Ok great - thanks. I think I will use r.carve to carve the streams into the DTM first and then use this procedure you have described. Do you with the seed raster (for r.lake) if I convert the stream to raster, will ever cell be a seed point (i.e. Water will be placed into every cell at a height of x?)? If so, this is exactly what I need.
Thanks On Luan 17 MFómh 2018 at 04:22, Vaclav Petras <[email protected]> wrote: > Hi Shane, > > On Fri, Sep 14, 2018 at 5:03 PM Shane Carey <[email protected]> wrote: > >> >> Does anyone know is it possible to calculate the elevation above a river >> channel (actual river network that was digitised as opposed to being >> extracted from a DTM) from a DTM and create a polygon from it. >> >> I need to calculate heights of 1m and 3m above a river channel on both >> sides of the channel and create a polygon from it. >> > > Maybe Height Above Nearest Drainage (HAND) is what you are looking for. It > is basically done using: > > r.stream.distance stream_rast=streams direction=drainage > elevation=elevation method=downstream difference=above_stream > > See full workflow here: > > > https://grasswiki.osgeo.org/wiki/From_GRASS_GIS_novice_to_power_user_(workshop_at_FOSS4G_Boston_2017)#Hydrology:_Estimating_inundation_extent_using_HAND_methodology > > You will need to do some post processing with r.lake or r.mapcalc and > r.to.vect to get the polygons. > > Since you already have the streams and you want to use them, you need to > convert them to raster and then also get stream direction for > r.stream.distance by reclassifying, e.g. with r.mapcalc, output from > something like this: > > v.to.rast in=streams output=streams_dir use=dir > > which you can see in context here (different hydrology tool): > > > http://ncsu-geoforall-lab.github.io/geospatial-modeling-course/grass/simwe.html > > However, I would be careful about how the streams fit with the terrain you > are using to be sure results make sense. > > Vaclav > > -- Le gach dea ghui, *Shane Carey* *GIS and Data Solutions Consultant*
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