True Heading

[Gerald I. Evenden]

>Date: Thu, 22 Jul 93 12:30:32 -0500
>From: Darrell McCauley <mccauley at ecn.purdue.edu>
>Sender: mccauley at ecn.purdue.edu
>To: grassu-list at max.cecer.army.mil
>Subject: Re: True Heading
>
>Gerald I. Evenden (gie at charon.er.usgs.gov) writes on 22 Jul 93:
>>Thus for any program that is otherwise operating on the data in
>>cartesian space (x-y or easting-northing) considerable computational
>>code must be in the program to inverse project the selected points
>>back to geographic as well as code to perform geodesic computations.
>
>so would anyone like to be a hero and add azimuth calculations
>to GRASS? Sounds like there's a heap o' folk that would find
>it useful (myself included).

I suspect that for the average analytic program working in cartesian
space this may be a bit of a task.  Details of the relationship between
the cartesian-geographic transformation associated with the data
(projection parameters) must be known and the program must have
either the inverse projection code for all supported projections built
in or have a method to communicate to a process that can do the
conversion.

Code for projection and geodesic computations exist, either in GRASS or
PROJ.4, but that's the easy part.  But ripping appart an application
to insert user interface and control seems very sticky.

A short term solution is to use program "geod" in the PROJ.4 system
for distance and azimuth between geographic points IF you can get
the cartesian application to spit out these values.

If, as you say, there is a considerable demand to such a service,
then programs where it is desirable should be identified and further
analysis given to such implementation.  But so far I have only seen
two or three parties discussing it.

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