[GRASSLIST:4332] Re: GIS visitor requests directions

[Eric G. Miller]

On Tue, Aug 20, 2002 at 01:04:05PM +1000, Nigel McFarlane wrote:
> 
> Hi,
> 
> ahh - I'm buried in terms again. I thought that "datum' related
> to Z-elevation only. I don't understand how arguments about different
> datums (data?) have any bearing on what 2D projections are
> supported. I don't know what NAD27 and NAD83 are.

Both.  Datum is the singular of data (also, the basis for measuring
something).  In cartography/geodesy, it specifies a reference 
frame, which may only be horizontal, or vertical or both.

NAD = North American Datum. These are horizontal datums *only*.
Typical vertical datum used in the US is National Vertical 
Geodetic Datum of 1929.

> For world-scale maps, elevation and variations of the order of
> meters (or 10s or 100s of meters) aren't an issue so even if
> the datum distorts the X,Y directions, it doesn't matter for me.
> 
> My question is: can GRASS take a set of data in one
> 2D projection (even if its just lat-long) and project that
> data into another 2D projection, regardless of which projections
> the two are, and regardless of what part of the earth the data
> covers?

The answer is yes, much of the time.  The geographic extents of
the projections should both be able to contain all of the data.

> If so, I'm doing it wrong because I can't calculate
> Northing and Easting boundaries exactly to start with.

??? 

> Or, as this thread seems to indicate, are projections only
> supported on a case-by-case basis? (in which case, back to manifold)

That is always the case, but GRASS supports a great number of
projections via the proj4 library.  It just doesn't, yet, support
many datum transformations. 

If this stuff confuses you, have a look at:

http://www.colorado.edu/geography/gcraft/contents.html

-- 
eric G. Miller <egm2 at jps.net>