# [GRASS-user] Elevation above a river

Shane Carey careyshan at gmail.com
Sat Sep 22 06:05:37 PDT 2018

```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 Carey*
*GIS and Data Solutions Consultant*

On Sun, Sep 16, 2018 at 8:46 PM, 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
>
> # 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
>>
>> Thanks
>>
>> On Domh 16 MFómh 2018 at 17:21, Johannes Radinger <
>>
>>> 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
>>>> grass-user at lists.osgeo.org
>>>> https://lists.osgeo.org/mailman/listinfo/grass-user
>>>
>>> --
>> Le gach dea ghui,
>> *Shane Carey*
>> *GIS and Data Solutions Consultant*
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20180922/8f097366/attachment-0001.html>
```