# [GRASS-user] Elevation above a river

Sat Sep 22 06:39:59 PDT 2018

```Sorry I ment method=mode or median as stated by Markus N.

Sent from my Samsung Galaxy smartphone.
-------- Original message --------From: Shane Carey <careyshan at gmail.com> Date: 22/09/2018  15:05  (GMT+01:00) To: Johannes Radinger <johannesradinger at gmail.com> Cc: Markus Neteler <neteler at osgeo.org>, GRASS user list <grass-user at lists.osgeo.org> Subject: Re: [GRASS-user] Elevation above a river
This works a treat - super stuff - thanks Johannes.
Could you point me in the direction of an algorithim with Grass that can do the following:
# Clean some of the spurious cells etc. (using some kind of neighbourhood filtering)
Thanks very much

Le gach dea ghui,
Shane CareyGIS and Data Solutions Consultant

On Sun, Sep 16, 2018 at 8:46 PM, Johannes Radinger <johannesradinger at gmail.com> wrote:
That is were the third step should follow, i.e. using r.mapcalc to identify all the cells that are 1m higher than the grown river (irrespective in which distance from the river these cells are located). The rasterized area could then be translated into a vector format using r.to.vect.
Here a small example of how the working flow could be using the North Caroline example dataset:############### Set regiong.region raster=elevation at PERMANENT
# Extract elevation of the streamsr.mapcalc --o expression="streams_elevation = if( streams_derived at PERMANENT, elevation at PERMANENT,null())"
# Grow stream_elevation map by a some hundred metersr.grow -m --overwrite input=streams_elevation output=streams_elevation_grow radius=500
# Calculate difference between original elevatoin and stream channel elevationr.mapcalc expression="stream_elevation_diff = elevation at PERMANENT - streams_elevation_grow" --overwrite
# Extract areas that are than x meters higher than stream channel elevationr.mapcalc expression="stream_elevation_diff_smaller1 = if( stream_elevation_diff < 1,1,null())" --overwrite
# Maybe here it needs some cleaning of spurious cells etc. (using some kind of neighbourhood filtering)
# Raster areas to a vector area (with smoothed corners; s-flag)r.to.vect -s --overwrite input=stream_elevation_diff_smaller1 output=stream_elevation_diff_smaller1_area type=area##############
HTH/Johannes
On Sun, Sep 16, 2018 at 8:53 PM Shane Carey <careyshan at gmail.com> wrote:
Hey Johannes,
Thanks for your reply. How does r.grow work if let's say the height above the river reaches 1m at 3meters away from the river. And in an other area it reaches the 1meter height at 2meters away from the river. Is it able to follow that line?
Thanks
On Domh 16 MFómh 2018 at 17:21, Johannes Radinger <johannesradinger at gmail.com> wrote:
To me this looks like a flooding-related question, i.e. to extract the shore lines of a river if it's water level is raised by 1m or 3m?Maybe (1) extract the raster cells of the elevation map that represents the river channel, (2) then apply r.grow and (3) then r.mapcalc to subtract the grown river channel from the original elevation map. /Johannes
On Sun, Sep 16, 2018 at 5:16 PM Markus Neteler <neteler at osgeo.org> wrote:
Hi,

Shane Carey <careyshan at gmail.com> schrieb am Fr., 14. Sep. 2018, 23:02:
Hi All,
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.
This isn't clear to me. Could you elaborate?
BestMarkus

Thanks all.

Le gach dea ghui,
Shane CareyGIS and Data Solutions Consultant

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Le gach dea ghui,
Shane CareyGIS and Data Solutions Consultant

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