NAME¶
gm_georect - 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.
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.
SEE ALSO¶
gis.m manual
gis.m: ANIMATION TOOL
gis.m: PROFILE TOOL
i.points
i.vpoints
i.rectify
g.proj
g.setproj
m.proj
r.proj
v.proj
AUTHORS¶
Michael Barton, Arizona State University, Tempe, Arizona, USA
Last changed: $Date: 2009-09-12 17:51:39 +0200 (Sat, 12 Sep 2009) $
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