[GRASS-user] how to get instantaneous heading of a line at a pt?

[Hamish]

Hamish:
> > I have used v.to.points to create a series of points that fall along a
> > line. Now I want to upload to a DB column in the points map the
> > instantaneous heading of the original line at that sample point.

Markus:
> This functionality might be added to v.to.db - perhaps
> using Vect_get_node_line_angle():
> http://download.osgeo.org/grass/grass6_progman/level__two_8c.html#a16

hmmm. at a node there are two lines, the fn wants to know which one.

I have a point somewhere along the line (between two vertices) and want
to #1 find the polyline, #2 extract the current linear feature (between
two nearest connected vertices), and #3 calc the angle of that line. I
see now the corner case of if the point falls exactly on a vertex or a
node you will have to average to angles of the lines on either side.

I don't think v.to.db could help other than the overall bearing from the
polyline's start node to end node. Remember a single line cat can only
hold a single value.

'v.distance upload=to_angle' might be something...


Hamish