[geos-devel] Possible difference anomaly

[Darafei "Komяpa" Praliaskouski]

Hi,

last time I researched the topic generating a set of administrative
boundaries from different tataset pprepair was a lifesaver, patching up the
planar partition of my heterogeneous coverage:
https://github.com/tudelft3d/pprepair


ср, 2 янв. 2019 г. в 19:33, Paul Ramsey <pramsey at cleverelephant.ca>:

> No, assumptions are not quite correct.
> It’s possible you’ll get an introduced vertex in the clipped geometry that
> is not a component of the clipping geometry and is also slightly inside one
> of the clipping geometry’s edges… I think in order to handle this we’d need
> to have a function that takes in both geometries and adjusts them both. If
> you’re only generating one geometry, it seems like it is always possible to
> have cases with edge mismatches. Maybe for your case if you difference to
> to get your input geometry, and then difference *that* from all the stored
> polygons you already have, which would hopefully result in an output
> collection in which all neighbours have the same vertices in the same
> places.
>
> P
>
>
> On Jan 2, 2019, at 7:48 AM, Paul Meems <bontepaarden at gmail.com> wrote:
>
> Hi List,
>
> I'm creating irregular polygons and I don't want to let them overlap.
> So before adding the polygon to my list of 'good' polygons, I do an
> overlap check first.
> When the polygon overlaps with an existing polygon I do a clip difference
> and add the clipped polygon to my good list.
>
> Sometimes two polygons still overlap after a clip difference.
>
> I managed to extract two sets that barely overlap. The first set doesn't
> overlap anymore after clipping the second one is still overlapping.
>
> Set 1:
> shp = POLYGON ((259449.071118579 543868.469050065,259460.460699968
> 543878.23006619,259463.245769758 543873.768560297,259455.578068317
> 543860.876455946,259449.071118579 543868.469050065))
> shp2 = POLYGON ((259463.243735243 543856.317227775,259458.0148573
> 543864.973555723,259462.906856719 543873.198728218,259471.741696109
> 543861.450443661,259463.243735243 543856.317227775))
> shp3 (clipped result) = POLYGON ((259463.243735243
> 543856.317227775,259458.014857301 543864.973555722,259462.906856719
> 543873.198728217,259471.741696109 543861.450443661,259463.243735243
> 543856.317227775))
>
> Set 2:
> shp = POLYGON ((259073.014081524 544320.092508488,259085.267633063
> 544321.677731439,259085.942168897 544320.308101652,259074.192475622
> 544310.983687036,259073.014081524 544320.092508488))
> shp2 = POLYGON ((259087.677538158 544333.393525226,259097.260135269
> 544321.853428493,259090.546766392 544316.354731347,259079.984140336
> 544327.005127152,259087.677538158 544333.393525226))
> shp3 (clipped result) = POLYGON ((259087.677538158
> 544333.393525226,259097.260135269 544321.853428493,259090.546766392
> 544316.354731347,259085.267633064 544321.677731438,259085.267633063
> 544321.677731439,259085.267633062 544321.677731439,259079.984140336
> 544327.005127152,259087.677538158 544333.393525226))
>
> I'm using C# with MapWinGIS to do the actions:
>             if (shp.Overlaps(shp2))
>             {
>                 var shp3 = shp2.Clip(shp, tkClipOperation.clDifference);
>                 Assert.IsFalse(shp.Overlaps(shp3), "New shape still
> overlaps");
>             }
>
> MapWinGIS is using the GDAL v2.3.3 binaries from GisInternals which in
> turn is using geos-3.4
> MapWinGIS is directly porting its functions to the GEOS functions:
>
> https://github.com/MapWindow/MapWinGIS/blob/master/src/COM%20classes/Shape.cpp#L1194
>
> https://github.com/MapWindow/MapWinGIS/blob/master/src/COM%20classes/Shape.cpp#L1302
>
> Before I report this as a defect I want to know if my workflow and
> expectations are correct.
>
> Thanks,
>
> Paul
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>
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-- 
Darafei Praliaskouski
Support me: http://patreon.com/komzpa
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