Ubuntu Manpage: i.rectify - Rectifies an image by computing a coordinate transformation for each pixel in

NAME

       i.rectify  - Rectifies an image by computing a coordinate transformation for each pixel in
       the image based on the control points.

KEYWORDS

       imagery, rectify

SYNOPSIS

       i.rectify
       i.rectify help
       i.rectify  [-ca]  group=name   [input=name[,name,...]]    extension=string   order=integer
       [res=float]   [memory=memory in MB]   [method=string]   [--verbose]  [--quiet]

   Flags:
       -c
           Use current region settings in target location (def.=calculate smallest area)

       -a
           Rectify all raster maps in group

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       group=name
           Name of input imagery group

       input=name[,name,...]
           Name of input raster map(s)

       extension=string
           Output raster map(s) suffix

       order=integer
           Rectification polynom order (1-3)

       res=float
           Target resolution (ignored if -c flag used)

       memory=memory in MB
           Amount of memory to use in MB
           Default: 300

       method=string
           Interpolation method to use
           Options: nearest,bilinear,cubic,bilinear_f,cubic_f
           Default: nearest

DESCRIPTION

       i.rectify  uses  the  control  points  identified  in i.points or i.vpoints to calculate a
       transformation matrix based on a  first,  second,  or  third  order  polynomial  and  then
       converts x,y cell coordinates to standard map coordinates for each pixel in the image. The
       result is a planimetric image with a transformed  coordinate  system  (i.e.,  a  different
       coordinate system than before it was rectified).

       i.points  or  i.vpoints  must  be  run before i.rectify, and both programs are required to
       rectify an image. An image must be rectified before it can reside in a standard coordinate
       LOCATION,  and  therefore be analyzed with the other map layers in the standard coordinate
       LOCATION. Upon completion of i.rectify, the rectified image is  deposited  in  the  target
       standard coordinate LOCATION. This LOCATION is selected using i.target.

Program Prompts

The first prompt in the program asks for the name of the group containing the files to be
rectified.
Enter the group containing files to be rectified
Enter 'list' for a list of existing imagery groups
Enter 'list -f' for a verbose listing
Hit RETURN to cancel request
>
This is the same imagery group that was selected in i.points or i.vpoints and the group
that contains the raster maps with the marked points and their associated map
coordinates. You are then asked to select the raster map(s) within the group to be
rectified:
Please select the file(s) to rectify by naming an output file
spot1.1 in mapsetname .............
spot1.2 in mapsetname .............
spot1.3 in mapsetname .............
spotclass1 in mapsetname spotrectify1.

spotreject1 in mapsetname .............
(enter list by any name to get a list of existing raster maps)
AFTER COMPLETING ALL ANSWERS, HIT <ESC> TO CONTINUE
(OR<Ctrl-C> TO CANCEL)
More than one raster map may be rectified at a time. Each cell file should be given a
unique output file name.

Next, you are asked to select one of two windows regions:
Please select one of the following options
1. Use the current window in the target location
2. Determine the smallest window which covers the image
>
The i.rectify program will only rectify that portion of the image or raster map that
occurs within the chosen window region, and only that portion of the cell file will be
relocated in the target database. It is important therefore, to check the current mapset
window in the target LOCATION if choice number one is selected.

If you are rectifying a file with plans to patch it to another file using the GRASS
program r.patch, choose option number one, the current window in the target location.
This window, however, must be the default window for the target LOCATION. When a file
being rectified is smaller than the default window in which it is being rectified, zeros
are added to the rectified file. Patching files of the same size that contain 0/non-zero
data, eliminates the possibility of a no-data line the patched result. This is because,
when the images are patched, the zeros in the image are "covered" with non-zero pixel
values. When rectifying files that are going to be patched, rectify all of the files
using the same default window.

Select the order of transformation desired with the order option:
1st Order 2nd Order 3rd Order
The program will immediately recalculate the RMSE and the number of points required.

Linear affine transformation (1st order transformation)
x' = ax + by +c
y' = Ax + Bt +C

The a,b,c,A,B,C 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
images, and (2) the terrain or camera distortion effect can be ignored.

Polynomial Transformation Matrix (2nd, 3d order transformation)
The ANALYZE function has been changed to support calculating the registration coefficients
using a first, second, or third order transformation matrix. The number of control points
required for a selected order of transformation (represented by n) is
((n + 1) * (n + 2) / 2)

or 3, 6, and 10 respectively. It is strongly recommended that one or more additional
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 RMS
error values for all the included control points are immediately recalculated when the
user selects a different transformation order from the menu bar. The polynomial equations
are performed using a modified Gaussian elimination method.

Resampling method
The rectified data is resampled with one of five different methods: nearest, bilinear,
cubic, bilinear_f or cubic_f.

The method=nearest method, which performs a nearest neighbor assignment, is the fastest of
the three resampling methods. It is primarily used for categorical data such as a land use
classification, since it will not change the values of the data cells. The method=bilinear
method determines the new value of the cell based on a weighted distance average of the 4
surrounding cells in the input map. The method=cubic method determines the new value of
the cell based on a weighted distance average of the 16 surrounding cells in the input
map. The method=lanczos method determines the new value of the cell based on a weighted
distance average of the 25 surrounding cells in the input map.

The bilinear, cubic and lanczos interpolation methods are most appropriate for continuous
data and cause some smoothing. These options should not be used with categorical data,
since the cell values will be altered.

In the bilinear, cubic and lanczos methods, if any of the surrounding cells used to
interpolate the new cell value are NULL, the resulting cell will be NULL, even if the
nearest cell is not NULL. This will cause some thinning along NULL borders, such as the
coasts of land areas in a DEM. The bilinear_f, cubic_f and lanczos_f interpolation methods
can be used if thinning along NULL edges is not desired. These methods "fall back" to
simpler interpolation methods along NULL borders. That is, from lanczos to cubic to
bilinear to nearest.

If nearest neighbor assignment is used, the output map has the same raster format as the
input map. If any of the other interpolations is used, the output map is written as
floating point.

Program Execution
Note: The rectified image or rectified raster maps will be located in the target LOCATION
when the program is completed. The original unrectified files are not modified or
removed.

NOTES

       i.rectify uses nearest neighbor resampling during the transformation choosing  the  actual
       pixel  that  has  its centre nearest to the point location in the image. Advantage of this
       method is that the pixel brightness of the image  is  kept  as  i.rectify  rearranges  the
       geometry of the image pixels.

       If i.rectify starts normally but after some time the following text is seen:
       GIS ERROR: error while writing to temp file
       the user may try the flag -c (or the module needs more free space on the hard drive).

AUTHORS

       William R. Enslin, Michigan State University, Center for Remote Sensing

       Modified for GRASS 5.0 by:
       Luca Palmeri (palmeri@ux1.unipd.it)
       Bill Hughes
       Pierre de Mouveaux (pmx@audiovu.com)
       CMD mode by Bob Covill

       Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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