[GRASS-user] Elevation above a river

[Johannes Radinger]

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 region
g.region raster=elevation at PERMANENT

# Extract elevation of the streams
r.mapcalc --o expression="streams_elevation = if( streams_derived at PERMANENT,
elevation at PERMANENT,null())"

# Grow stream_elevation map by a some hundred meters
r.grow -m --overwrite input=streams_elevation output=streams_elevation_grow
radius=500

# Calculate difference between original elevatoin and stream channel
elevation
r.mapcalc expression="stream_elevation_diff = elevation at PERMANENT -
streams_elevation_grow" --overwrite

# Extract areas that are than x meters higher than stream channel elevation
r.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?
>>>
>>> Best
>>> Markus
>>>
>>>
>>> Thanks all.
>>>>
>>>> Le gach dea ghui,
>>>> *Shane Carey*
>>>> *GIS and Data Solutions Consultant*
>>>> _______________________________________________
>>>> grass-user mailing list
>>>> grass-user at lists.osgeo.org
>>>> https://lists.osgeo.org/mailman/listinfo/grass-user
>>>
>>> _______________________________________________
>>> grass-user mailing list
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>>
>> --
> Le gach dea ghui,
> *Shane Carey*
> *GIS and Data Solutions Consultant*
>
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