No subject

> Let me re-phrase the question. Forget about spheres
> and ellipsoids and earth for a second. I am coding up
> an algorithm that was written for points in a plane
> (call it x-y).  A line extends through two points,
> (x1,y1) and (x2,y2), both of which are in the first
> quadrant of a Cartesian coordinate system.  This line
> crosses the y-axis at some point. The angle 't'
> between the line and the axis is what I am interested
> in.
Ok.No problem so far.

> [figure deleted]
> Calculation of the angle is straightforward in the
> x-y plane.  Now enter the complexities of geo
> referenced data, an funny-shaped earth, etc. How would
> this angle be calculated using GRASS library
> functions? (Assuming that the data is in one of 
> the supported coordinate systesms.)

Sorry for butting in like this, but one of the problems seems to be that,
on anything _but_ a plane, two points do _not_ uniquely define a 'straight'
line. So before you can start computing your angle t, you need to specify
what sort of straight line you want (their names tend to be even more funny
than the shapes they are defined on).

Martin
--
Martin Ameskamp, Inst. f. Informatik I (Computing Dept.)
Kiel University, Olshausenstr. 40, 24118 Kiel, Germany
Fax: ++49 431 8804054, Voice: ++49 431 8804474, 
email: ma at informatik.uni-kiel.d400.de