Ubuntu Manpage: gm_georect - GEORECTIFY TOOL (gis.m)

NAME

       gm_georect - GEORECTIFY TOOL (gis.m)

       GEORECTIFY TOOL (gis.m)

GEORECTIFY TOOL (gis.m)

In order to use and overlay different forms of geospatial data in a GIS, they must use the
same coordinate systems and be projected identically. The Georectify Tool uses multiple
GRASS modules to georectify spatial data (map or image) that is not originally coded in an
earth-based coordinate system (e.g., latitude/longitude or UTM). The Georectify Tool can
transform the simple x/y coordinates of the original data by shifting, rotating, and
warping them as needed for the original data to match other georectified data from the
same location on earth.
The basic procedure is to identify a set of ground control points (GCPs) in the
ungeorectified data. The georectified coordinates for the points are identified. A
mathematical transformation is calculated by regressing the original x/y coordinates
against the georectified coordinates for the same points. This transformation is then
applied to all the data.
The Georectify Tool can georectify both raster and vector maps. Raster maps can be
transformed using 1st, 2nd, and 3rd order polynomial transformations; vector maps are
transformed only with a 1st order polynomial transformation. The georectified coordinates
of GCPs can be entered by typing or can be extracted by clicking the equivalent locations
with a mouse in a displayed georectified map.

Using the Georectify Tool
Map Preparation
The unrectified map or image (e.g., scanned image or CAD drawing) must be imported into a
GRASS XY location.
In order to be georectified, the unrectified map or image must be added to a imagery
group. An imagery group holds a single map/image or a set of maps/images that represent
different views of exactly the same locale. All maps or images in a single imagery group
must be exactly aligned (e.g., as in the different spectral bands of a multi-channel
satellite image or layers of a single CAD drawing). You can create an imagery group from
within the Georectify Tool if desired.

Setup
Restart GRASS, selecting the location/mapset where you want the map you will georectify to
END UP (i.e., the projected TARGET location). Open any map display (or displays) that you
want to use for extracting geographic coordinates by clicking with a mouse. Make sure that
the computational region extents and resolution are set to the values you want for the
newly georectified map.

Select Georectify from the File menu.

The Georectify Tool startup lets you select the type of map to georectify, the
location/mapset of the map to georectify, the image group of the map to georectify, and a
map to use for marking GCPs.

1
click the radio button to georectify either raster or vector maps (you can only
georectify raster OR vector maps in a single georectification session),

2
select the source (XY) location/mapset of the map to be georectified,

3 if needed, create an imagery group for the map to georectify. (the imagery group
target is automatically set to the current location and mapset),

4
select the imagery group containing the map to georectify,

5
select the map/image to display for marking GCPs interactively (a raster or vector
in the group you want to georectify),

6
click the "Start Georectifying" button to begin marking GCPs.

GCP Marking
This opens a special map display with an ungeorectified map to use for marking GCPs, and a
GCP manager window. The GCP map display can be zoomed and panned like a normal map
display.
The GCP manager window holds the x/y coordinates and geographic coordinates of each GCP,
and displays the error for each GCP (i.e., the distance that the actual GCP marked
deviates from its expected placement using a transformation equation).

1
Click in an empty x/y entry box in the GCP manager to begin marking a new GCP,

2
Click on the ungeorectified map to mark a GCP and automatically enter its x/y
coordinates in the entry box. (Your cursor will automatically jump to the
corresponding geographic coordinate entry box for the same GCP),

3 Enter the corresponding geographic coordinates for the GCP OR click on the same
place in a georectified map (in a normal map display) to automatically extract the
geographic coordinates and enter them into the geographic coordinate entry box in
the GCP manager window (Note: use the pointer tool in the map display window)

GCP ManagementContinue marking GCPs until you
have enough for an accurate georectification. At least 3 GCPs are needed for a simple 1st
order (affine) transformation that can shift and rotate a map; at least 6 GCPs are needed
for a 2nd order (polynomial) transformation that will do simple warping in addition to
shifting and rotating; at least 10 GCPs are needed for a 3rd order (polynomial)
transformation that will do complex warping.
Any GCP can be edited by typing new values or by selecting the appropriate entry box and
clicking on the GCP-marking display or georectified map display.
GCPs can be excluded and not used for calculating the transformation equation or the total
RMS error by unchecking the box in the "use" column at the left of each GCP entry line.
Only active GCPs (i.e., with the "use" box checked) will be used for computing the
georectification and total error.
You can delete all inactive GCPs (i.e., with unchecked "use" boxes) by clicking the eraser
tool in the GCP manager tool bar.
Total RMS (root mean square) error is calculated from all active GCPs. Click the RMS tool
in the GCP manager tool bar to update the total RMS error after changing or
including/excluding a GCP. Excluding a GCP with an especially high individual error can
reduce overall RMS error, but will also change the individual errors of all other GCPs.
The lower the RMS error, the more accurate the final georectification will be.
All active GCPs can be saved to a POINTS file, associated with the imagery group for the
map(s) to be georectified. If a saved POINTS file exists, prior to a georectification
session the GCP values in the POINTS file will automatically be entered into the GCP
manager window when a georectification session is started.

Georectification
When you are satisfied with your selection of GCPs and RMS error...

1 Select the type of georectification you want to perform (1st, 2nd, or 3rd order for
rasters; 1st order only for vectors). Remember, at least 3 points are needed for
1st order georectification, 6 points for 2nd order, and 10 points for 3rd order.

2 Press the Georectification button on the GCP manager tool bar. Depending on the
number of cells in a raster map or nodes in a vector map, georectification may take
some time.

AUTHORS

       Michael Barton, Arizona State University, Tempe, Arizona, USA

       Last changed: $Date: 2009-09-12 08:51:39 -0700 (Sat, 12 Sep 2009) $

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GEORECTIFY TOOL (gis.m)

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