[GRASS-user] Re: grass-user Digest, Vol 55, Issue 64

[Richard Chirgwin]

On 30/11/10 3:02 AM, grass-user-request at lists.osgeo.org wrote:
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> Today's Topics:
>
>     1. Re: Closed Areas (Markus Metz)
>     2. Re: Closed Areas (pipppoo)
>     3. Avoiding using specific-machine language in winGRASS
>        (Franz Schiller)
>     4. Re: Closed Areas (pipppoo)
>     5. Re: Closed Areas (pipppoo)
>     6. Re: Avoiding using specific-machine language in winGRASS
>        (Philipp Glira)
>     7. Re: Re: Closed Areas (Martin Landa)
>     8. Re: Re: Closed Areas (Markus Metz)
>     9. WinGRASS: Error when importing GRIB files (Ant?nio Rocha)
>    10. Re: Closed Areas (pipppoo)
>    11. Re: Avoiding using specific-machine language in winGRASS
>        (Markus Neteler)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 29 Nov 2010 11:16:59 +0100
> From: Markus Metz<markus.metz.giswork at googlemail.com>
> Subject: Re: [GRASS-user] Closed Areas
> To: pipppoo<pipppoo at gmail.com>
> Cc: grass-user at lists.osgeo.org
> Message-ID:
> 	<AANLkTin0uA=13+axf7b_qrDOw2OT2hoyuJ=zz5Ht4+mX at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> pipppoo wrote:
> [...]
>> Number of duplicate centroids: 116
>> Number of areas without centroid: 116
> That looks like there are overlapping areas. For these, a centroid can
> not be unambiguously assigned.
>
>> v.category complete. 597 features modified.
>>
>> The output says that to 116 areas a centroid can't be added.
>> Thus i controlled with v.digit, if the boundaries are closed:
>> http://osgeo-org.1803224.n2.nabble.com/file/n5783822/Screenshot-v.digit_-_FNF%40PERMANENT.png
>>
>> How you can see, there are just green crosses, that means that every
>> boundary should be closed? Or not?
>> But why then there are 116 areas without centroid after runnig v.centroids?
>>
> Because there are also 116 duplicate area centroids. The total number
> of centroids is identical to the total number of areas.
>
> You would have to clean the area vector without centroids first with
> the tools break,rmdupl, maybe also with rmsa: repeat
> tool=break,rmdupl,rmsa until rmsa does no more modifications.
>
> Markus M
>
>> Thanks for any help in advance!
>>
>> cheers,
>> pipppoo
>> --
>> View this message in context: http://osgeo-org.1803224.n2.nabble.com/Closed-Areas-tp5783822p5783822.html
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> ------------------------------
>
> Message: 2
> Date: Mon, 29 Nov 2010 02:33:38 -0800 (PST)
> From: pipppoo<pipppoo at gmail.com>
> Subject: [GRASS-user] Re: Closed Areas
> To: grass-user at lists.osgeo.org
> Message-ID:<1291026818544-5783892.post at n2.nabble.com>
> Content-Type: text/plain; charset=us-ascii
>
>
> Thanks for you help!
>
> What I really need is one area for every boundary. The reason is that I want
> to compute the overlap of the areas, just like done here:
> http://grass.osgeo.org/wiki/Vector_Overlapping_Areas
> http://grass.osgeo.org/screenshots/vector.php (Overlapping Areas)
>
> So the option with tool:break,rmdupl probably won't work. Is that right?
>
> Has anybody another idea?
>
> This is a closer screenshot of the overlapping areas (randomly colored):
> http://osgeo-org.1803224.n2.nabble.com/file/n5783892/Screenshot-Map_Display_1.png
>
> cheers,
> pipppoo
>
>
Pipppoo,

Can I first ask: what was the original source of the map? Did you create 
in v.digit, or was it imported from somewhere else?

It would be complex to calculate the overlaps, but perhaps possible. If 
you can get a single area extracted, you could upload its area - with no 
overlap - then get its overlap area, and compare the two.

I'm going to give, as an example, two overlapping circles, created as 
buffers around points.

for i in `v.db.select PointMap column=cat -c`; do
v.extract PointMap output="PointMap_"$i where="cat=$i";
v.buffer "PointMap_"$i output="Buffer_"$i distance=<number>;
done

This creates one buffer for each point in PointMap.

for i in `g.mlist vect pattern=Buffer*`; do
v.db.addtable $i;
v.db.addcol $i columns="Area double";
v.to.db $i option=area units=k column=Area
done

I now have the area associated with each individual polygon.

v.overlay ainput=Buffer_1 atype=area binput=Buffer_2 btype=area 
operator=and output=Overlay

The overlay will have three areas: the non-overlapping part of Buffer_1 
and Buffer_2, and the overlapping area. The original areas uploaded by 
v.to.db will be in the Overlay map's table, as a_area and b_area.

v.db.addcol Overlay columns="New_Area double"
v.to.db Overlay option=area units=k column=New_Area

This will put area values into the table for the non-overlapping and 
overlap polygons. It's now easy to calculate the overlap.

The point of this ramble is: if your overlapping areas exist as 
individual polygons, you can solve the problem of calculating overlap.

Cheers,
Richard Chirgwin