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