Date: Wed, 7 Jul 93 22:05:32 -0500
Message-Id: <9307080305.AA14794@bushland.ecn.purdue.edu>
From: Darrell McCauley <mccauley@ecn.purdue.edu>
Sender: mccauley@ecn.purdue.edu
To: grassp-list@max.cecer.army.mil
In-Reply-To: <9307080100.AA00245@charon.er.usgs.gov>
Subject: Re: basic projection questionGerald I. Evenden (gie@charon.er.usgs.gov) writes on 7 Jul 93:
...
coordinate system:
x,y (for imagery and other unreferenced data)
most imagery I'm familiar with is "referenced" to geographic location!
UTM
State Plane
both UTM and State Plane are x-y cartesian "systems"
Latitude-Longitude
geographic system
other projection
a projection per se is NOT a coordinate system.
...
Second restatement (someone help - I must not be conveying this
correctly). This is probably more detail than is useful, but
I must be leaving something out somewhere.1. I calculate the following statistic for x,y,z data:
(x and y are location - z is perhaps an elevation,
or a concentration, or whatever).for all points separated by the vector h,
sum the squared differences in z values
and divide by the number of sample points.
call this value '2G'.do this again for another vector (the same
direction as h, but with the magnitude incremented).This gives values of '2G' at several increments
of the magnitude of h (called "lags").Plot values of '2G' on one axis and ||h|| on another
axis. This is called a variogram.2. I want to code this into GRASS as a sites program. Some
users may have some data that's not in an x,y coordinate
system, and I'd my program to be able to deal with this.3. As I understand it, distances are calculated like so:
G_begin_distance_calculations();
G_distance (x1, y1, x2, y2);
This gets me the magnitude of h - G_distance is supposedly
smart enough to take care of the details.4. WHAT GRASS LIBRARY FUNCTIONS SHOULD I USE SO THAT I CALCULATE
THE ANGLE OF h?--Darrell
Ah, full circle. Depends if you are in geographic space or cartesian
space! 
Sorry, there is no relationship between the angle in cartesian space
and the angle in geographic space UNLESS one knows the details of
the transformation between the two spaces. One also does not flip
between cartesian and spherical coordinate operators without further
qualifications. As I pointed out, the angle and the distance must
be qualified as either loxodrome or geodesic in spherical space.
I suggest you stick to cartesian space.
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