I have a polygon defined as follows:
segment from 43:29:02 N 003:50:39 E to 43:34:12 N 003:43:04 E
arc(CW): center (43:34:49 N 003:58:16 E) from 43:34:12 N 003:43:04 E to 43:45:45 N 003:59:56 E
segment from 43:45:45 N 003:59:56 E to 43:40:01 N 004:06:40 E
arc(CCW): center(43:34:49 N 003:58:16 E) from 43:40:01 N 004:06:40 E to 43:29:02 N 003:50:39 E
The corresponding shapefile can be found at http://bordel.kataplop.net/mont2.zip
EDIT corresponding WKT:
POLYGON ((3.8441666666666667 43.4838888888888917, 3.7177777777777781 43.5700000000000003,3.7177777777777781 43.5700000000000003, 3.7175416922400166 3.5735823218689617, 3.7174122630334763 43.5767891717197671, 3.7173600981542299 3.5799971695780499, 3.7173852258405691 43.5832053383720393, 3.7174876507934393 3.5864127008216897, 3.7176673541599534 43.5896182797364418, 3.7179242935240802 3.5928210983129603, 3.7182584029045262 43.5960201804331646, 3.7186695927598370 3.5992145509620244, 3.7191577500007265 43.6024032360454115, 3.7197227380096543 3.6055852634076615, 3.7203643966676592 43.6087596626488931, 3.7210825423884666 3.6119254655418658, 3.7218769681598638 43.6150817063284819, 3.7227474435923660 3.6182274220155932, 3.7236937149751603 43.6213616526702097, 3.7247155053393355 3.6244834417140126, 3.7258125145283985 43.6275918362168653, 3.7269844192760679 3.6306858871895287, 3.7282308732913374 43.6337646498753315, 3.7295515073508057 3.6368271840406265, 3.7309459293982550 43.6398725542641515, 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I'm using shapely to handle my data, and it reports that this polygon is self intersecting. I checked with QGis (it is also complaining about this problem), and I don't see why it would be self-intersecting.
I've narrowed down the problem to the 2 "single points" of the second arc (beginning and end). It looks like the 2 segments are considered as crossing the second arc… Which is not true.
I'm not producing the data but I can ask for fixing it in case it is wrong. But in this case, I don't understand the problem (if any).
Is this an issue of some buggy python library (shared by shapely and qgis) or is it something else ?
Best Answer
If I zoom in here...
I see this:
I think the issue stems from the limitation that a shapefile cannot store arcs. Instead it does a linear approximation. When a polyline intersects (what is intended to be) a circular arc at a tangent the zig zag approximation results in an intersection.
I'd recommend storing the geometry in something that supports circular arcs, like a file geodatabase. There is an open source API available for it. When was Esri File Geodatabase API made available?