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

GRASS GRASS user
1

Hi,

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.

any ideas?

thanks,
Hamish

2

On Tue, Feb 17, 2009 at 5:05 AM, Hamish <hamish_b@yahoo.com> wrote:

Hi,

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.

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

Markus

3

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