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The desired projection of the target location is to be defined before starting the equalization according to the required geographical borders and resolution. In principle, any projection supported by GRASS can be defined. The resolution of the target location should not be too low. If the resolution of the target location is too low compared to the resolution of the data to be georeferenced, "resampling" during the georeferencing process will result in a low quality digital map (data loss). On the other hand, too high relative resolution will result in a high quality map requiring an unreasonable amount of storage space (file size).
If an analog map is to be imported into GRASS GIS, the scan resolution of the map also determines the resolution of the location which is to be chosen. This may seem very simple but requires some trial and error to find the balance between not too high (file size) and not too low (data loss) scan value. The following example shows how to proceed in order to adjust the location optimally on the geodata (or vice versa):
a) Sample calculation of the resolution for a 300dpi scan:
300dpi = 300rows/2,54cm = 118,11rows/cm
b) Calculation of the compatible raster resolution for a 1:25000 map scale:
Route_in_nature/scan rows_per_cm = 25000cm/118,11rows = 2.12m/row
If the location with its resolution is already defined, a conversion between scan resolution and geographical resolution will be necessary to determine the value for the scan resolution. The example shown above is thus counted back (5).
The first step is to create a XY location for the scanned raw map to be imported. The extent of the project region should at least comply with the number of pixel in X-(rows, height) and Y-(columns, width) direction of the map to be imported (e.g. use 'xv' for determination). It is important that the XY location be large enough to accomodate the raw data. GRASS allows the locations to be larger than needed, so when in doubt it is possible to "play it safe" and define a generous location (storage space does not cost extra!). The resolution is set to 1 so that each pixel of original image can be assigned to a raster field in GRASS. Usually the geographical resolution (e.g. in meters) determines the number of pixel of the scanned map. However, this connection is irrelevant in the projection-free XY location. Only later, with the transformation to another coordinate system, will the "genuine" resolution (specified by the scan resolution and the map scale) be assigned.
The second step is to create a target location into which the
georeferenced map is to be transformed. The target location will provide the projection, extent,
and resolution required by the project. In the simplest case a target
location already exists to which the georeferenced map will be added. The creation of a
target location is described in
chapter 4.5.1.
Procedure for several map segments
If a map consists of several segments, each segment should be imported independently into separate XY locations in order to exclude "slips of the pen" during the georeferencing. Otherwise, the map can be composed with an image editing program before importing into GRASS in order to import the map "as a whole" - but only if the scans are properly cut. This is only recommended if the data to be imported already lie properly one upon the other as image data. An example could be unreferenced satellite images, which have been supplied congruently for different spectral ranges.
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