How to calculate orientation of line segments using open source GIS?

Question

I'd like to calculate the orientation of line segments relative to north direction using open source tools. Which tools or functions would you recommend?

Answer 1

To get an angle from a line you just need to find the angle of the normalized direction. The Atan2 function is available on every computing platform I have used, even calculators. The basic idea is to get a normalized vector for the direction of that line then get the angle.

var normal = line.Direction.GetNormalized();

For your case since you need it to be north (+y hopefully) relative and possibly clockwise you could reverse the inputs to Atan2 like so:

var radians = Atan2(normal.x,normal.y);

And if you need counter-clockwise negate the result of Atan2. For degrees just multiply by 180 then divide by PI. Also note that when the result is negative you can add 2*PI.

if(radians < 0) { radians += 6.28... }

Edited: to correct an error for counter-clockwise.

Note: only works if North is always up.

Answer 2

To muddy the water a bit: what projection are your line segments in? The answer depends! If you're it's a Mercator projection, Dandy's answer works. (In a Mercator, lines are rhumb -- by definition, holding the same compass direction for their length.)

In general, however, a line on a map will not correspond to the same compass direction (azimuth) along its entire length -- so your question doesn't always have an answer.

It might be acceptable for you to just assume your lines are rhumb, or to calculate two directions -- one at each endpoint, or to compromise and calculate the direction at the midpoint . . . .

Answer 3

GRASS GIS (http://grass.osgeo.org/) offers native directed graphs (i.e. vector lines). See "Vector network" screenshots here. Furthermore, there is the m.cogo tool included.