[GRASS-user] ordering points along a line

GRASS GRASS user
,
1

Hi there

I’ve been looking into this, but not sure about the best way to do it:

I have a set of points that lie over a line (the line is a river course and points are intersections with contour lines). I want to order the points ‘downstream’, so I can make a long profile of the river (just sampling elevations from a DEM at line nodes gives too much noise, some papers suggest contours intersections work better).

Question is: how? After some searching I found the Network Analysis of QGIS, but still not sure if that’s the best approach. The river is very meandering, so nearest neighbour won’t work because the nearest node might not be the next one downstream

I’m open to suggestions

thanks

Carlos

–
Prof. Carlos Henrique Grohmann
Institute of Geosciences - Univ. of São Paulo, Brazil

  • Digital Terrain Analysis | GIS | Remote Sensing -

http://carlosgrohmann.com
http://orcid.org/0000-0001-5073-5572

Can’t stop the signal.

2

Hi,

as your points are intersections along the river course you could create a network within GRASS (v.net with points as nodes). I think then it should be possible to use a tool like v.net.distance to calculate distances between the nodes which is basically the basis for any ordering along the line. Unfortunately, I haven’t performed such analysis myself, so I can’t provide any example, but the manuals of the v.net.* tools should be helpful in this case.

cheers,
Johannes

···

On Mon, Nov 18, 2013 at 7:39 PM, Carlos Grohmann <carlos.grohmann@gmail.com> wrote:

Hi there

I’ve been looking into this, but not sure about the best way to do it:

I have a set of points that lie over a line (the line is a river course and points are intersections with contour lines). I want to order the points ‘downstream’, so I can make a long profile of the river (just sampling elevations from a DEM at line nodes gives too much noise, some papers suggest contours intersections work better).

Question is: how? After some searching I found the Network Analysis of QGIS, but still not sure if that’s the best approach. The river is very meandering, so nearest neighbour won’t work because the nearest node might not be the next one downstream

I’m open to suggestions

thanks

Carlos

–
Prof. Carlos Henrique Grohmann
Institute of Geosciences - Univ. of São Paulo, Brazil

  • Digital Terrain Analysis | GIS | Remote Sensing -

http://carlosgrohmann.com
http://orcid.org/0000-0001-5073-5572

Can’t stop the signal.

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3
···

Here’s a thought:
If you have the elevation of each of the points (from the contour lines) then this could be used to order the points (assuming the river never runs uphill :slight_smile: ). So it could be as easy as: v.out.ascii,
sort the list by the elevation, then v.in lines.

On 18/11/2013 20:39, Carlos Grohmann wrote:

Hi there

I’ve been looking into this, but not sure about the best way to do it:

I have a set of points that lie over a line (the line is a river course and points are intersections with contour lines). I want to order the points ‘downstream’, so I can make a long profile of the river (just sampling elevations from a DEM at line nodes gives too much noise, some papers suggest contours intersections work better).

Question is: how? After some searching I found the Network Analysis of QGIS, but still not sure if that’s the best approach. The river is very meandering, so nearest neighbour won’t work because the nearest node might not be the next one downstream

I’m open to suggestions

thanks

Carlos

–
Prof. Carlos Henrique Grohmann
Institute of Geosciences - Univ. of São Paulo, Brazil

  • Digital Terrain Analysis | GIS | Remote Sensing -

http://carlosgrohmann.com
http://orcid.org/0000-0001-5073-5572

Can’t stop the signal.

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