# [geos-devel] Possible difference anomaly

Paul Ramsey pramsey at cleverelephant.ca
Wed Jan 2 08:33:25 PST 2019

```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#L1194>
> https://github.com/MapWindow/MapWinGIS/blob/master/src/COM%20classes/Shape.cpp#L1302 <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
> _______________________________________________
> geos-devel mailing list
> geos-devel at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/geos-devel

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/geos-devel/attachments/20190102/84e515e4/attachment.html>
```