<html><head></head><body><div style="font-family: Verdana;font-size: 12.0px;"><div>Hello, </div>
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<div>I got some shp files with many polygons. Now I want to delete some of them, but I have not been able to do so.</div>
<div>I tried v.edit which does not seem to have any effect.</div>
<div>Then I went to the attribute table and tried both deleting the record and the feature and both times I got rid of the filling of the polygon, but still have the border. </div>
<div>I tried to delete these borders using the digitizing tool, but I did not find any way of doing so. </div>
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<div>Thanks a lot. </div>
<div>Best wishes, Andrea</div>
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<div style="margin:0 0 10px 0;"><b>Gesendet:</b> Montag, 16. Februar 2015 um 23:12 Uhr<br/>
<b>Von:</b> "Thomas Adams" <tea3rd@gmail.com><br/>
<b>An:</b> "Markus Metz" <markus.metz.giswork@gmail.com><br/>
<b>Cc:</b> "grass-user@lists.osgeo.org" <grass-user@lists.osgeo.org><br/>
<b>Betreff:</b> Re: [GRASS-user] Question about r.watershed and flow accumulation grid</div>
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<div>Markus,<br/>
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Thank you very much for you insights; Yes, I have seen significant differences between DEM derived streams and those from other sources. I have not attempted burning the streams into the DEM as Micha suggested. FORTUNATELY, I had a 100m DEM from the USGS that was fine for my uses; so, the problem area that I had was resolved and I did not have to resort to such drastic measures -- it was disheartening to think I might have to resort to that.<br/>
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So, what's really cool is that I have hydrologically modeled a good size basin (8332 sq km) at a 250 meter grid spacing; all the model parameter estimation was done using GRASS 70, as is the animation of the simulation. I'll be posting it to Vimeo later today. It's the same basin area I did previously that you saw, only the previous one had a much coarser (4km) spatial resolution.<br/>
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Thanks all!<br/>
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Tom
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<div class="gmail_quote">On Mon, Feb 16, 2015 at 2:42 PM, Markus Metz <span><<a href="markus.metz.giswork@gmail.com" target="_parent">markus.metz.giswork@gmail.com</a>></span> wrote:
<blockquote class="gmail_quote" style="margin: 0 0 0 0.8ex;border-left: 1.0px rgb(204,204,204) solid;padding-left: 1.0ex;"><span>On Sat, Feb 14, 2015 at 3:45 PM, Thomas Adams <<a href="tea3rd@gmail.com" target="_parent">tea3rd@gmail.com</a>> wrote:<br/>
></span><br/>
<span>> The reason I need to use the D8 (SFD) algorithm is that I am generating a<br/>
> file that identifies how pixels are connected hydraulically; the distributed<br/>
> hydrologic model I am using requires that this connectivity be unique; so a<br/>
> pixel must be uniquely connected to a single downstream pixel. Consequently,<br/>
> with the GRASS scripting I have done to produce the needed ascii output file<br/>
> I need, I think I have to use the D8 SFD algorithm.</span><br/>
<br/>
The connectivity (drainage output of r.watershed) is always unique, no<br/>
matter if you use D8 or MFD. In case of MFD, the predominant direction<br/>
is used.<br/>
<br/>
As Micha said, r.watershed does not produce breaks in flow<br/>
accumulation, it stops only at the edge of the current region and at<br/>
NULL cells.<br/>
<br/>
Modifying a DEM to match a river network can be dangerous, I would<br/>
recommend to not burn the whole river network into the DEM but modify<br/>
only those parts that really need modification. Usually you will never<br/>
get a perfect match between a river network created from different<br/>
sources and a stream network derived from a DEM.<br/>
<br/>
Markus M
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><br/>
> Cheers!<br/>
> Tom<br/>
><br/>
><br/>
> On Sat, Feb 14, 2015 at 5:02 AM, Micha Silver <<a href="micha@arava.co.il" target="_parent">micha@arava.co.il</a>> wrote:<br/>
>><br/>
>> Hi Thomas:<br/>
>><br/>
>> On 02/13/2015 10:28 PM, Thomas Adams wrote:<br/>
>><br/>
>> Micha,<br/>
>><br/>
>> No, after looking at what's going-on in more detail, I think the DEM is<br/>
>> too coarse (even at 90m), so the flow direction and accumulation is<br/>
>> mis-directed in one critical area of the watershed. I tried using r.carve,<br/>
