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#### NAME

gm_georect - GEORECTIFY TOOL (gis.m)

GEORECTIFY TOOL (gis.m)

#### GEORECTIFYTOOL(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.

#### SEEALSO

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 08:51:39 -0700 (Sat, 12 Sep 2009) \$

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