coordinates vs projections

[mike camann]

gie at charon.er.usgs.gov (Gerald (Jerry) I. Evenden) writes:

>>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.

Sorry for the confusion.  I capitalized LOCATION in an attempt to make
clear that I meant location in a GRASS sense: the subdirectory
$LOCATION.  When creating a new LOCATION, GRASS expects information
about the projection that will apply to that LOCATION.  And yes, it
requests that you select from one of the following *projections*:

	x,y (for imagery and other unreferenced data)
	UTM
	State Plane
	Latitude-Longitude
	other projection         (emphasis is mine)
	      ^^^^^^^^^^

The menu header *does* refer to them-- properly-- as coodinate systems,
but the final item lumps in a whole series of *projections* and
whatever you choose from this list (including lat-lon) is stored in the
DEFAULT_WIND file (under $LOCATION/PERMANENT) as the LOCATION's
*projection*.  You can check this for yourself.  Yes, Jerry, I do find
GRASS's behavior in this regard confusing.

I'm sorry if I gave the impression that I don't know the difference
between a coordinate system and a projection-- I do.

>>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.

>Again, lat-lon is *not* a projection.  We are confusing a Plate Carree
>projection with elemental definitions.

I didn't say that *I* think lat-lon is a projection, I said that GRASS
seems to think so, and I asked for clarification.  Again, look at the
DEFAULT_WIND file in any $LOCATION/PERMANENT that was originally
created with lat-lon coordinates.  Better yet, do g.region -p; it will
tell you that the *projection* is lat-lon.  That is the term GRASS
uses-- not a slip of the tongue on my part.  If you do g.region -p in a
UTM LOCATION, it will say thet the projection is UTM.  Finally, try to
run g.setproj.  It will confirm that the *projection* has already been
set, when all that has actually been explicitly entered was the
*coordinate system* at the time the LOCATION was created.  You can
check this for yourself by creating a dummy LOCATION.

>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.

Now that was helpful....

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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

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