v.rectify(1grass) | GRASS GIS User's Manual | v.rectify(1grass) |
v.rectify - Rectifies a vector by computing a coordinate transformation for each object in the vector based on the control points.
vector, rectify, level1, geometry
v.rectify
v.rectify --help
v.rectify [-3orb] input=name
output=name [group=name]
[points=name] [rmsfile=name]
[order=integer] [separator=character]
[--overwrite] [--help] [--verbose] [--quiet]
[--ui]
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 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).
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, 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.
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:
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.
x y z east north height status
If no group is given, the rectified vector will be written to the current 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.
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, followed by average deviations for each dimension and both forward and backward transformation and finally forward and backward overall RMSE.
x’ = a1 + b1 * x + c1 * y
y’ = a2 + b2 * x + c2 * y
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
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.
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 (represented by n) is
((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.
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
Markus Metz
based on i.rectify
Available at: v.rectify source code (history)
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