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using QGIS (or even GRASS) is it possible to label a contour lines shape with arrows that indicate the flow direction?

Briefly: I have a point shape with elevation values. I make an interpolation map and then I extract the contour lines.

Now how can I show the flow direction with arrows?

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When you interpolated, you probably created a regular grid of values--tantamount to a DEM. If possible, that's what you want to use for input, not the contour lines. (It is much easier to compute flow directions from a DEM than from a contour line layer.) –  whuber May 10 '13 at 15:10
Yes, in output I created a .asc file. How can I extract flow lines direction from a DEM file? –  matteo May 10 '13 at 15:24
Is there anything unsatisfactory about r.flow? What about the answer to this apparent duplicate question? Even more is available by searching the Web. –  whuber May 10 '13 at 15:27
With r.flow I receive a byte allocation problem and the r.fill.dir crashes. I'd be satisfied just with some label on the contour lines. I will try with saga –  matteo May 10 '13 at 15:53
Have you seen: gis.stackexchange.com/a/27131/187? Is that what you mean? –  underdark May 11 '13 at 16:13

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