>> but it is taking forever — after 15 minutes, no advancement of the progress<br/>
>> bar…<br/>
>><br/>
>><br/>
>> I don't see why carving into a DEM should cause r.watershed to run more<br/>
>> slowly, unless you have carved out only a section of some of the streams. If<br/>
>> your carving does not continue right to the outlet of the stream (i.e. to an<br/>
>> ocean, or to the edge of the region) then r.watershed would actually have to<br/>
>> fill in that carved out stream to find a flow path. That could cause the<br/>
>> performance hit.<br/>
>><br/>
>> Additionally, are you using GRASS 7.0? As you probably know some<br/>
>> substantial improvements to the algorithms were introduced in 7 for several<br/>
>> modules, r.watershed among them.<br/>
>><br/>
>> And third, why use the D8 flow direction when MFD is available (again in<br/>
>> GRASS 7.0)? That could also be causing what you refer to as breaks in the<br/>
>> channels. MFD is especailly good, I believe, in flat areas.<br/>
>><br/>
>> In any case, Keep up posted on your progress.<br/>
>> Thanks,<br/>
>> Micha<br/>
>><br/>
>><br/>
>> I did not want to have to do this for my testing, but I'll probably try at<br/>
>> 3 or 10 meter — lots of pixels for my basin! The problem, in general, for me<br/>
>> is that I want to apply my techniques at international locations where I<br/>
>> probably won't have the benefit of higher resolution DEMs, so I need to<br/>
>> develop something a bit more robust…<br/>
>><br/>
>> Cheers!<br/>
>> Tom<br/>
>><br/>
>> On Fri, Feb 13, 2015 at 1:15 PM, Micha Silver <<a href="micha@arava.co.il" target="_parent">micha@arava.co.il</a>> wrote:<br/>
>>><br/>
>>><br/>
>>> On 02/13/2015 08:48 PM, Thomas Adams wrote:<br/>
>>><br/>
>>> Stefan,<br/>
>>><br/>
>>> A fair question; since I know the stream topology from personal<br/>
>>> experience it is clear that there should be no break in the stream network<br/>
>>> and the flow accumulation grid should reflect that. I am seeing the flow<br/>
>>> accumulation values break at a point where they should continue to<br/>
>>> accumulate downstream.<br/>
>>><br/>
>>><br/>
>>> Are these breaks possibly caused by null pixels in the DEM?<br/>
>>><br/>
>>> Tom<br/>
>>><br/>
>>> On Fri, Feb 13, 2015 at 11:43 AM, Stefan Lüdtke <<a href="sluedtke@gfz-potsdam.de" target="_parent">sluedtke@gfz-potsdam.de</a>><br/>
>>> wrote:<br/>
>>>><br/>
>>>> -----BEGIN PGP SIGNED MESSAGE-----<br/>
>>>> Hash: SHA1<br/>
>>>><br/>
>>>> Hi Tom,<br/>
>>>><br/>
>>>> just out of curiosity, what do you mean by "break in the flow<br/>
>>>> accumulation"?<br/>
>>>><br/>
>>>> Cheers,<br/>
>>>><br/>
>>>> Stefan<br/>
>>>><br/>
>>>> On 02/13/2015 07:12 PM, Thomas Adams wrote:<br/>
>>>> > Hello all!<br/>
>>>> ><br/>
>>>> > I'm making use of the flow accumulation grid in GRASS 6.4.5<br/>
>>>> > generated from r.watershed using the SFD (D8) flow algorithm. The<br/>
>>>> > DEM has a 250m spatial resolution. What I'm getting is a break in<br/>
>>>> > the flow accumulation in a few locations which is causing me<br/>
>>>> > serious problems with subsequent processing (with help from some<br/>
>>>> > here, I have put together some scripting to generate a pixel<br/>
>>>> > connectivity file for a distributed hydrologic model).<br/>
>>>> ><br/>
>>>> > Besides going to a higher resolution DEM, are there any thoughts as<br/>
>>>> > to how I can eliminate these flow accumulation breaks?<br/>
>>>> ><br/>
>>>> > Thank you, Tom<br/>
>>>> ><br/>
>>>> > --<br/>
>>>> ><br/>
>>>> ><br/>
>>>> ><br/>
>>>> > _______________________________________________ grass-user mailing<br/>
>>>> > list <a href="grass-user@lists.osgeo.org" target="_parent">grass-user@lists.osgeo.org</a><br/>
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>>>> ><br/>
>>>><br/>
>>>> - --<br/>
>>>> Stefan Lüdtke<br/>
>>>><br/>
>>>> Section 5.4- Hydrology<br/>
>>>> Tel.: <a>+49 331 288 2821</a><br/>
>>>> Fax: <a>+49 331 288 1570</a><br/>
>>>> Email: <a href="sluedtke@gfz-potsdam.de" target="_parent">sluedtke@gfz-potsdam.de</a><br/>
>>>><br/>
>>>> Helmholtz-Zentrum Potsdam<br/>
>>>> Deutsches GeoForschungsZentrum GFZ<br/>
>>>> (GFZ German Research Centre for Geoscience)<br/>
>>>> Stiftung des öff. Rechts Land Brandenburg<br/>
>>>> Telegrafenberg, 14473 Potsdam<br/>
>>>> - -------------------<br/>
>>>><br/>
>>>> PGP Public Key: <a href="http://bit.ly/13d9Sca" target="_blank">http://bit.ly/13d9Sca</a><br/>
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>>><br/>
>>><br/>
>>><br/>
>>><br/>
>>> --<br/>
>>><br/>
>>><br/>
>>><br/>
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