AUTHOR
Markus Metz
.PP
based on i.rectify
.SH SOURCE CODE
.PP
Available at:
v.rectify source code
(history)
.PP
Accessed: Sunday Dec 17 17:22:29 2023
.PP
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.PP
© 2003\-2023
GRASS Development Team,
GRASS GIS 8.3.1 Reference Manual