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After this excursion though the world of projections, which play an important role for working with geographical information systems, it is time to introduce the process of defining project regions (Locations) in GRASS (see Chapter 4.1).
To create a Location the following parameters must be known:
After starting GRASS (see Chapter 4.1) click with the mouse on the button Create New Location for creating a new location. Now, the text-based mode is opened (see Fig. 8). This is equivalent to starting GRASS with the setting grass60 -text. When a session is ended, the start mode is saved and become the default start mode. The start mode can be changed using the option grass60 -gui or -text.
Three entries must be made in the start window to define a new Location.
Please remember: The mapset PERMANENT in which the general information of the project region is saved in individual files will automatically be created by GRASS (see Chapter 2.3) - even if you have entered an other name in the sector mapset.
Subsequently, the keystrokes <ESC><ENTER>, that is typical for GRASS, leads you to the screen for defining the project region. The creation of a Gauß-Krüger Location is described in chapter 4.5.2. The procedure for defining other coordinate systems is similar.
Advice: The values for the projection of a location can always be set on the previously defined default values via the module g.region -d. While working with GRASS the module g.region -p lists the projection and resolution values currently being used.
The Gauß-Krüger coordinate system uses a transverse mercator projection. Accordingly, the cylinder - rotated 90^ - is transversely put over the Bessel ellipsoid. Distortions occur in the border area (max. 12 cm/km on a length of 2^). This means the Gauß-Krüger sheet sides are not displayed parallel to the sides of the paper in topographical maps using this projection.
Now, the first task would be to choose the desired projection system from the menu. However, the Gauß-Krüger system is not listed therefore we must select 'other'. These are the steps to be followed:
- specify projection name: tmerc (Transverse Mercator)
- specify ellipsoid name: bessel (Bessel Ellipsoid)
- Do you want to specify a map datum for this location?
potsdam
- Enter Central Parallel [lat_0] (23N): 0N
- Enter Central Meridian [lon] (96W): 9E
- Enter Scale Factor at the Central Meridian: 1
- Enter False Easting: 3500000 (3 because 9E is Central Meridian)
- Enter plural form of units: meters
During parameter input, it is sometimes possible to display the projection parameter supported by GRASS via the entry list.
After the projection parameters have been entered, set border coordinates of the project region. The boundaries are set to: North = 5801000, South = 5787000, West = 3427000, and East = 3445000. The Gauß-Krüger values (defined exactly in meters) of the topographic maps are attached with 3 zeros. For more precision a point is following and afterwards the decimal places (see Fig. 12).
The specification for the resolution (raster cell in meters) in
east-west and north-south direction is entered in the same entry
mask. This is the standard raster resolution (default) of
the created location. Thus, it has no importance for the sharply
illustrated sites and vector data and can always be changed with
GRASS while working. It is recommended to take into consideration
that the resolution of raster data has an enormous influence
on the necessary calculating and memory requirements.
As usual, complete the data entry by pressing <ESC><ENTER> and you will get back to the beginning. Confirm the creation of the mapset with "yes" because the name for the mapset was already been indicated. An additional confirmation of all the data will finally exit this page. Now, the project region is installed and active. This can be seen because GRASS Prompt is displayed in the Shell. The command g.region -p can be used to verify the region data.
Here is an example of a Gauß-Krüger project region:
| projection: | 99 (Transverse Mercator) |
| zone: | 0 |
| datum: | potsdam |
| ellipsoid: | bessel |
| north: | 5801000 |
| south: | 5787000 |
| west: | 3427000 |
| east: | 3445000 |
| nsres: | 2.5 |
| ewres: | 2.5 |
| rows: | 5600 |
| cols: | 7200 |
Creating an xy-location, which is necessary for e.g. intermediate
steps for geocoding of non-referenced map scans, is much
more simple because no projection parameters need to be provided.
After entering 'A' at the question concerning the coordinate system
- coordinate system for location: x,y (A)
it is directly be followed by the definition of the border coordinates (columns or width, rows or height) of the project region. The border coordinates are defined by the pixel extent of the non-referenced map(s) to be imported. These can be determined among other things with the program xv (Windows -> Image Info). Starting from the defined attachment in the left edge below only a north and an east value is added according to the rows and columns of the original picture. The raster resolution must consequently be 1.
This procedure will be discussed again in the context of georeferencing a scanned map (see chapter 6).
Here is an example of a XY project region (g.region -p):
| projection: | 0 (x,y) |
| zone: | 0 |
| north: | 8000 |
| south: | 0 |
| west: | 0 |
| east: | 8000 |
| nsres: | 1 |
| ewres: | 1 |
| rows: | 8000 |
| cols: | 8000 |
The creation of an UTM location (Universal Transverse Mercator)
resembles the creation of a Gauß-Krüger location (but with a
smaller cylinder). In this case, it is not referred to a
"Berührzylinder" with the basic bessel ellipsoid but to a
"Schnittzylinder" with the basic WGS 84 ellipsoid. With respect to the
Gauß-Krüger projection, the parameters that have changed are shown below:
- coordinate system for location: UTM (C)
- specify ellipsoid name: z.B.: wgs84 (world geodetic system 1984)
- do you want to specify a map datum for this location? z.B.: wgs84
- Enter Zone 32 (UTM zone for Germany)
- Is this South Hemisphere? n
Afterwards the coordinates of the project region are defined. The entry is in meters, which means a 7-digit number for the northern value and a 6-digit for the eastern value.
Here is an example of a UTM project region (g.region -p):
| projection: | 1 (UTM) |
| zone: | 32 |
| datum: | potsdam |
| ellipsoid: | wgs84 |
| north: | 6100000 |
| south: | 5880000 |
| west: | 500000 |
| east: | 630000 |
| nsres: | 12.5 |
| ewres: | 12.5 |
| rows: | 17600 |
| cols: | 10400 |
A latitude-longitude location contains data in latitudes and
longitudes (0-90^ north and/or south, 0-180^ east and/or west),
expressed in the sexagesimal system (degree:minutes:seconds,
letter indicating the orientation) or decimal system (pos./neg.
decimal degrees). The entry of the border coordinates
and standard raster resolution can be in the decimal degrees system
or sexagesimal system.
Here is an example of a lat-lon project region (g.region -p):
| projection: | 3 (Latitude-Longitude) |
| zone: | 0 |
| datum: | unknown (default: WGS84) |
| ellipsoid: | unknown (default: WGS84) |
| north: | 90N |
| south: | 90S |
| west: | 180W |
| east: | 180E |
| nsres: | 0:04:48 |
| ewres: | 0:04:48 |
| rows: | 2250 |
| cols: | 4500 |
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![\includegraphics[clip=true, width=0.75\textwidth]{finalpix/projdef.eps}](img12.png)