basic projection question

[Darrell McCauley]

Gerald I. Evenden (gie at charon.er.usgs.gov) writes on 7 Jul 93:

>You have totally lost me.

and you were the only reply :-(

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.

     |           * (x2, y2)
     |          /
     |         /
     |        *  (x1, y2)
     |       /
     |      /
     -----------------------
     |    /
     |   /
     |t /
     | /
     |/
     |
    /|
   / |

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.)

I suspect that most users of this program that I'm
writing will never have anything by x-y
(unreferenced) data, but I was hoping to do what
is right-n-proper by including support for lat-long
data.

--Darrell