[GRASSLIST:5402] RE: How to choose initial parameters?

[John Gillette]

I sympathize because I went through this.

Here's the answers:

(Q) How do I come up with all of that information 
    necessary to creating a database?

(A) Check the Metadata for the data set.  The Metadata
    should contain the projection, datum and ellipsoid
    data.  See examples at bottom.

(Q)  Back when I first set my GRASS database up, I read 
     somewhere that I should use the wgs84 ellipsoid.
     How do you know which to use, and what happens if 
     you use the wrong one?

(A)  The meta data may have the ellipsoid data.  However,
     the ellipsoid is tied to the datum.  For a given datum,
     a certain ellipsoid is used.  In the US, NAD83 datum
     uses GRS80 ellipsoid and NAD27 uses Clarke66 (as in
     1866).  Search the web and read about Datum and
     Ellipsoid.  If you get it wrong, nothing happens.
     Grass shows everything in its display as X,Y data.
     The importance of getting it correct is if you
     transform from one datum or projection to another 
     or if you try to display 2 different datum or 
     projections in the same mapset and expect them to 
     line up.

(Q) Since I don't have a database setup yet, I can't 
    run the GRASS commands to get information from the map data.

(A) Doesn't matter too much.  Just set up a location.
    Let's say you have sdts data you want to import.
    use m.sdts.read to read the files.  Poke around and
    you will find the metadata you need.  If your location
    isn't correct, stop grass and restart creating the
    location you need.  No harm done.

The nice thing about the spearfish data set is that you can 
start with something and play till your hearts content. I
loaded in all kinds of stuff: geotiff, bil, shape files, DRG,
DLG till I figured out what I was doing.  It won't break 
anything.  You can erase it later if you want.
     
---------------------
EXAMPLES:

  Spatial_Reference_Information:
  Horizontal_Coordinate_System_Definition:
    Horizontal_Coordinate_System: Grid
     Grid_Coordinate_System_Name: UTM
     Planar_Distance_Units: METERS
      one_Number: 19
          Horizontal_Datum_Name:  NAD83
          Ellipsoid_Name:  GRS1980
          X-Shift:  0.0000000000
          Y-Shift:  0.0000000000

Spatial_Reference_Information: 
  Horizontal_Coordinate_System_Definition: 
    Planar: 
      Map_Projection: 
        Map_Projection_Name: Albers Conical Equal Area 
        Albers_Conical_Equal_Area: 
          Standard_Parallel: 55 
          Standard_Parallel: 65 
          Longitude_of_Central_Meridian: -154 
          Latitude_of_Projection_Origin: 50 
          False_Easting: 0.00000 
          False_Northing: 0.00000 
        Planar_Coordinate_Information: 
          Planar_Coordinate_Encoding_Method: coordinate pair 
          Coordinate_Representation: 
            Abscissa_Resolution: 1.0 
            Ordinate_Resolution: 1.0 
          Planar_Distance_Units: METERS 
      Geodetic_Model: 
        Horizontal_Datum_Name: NAD27 
        Ellipsoid_Name: Clarke 1866 
        Semi-major_Axis: 6378206.4 
        Denominator_of_Flattening_Ratio: 294.98