Re: Bugs in r.stream.extract

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Hi Ken

Many thanks for your reply. your time and effort are much appreciated.

I use -a option to ensure positive flow accumulations and using SFD, but
still got different results.

The reason why needs to change flow direction is that the DEM imperfect,
especially when there is a lot of lakes in the watershed. Without change
flow direction, the lake will be divided into different subbasins.

And the reason why I want to start from flow direction instead of DEM is
that there are several hydrological pre-conditioned datasets, the flow
direction in this dataset already adjusted for most of the lakes.

---- Some personal suggestions----
I find a solution for my work, but based on this experience feels like
divide functions like r.stream.extract into four functions
might provide user more feasibility:  such as a function that defines flow
direction DEM, a function define flow accumulation from flow direction, a
function define streams from flow accumulation and a function define
subbasin based on flow direction and stream network.

These are just personal suggestions.

Cheers
Ming

Ken Mankoff <[email protected]> =E4=BA=8E2020=E5=B9=B411=E6=9C=8825=E6=97=
=A5=E5=91=A8=E4=B8=89 =E4=B8=8B=E5=8D=886:01=E5=86=99=E9=81=93=EF=BC=9A

>
> On 2020-11-25 at 04:17 -08, ming han <[email protected]> wrote...
> > And another problem I got is that the flow accumulation I got from
> > r.accumulate and r.watershed is different when r.accumulate using flow
> > direction from r.watershed. Again is there anyway r.watershed supports
> > using flow direction, so we can get the consistent result.
>
> There may be reasons for these differences? For example, if SFD v. MFD, o=
r
> the "-a" flag to r.watershed?
>
> > We need this when we need to adjust the flow direction from
> > r.watershed or r.stream.extract. and then we need to determine new
> > flow accumulation with an adjusted flow direction dataset. If the
> > result is inconsistent, not sure what is the solution is.
>
> It isn't clear why you are adjusting the flow direction. Is this required=
?
>
> >> But, If I only want to use flow direction to drive streams, which
> >> function I should use?
>
> https://grass.osgeo.org/grass78/manuals/r.water.outlet.html but this only
> works for 1 outlet.
>
>   -k.
>

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<div dir=3D"ltr"><div>Hi Ken=C2=A0</div><div><br></div><div>Many thanks for=
 your reply. your time and effort are much appreciated.=C2=A0<br></div><div=
><br></div>I use -a option to ensure positive=C2=A0flow accumulations and u=
sing SFD, but still got different results.=C2=A0 =C2=A0<div><br></div><div>=
The reason why needs to change flow direction is that the DEM imperfect, es=
pecially=C2=A0when there is a lot of lakes in the watershed. Without change=
 flow direction, the=C2=A0lake will be divided into different subbasins.=C2=
=A0</div><div><br></div><div>And the reason why I want to start from flow d=
irection instead of DEM is that there are several hydrological pre-conditio=
ned datasets, the flow direction in this dataset already adjusted for most =
of the lakes.=C2=A0=C2=A0</div><div><br></div><div>---- Some personal sugge=
stions----</div><div>I find a solution for my work,=C2=A0but based on this =
experience feels like divide=C2=A0functions like r.stream.extract into four=
 functions might=C2=A0provide=C2=A0user more feasibility:=C2=A0 such as a f=
unction that defines=C2=A0flow direction DEM, a function define=C2=A0flow a=
ccumulation from flow direction, a function define streams from flow accumu=
lation and a function define subbasin based on flow direction and stream ne=
twork.=C2=A0 =C2=A0</div><div><br></div><div>These are just personal sugges=
tions.=C2=A0</div><div></div><div>=C2=A0=C2=A0</div><div>Cheers</div><div>M=
ing=C2=A0=C2=A0</div></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" =
class=3D"gmail_attr">Ken Mankoff &lt;<a href=3D"mailto:[email protected]">m=
[email protected]</a>&gt; =E4=BA=8E2020=E5=B9=B411=E6=9C=8825=E6=97=A5=E5=91=
=A8=E4=B8=89 =E4=B8=8B=E5=8D=886:01=E5=86=99=E9=81=93=EF=BC=9A<br></div><bl=
ockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-lef=
t:1px solid rgb(204,204,204);padding-left:1ex"><br>
On 2020-11-25 at 04:17 -08, ming han &lt;<a href=3D"mailto:[email protected]=
om" target=3D"_blank">[email protected]</a>&gt; wrote...<br>
&gt; And another problem I got is that the flow accumulation I got from<br>
&gt; r.accumulate and r.watershed is different when r.accumulate using flow=
<br>
&gt; direction from r.watershed. Again is there anyway r.watershed supports=
<br>
&gt; using flow direction, so we can get the consistent result.<br>
<br>
There may be reasons for these differences? For example, if SFD v. MFD, or =
the &quot;-a&quot; flag to r.watershed?<br>
<br>
&gt; We need this when we need to adjust the flow direction from<br>
&gt; r.watershed or r.stream.extract. and then we need to determine new<br>
&gt; flow accumulation with an adjusted flow direction dataset. If the<br>
&gt; result is inconsistent, not sure what is the solution is.<br>
<br>
It isn&#39;t clear why you are adjusting the flow direction. Is this requir=
ed?<br>
<br>
&gt;&gt; But, If I only want to use flow direction to drive streams, which<=
br>
&gt;&gt; function I should use?<br>
<br>
<a href=3D"https://grass.osgeo.org/grass78/manuals/r.water.outlet.html" rel=
=3D"noreferrer" target=3D"_blank">https://grass.osgeo.org/grass78/manuals/r=
.water.outlet.html</a> but this only works for 1 outlet.<br>
<br>
=C2=A0 -k.<br>
</blockquote></div>

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