coordinates vs projections

[Gerald I. Evenden]

>Date: Tue, 18 Jan 1994 17:07:20 -0500
>From: mike camann <camann at pick.uga.edu>
>To: grassu-list at 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 at dial.pick.uga.edu
>Department of Entomology                camann at phoenix.cs.uga.edu
>University of Georgia                   (706) 542-1388
>Athens, GA 30602                        (706) 542-2276
>
>%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
voice: (508)563-6766          Postal: P.O. Box 1027
  fax: (508)457-2310                  N.Falmouth, MA 02556-1027