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 <bontepaar...@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
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