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Is there a way to clip a linestring based on distance? For example, linestring from a to b is 150 meters. I would like to get the linestring from a to a point that is is 75 meters away from a following the a-b linestring.

What I have thought until now is to get the line NpointCount and start adding together small line segments until I get close to 75 meters, but still, how am I supposed to add the final line segment?

I hope I am making sense here..

thanks in advance!

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1 Answer

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In PostGIS it's a two-stage process. First you need to find the overall length of your line geometry with ST_length()

Say that produces a value of 150. Then you need to divide your desired length, 75, by the total length, giving you 0.5. Finally, with that value call ST_Line_Interpolate_Point() or ST_Line_Substring() to return the substring rather than a point.

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thx, i ll give it a try! – Antony Jan 26 '12 at 15:57
My bad, I meant ST_Line_Substring() postgis.org/docs/ST_Line_Substring.html not ST_Line_Interpolate_Point() – MerseyViking Jan 26 '12 at 16:12
Right, your suggestion works fine but it seems that postgis has problems converting some specific multilinestrings into linestrings which is necessary before calling the st_line_substring function. Is there a way to come around this postgis issue? – Antony Jan 27 '12 at 16:02
If your lines are as multilines, you'll need to split them up first. You can use ST_Dump (postgis.refractions.net/docs/ST_Dump.html) and pass the results into your query. – MerseyViking Jan 27 '12 at 16:09
You are right, some of my lines have a double ending like a double fork and that is probably causing trouble to postgis...Anyway, thank you very much for your help! – Antony Jan 27 '12 at 16:30

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