[GRASS-user] floodplain creation

[Shane Carey]

Hi Vaclav,

I have checked the output from r.watershed with what you have calculated
and I do see differences in the value obtained by both. Could it be a
rounding issue perhaps? The only other difference I see is that the output
from r.watershed is a complete raster as opposed to just the river network.
See image attached to explain what I mean. Does that make any difference?

[image: image.png]
Le gach dea ghui,
*Shane Carey*
*GIS and Data Solutions Consultant*


On Tue, Oct 2, 2018 at 12:07 PM Shane Carey <careyshan at gmail.com> wrote:

> Ok cool. When I I try to run the r.stream.distance with this new new
> stream direction layer (that I calculated with  r.mapcalc
> "streams_direction_8 = int((streams_direction + 45) / 45)" ),
>
> it just hangs on the part that says: "caclulating downstream parameters".
> So I am not sure what is going on here - have you any ideas on this?
>
> Also, could you explain a little about what this expression is doing:
> (streams_direction + 45) / 45)
>
> Thanks Vaclav
>
>
> Le gach dea ghui,
> *Shane Carey*
> *GIS and Data Solutions Consultant*
>
>
> On Mon, Oct 1, 2018 at 4:29 PM Vaclav Petras <wenzeslaus at gmail.com> wrote:
>
>>
>> On Sat, Sep 29, 2018 at 1:55 PM Shane Carey <careyshan at gmail.com> wrote:
>>
>>> My last bit of the puzzle is to create the flow direction raster. How do
>>> I reclassify in order to get the stream direction using r.mapcalc?
>>>
>>
>> Hi Shane,
>>
>> You are welcome. What you need to do are 2 steps: 1) vector to raster
>> conversion with direction to be used for raster values:
>>
>> v.to.rast input=streams output=streams_direction use=dir
>>
>> The v.to.rast manual says: "line direction in degrees CCW from east" [1].
>> However, the r.stream.distance manual specifies the drainage direction to
>> be: "...zero and negative values are valid direction data only if they vary
>> from -8 to 8 (CCW from East in steps of 45 degrees). Flow direction map
>> shall be of integer type (CELL)." So you need to reclassify.
>>
>> [1] https://grass.osgeo.org/grass74/manuals/v.to.rast.html
>> [2] https://grass.osgeo.org/grass74/manuals/addons/r.stream.distance.html
>>
>> The reclassification with r.mapcalc would be something like the
>> following, but I didn't really check and it may be wrong. Alternative would
>> be to use r.recode.
>>
>> r.mapcalc "streams_direction_8 = int((streams_direction + 45) / 45)"
>>
>> So please double check the output from the above and compare it with what
>> you get from r.watershed.
>>
>> For completeness, here is what I wrote before:
>>
>> """
>> Yes, just convert your streams to raster (presence-absence) and a raster
>> for flow direction as I mentioned here:
>>
>> https://lists.osgeo.org/pipermail/grass-user/2018-September/079135.html
>>
>> 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
>> """
>>
>>
>>> I am just coping in what you said from your last email Vaclav so that
>>> you know which part I am stuck on. This is really super - thanks again
>>>
>>> [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]
>>>
>>> Le gach dea ghui,
>>> *Shane Carey*
>>> *GIS and Data Solutions Consultant*
>>>
>>>
>>> On Fri, Sep 28, 2018 at 3:02 PM Vaclav Petras <wenzeslaus at gmail.com>
>>> wrote:
>>>
>>>>
>>>> On Thu, Sep 27, 2018 at 12:10 PM Shane Carey <careyshan at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>> Is there anyway of the algorithm just being able to create a floodlain
>>>>> for actual real rivers as opposed to the derived river dataset?
>>>>>
>>>>
>>>> Yes, just convert your streams to raster (presence-absence) and a
>>>> raster for flow direction as I mentioned here:
>>>>
>>>> https://lists.osgeo.org/pipermail/grass-user/2018-September/079135.html
>>>>
>>>> However, you need to be cautious about fit between the existing streams
>>>> and the DEM. Definitively check how your existing streams align with the
>>>> derived ones (where there are both streams).
>>>>
>>>>
>>>>>
>>>>> On Máirt 25 MFómh 2018 at 15:49, Shane Carey <careyshan at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Image attached shows rivers (black line) and floodplain of where it
>>>>>> thinks there are rivers and in some cases where there may not be rivers.
>>>>>>
>>>>>
>>>> To use the derived streams and get rid of parts you don't want, you
>>>> could buffer the existing streams and preserve only the derived streams
>>>> which are in that buffer.
>>>>
>>>> However, you should really look at why they are different and
>>>> specifically what is closer to reality.
>>>>
>>>
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