Date: Tue, 18 Jan 1994 17:07:20 -0500
From: mike camann <camann@pick.uga.edu>
To: grassu-list@max.cecer.army.mil
Subject: coordinates vs projections
I am hesitant to comment upon this because of the apple-oranges
confusion going on here. But ...
GRASS users--
This may be a FAQ, but I haven't seen any recent discussion about it,
so I'll ask anyway. Last week I posted a question regarding how to set
up a LOCATION using a transverse Mercator projection and a lat-lon
coordinate system. I gather, after some experimentation, that you
can't (*please* correct me if I'm wrong). GRASS expects transverse
Mercator locations to be referenced in meters. This raises several
important issues:
The above statement is confusing. What is meant by "set up a LOCATION?"
Indeed, latitude and longitude is a "coordinate system." Cartesian,
(x-y or easting-northing) is also a coordinate system---for a plane.
A "projection" is a mathematical function for converting lat-lon spherical
system to and from a cartesian system. This conversion process is
imperfect and thus there are a wide variety of projections, of which
Transverse Mercator is one of hundreds.
My assumed definition of a location is defined in terms of lat-lon
coordinates on the sphere or x-y coordinates on the plane. The
relationship of the lat-lon to the x-y is dependent upon the
projection used and it associated parameters. A bridge or a well
have coordinates and that defines the location.
1) As near as I can tell, GRASS regards lat-lon as a *projection*.
Furthermore, it seems to view lat-lon as a square projection, where all
the grid lines intersect at 90 degrees. Square-grid projections of the
Earth's surface don't make great maps because of the enormous
distortion they introduce in northern and southern latitudes. Besides,
latitude and longitude are not a projection at all: they're a
coordinate referencing system, one that uses sexagesimal notation to
reference points. The coordinate system should be independent of the
projection used to project the elliptical surface of the Earth onto a
plane.
Again, lat-lon is *not* a projection. We are confusing a Plate Carree
projection with elemental definitions.
As far as the sexagesimal notations, one can easily get around this
by using decimal degrees which are acceptable to many programs. But
the lat-lon notation system is merely a matter of tradition, with
no other build in meaning.
Lastly, a location or cite is totally independent coordinate variable.
The projection only has meaning when defining the interrelation
between its spherical and cartesian representations.
2) The GRASS restriction that transverse Mercator projections
Much confused material omitted.
another application.
Thanks in advance. I hope this stimulates some discussion about
GRASS's apparent confusion over coordinate systems vs map projections,
and that someone can answer my specific questions about how to define
the DEFAULT_WIND under a transverse Mercator projection.
May I kindly suggest that the problem may not lie with the CERL
folks and the lack of discussion is due to the fact that few seem
seem to have so much trouble with these problems.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Michael Camann camann@dial.pick.uga.edu
Department of Entomology camann@phoenix.cs.uga.edu
University of Georgia (706) 542-1388
Athens, GA 30602 (706) 542-2276%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Gerald (Jerry) I. Evenden Internet: gie@charon.er.usgs.gov
voice: (508)563-6766 Postal: P.O. Box 1027
fax: (508)457-2310 N.Falmouth, MA 02556-1027