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I have QGIS and GRASS installed . Basically i need this program for making maps something like this: https://www.youtube.com/watch?v=5HuPX7JS7ZA

I downloaded DEM from http://glcfapp.glcf.umd.edu:8080/esdi/ for area i want to map and i apply it on google maps. I used GRASS until i couldn't figure out how to put colours for my heights and rivers. Problem with colors i solved using Terrain analysis but still dont know how to put rivers. (I didnt get how any of tools here work - http://grasswiki.osgeo.org/wiki/Hydrological_Sciences guess too complicated for me)

do you know any step by step simple guide for putting rivers in map, or even better video description ? Links where i can find vector data for very detailed hydrological network that i can overlay on DEM would help a lot?

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  • Please edit your posting to say if you want to derive the hydrological network from the DEM or simply overlay a DEM with vector data?
    – markusN
    Nov 18, 2014 at 9:35
  • Deriving hydrological network from the DEM with r.watershed is to complicated so vector data would be great if someone knows where i can find detail data (basically with every little river and stream)?
    – Mickey
    Nov 18, 2014 at 15:32
  • Well, you can use r.watershed to calculate the basins. To derive streams, use the r.stream modules, see grasswiki.osgeo.org/wiki/R.stream.*
    – markusN
    Nov 18, 2014 at 19:53
  • or is there a way to draw rivers by hand?
    – Mickey
    Nov 19, 2014 at 15:41
  • Both QGIS and GRASS GIS offer digitizers.
    – markusN
    Nov 19, 2014 at 16:19

2 Answers 2

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in GRASS you can do that (extract rivers) from a DEM (which gives you informations only regarding "height" of the single pixel - there are not rivers "included" in this kind of information). You can use GRASS by Qgis too (there is a useful GRASS plugin) and the tool to extract rivers is called "r.watershed". As rightly @underdark said, you need to decide WHERE your river begins - GRASS gives you the possibility to do that by choosing a value for the parameter "threshold" (required in the tool r.watershed): this parameter only says to the GIS the minimum number of cells which drains water into the head of the basin - you can try different values for this parameter in order to obtain the detail you want for your river network.

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  • yes i did use GRASS by QGIS otherwise its to complicated, and i also used r.watershed but still didnt menage to extract rivers. I couldnt find any info on how to use that tool by QGIS. Maybe im putting wrong value in threshold. whats optimal value? because all i got was 3 or 4 different files that i dont know how to implement or what to do with them next.
    – Mickey
    Nov 17, 2014 at 15:24
  • Mickey, you are using the wrong tool here. Look at grasswiki.osgeo.org/wiki/R.stream.*
    – markusN
    Nov 18, 2014 at 19:54
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If you want to create a map, forget GRASS and use QGIS instead. Load both the rivers and the elevation data. Double-click on the layer names and go to the Style section to change the colors and you are all set.

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  • Ok thanks but how would i do that? On that site above i found just DEM which dont have rivers or i dont know how to load them. Ill try double-click on layer for colors although Terrain analysis works fine.
    – Mickey
    Nov 16, 2014 at 16:27
  • Are you hoping to compute rivers from the DEM? Usually, one would get a river dataset and just overlay it on top of the DEM.
    – underdark ♦
    Nov 16, 2014 at 17:08
  • yes i was. probably dum thought but im new to this. were can i find river data for the same area as in DEM ?
    – Mickey
    Nov 16, 2014 at 17:17
  • Really depends on the area and resolution you need, i.e. how small rivers should be included. Openstreetmap might have them.
    – underdark ♦
    Nov 16, 2014 at 17:25
  • i need them very detailed, if possible like in google maps. And if i find hydrographic map how do i insert it? just like DEM like layer on top?
    – Mickey
    Nov 16, 2014 at 20:03

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