[GRASSLIST:4332] Re: GIS visitor requests directions
Eric G. Miller
egm2 at jps.net
Tue Aug 20 00:30:19 EDT 2002
On Tue, Aug 20, 2002 at 01:04:05PM +1000, Nigel McFarlane wrote:
> 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
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:
eric G. Miller <egm2 at jps.net>
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