[GRASS-user] Elevation above a river

[Shane Carey]

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 <wenzeslaus at gmail.com> wrote:

> Hi Shane,
>
> On Fri, Sep 14, 2018 at 5:03 PM Shane Carey <careyshan at gmail.com> 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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