[GIS] Looking for guidance to create a hydrological map with QGIS and/or GRASS

cartographygrassqgisrivers

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?

Best Answer

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.

Related Solutions

[GIS] Create filled contour map with GRASS

After using v.surf.rst, it's only a question about how to display the resulting cost raster.

GRASS offers r.colors to specify a color map for a raster. As far as I can see, the normal behavior of GRASS is to interpolate colors, so you you'll have to work around this:

r.colors map=travel_costs color=rules 
0 green
100 green 
101 yellow
200 yellow
201 red 
300 red 
end

[GIS] How to extract elevation values at line start and end points from a DEM

One of the easier ways to do this is with a spatial database such as PostGIS. Load your raster into the database using raster2pgsql (which is installed with PostGIS) and then load your rivers with shp2pgsql (also installed with PostGIS).

Then, you can run a simple query that samples the elevation model at the end points of all your lines:

select 
    st_value(r.rast, st_startpoint(geom)) as a, 
    st_value(s.rast, st_endpoint(geom)) as b 
from 
    rivers, dem r, dem s 
where 
    st_contains(st_convexhull(r.rast), st_startpoint(geom)) and      
    st_contains(st_convexhull(s.rast), st_endpoint(geom));

I've named the DEM twice in the from clause because the ends of the line may be contained by two different tiles in the raster, and each tile will have to be matched independently. That's why there are two st_contains calls.

The great thing about PostGIS is that you can automate this kind of processing over millions of records with a minimum of effort.

Note that this works if the line geometry is LineString. If they're MultiLineStrings, you'll have to refine the query, or use st_dump to turn them into LineStrings.

Best Answer

Related Solutions

[GIS] Create filled contour map with GRASS

[GIS] How to extract elevation values at line start and end points from a DEM

Related Question