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Inserting a geojson polygon in a field indexed with 2dsphere fails, claiming two separate parallel lines of constant latitude intersect. Full repro in 5.0.9 below.
mills.wj@gmail.com commented on Wed, 1 Nov 2023 15:12:07 +0000: Hi adam.harrison@mongodb.com, thanks for taking the time! I'm really surprised by this, there must be something I don't understand about how MongoDB is interpreting these coordinates. I understand this geometry is on a sphere, but even (or especially) on a sphere, lines of constant latitude do not intersect. The only conclusion I can draw here, is that a line segment such as [101.5625, -45.25], [76.5625, -45.25] is not interpreted as being a line of constant latitude even though it starts and ends on the same circle. What is the correct way to define a polygon edge of constant latitude for a 2dsphere index? I suppose we can approximate it by putting lots of vertexes along the line rather than traversing 25 degrees at a time, but I'd rather avoid this because I suspect this is going to bloat the index size. Thanks again for your attention on this. adam.harrison commented on Mon, 30 Oct 2023 22:12:57 +0000: Hi Bill, Thanks for the clear reproduction! This looks like it is expected behavior. Note that 2dsphere indexes perform geospatial operations on an earth-like sphere. While these specific edges are parallel on an X-Y coordinate plane, they are actually intersect when mapped over a sphere like surface. I've attached screenshots showing both of these edges graphed across a sphere and the corresponding intersection that would be generated a result. We hope this helps!
> db.createCollection('xx') { "ok" : 1 } > db.xx.createIndex({geolocation:'2dsphere'}) { "numIndexesBefore" : 1, "numIndexesAfter" : 2, "createdCollectionAutomatically" : false, "ok" : 1 } > db.xx.insertOne({'_id': 'busted', geolocation: {'type': 'Polygon', 'coordinates': [[[101.5625,-45.25],[76.5625,-45.25],[82.1875,-45.75],[89.6875,-45.75],[101.5625,-45.25]]]}}) WriteError({ "index" : 0, "code" : 16755, "errmsg" : "Can't extract geo keys: { _id: \"busted\", geolocation: { type: \"Polygon\", coordinates: [ [ [ 101.5625, -45.25 ], [ 76.5625, -45.25 ], [ 82.1875, -45.75 ], [ 89.6875, -45.75 ], [ 101.5625, -45.25 ] ] ] } } Loop is not valid: [ [ 101.5625, -45.25 ], [ 76.5625, -45.25 ], [ 82.1875, -45.75 ], [ 89.6875, -45.75 ], [ 101.5625, -45.25 ] ] Edges 0 and 2 cross. Edge locations in degrees: [-45.2500000, 101.5625000]-[-45.2500000, 76.5625000] and [-45.7500000, 82.1875000]-[-45.7500000, 89.6875000]", "op" : { "_id" : "busted", "geolocation" : { "type" : "Polygon", "coordinates" : [ [ [ 101.5625, -45.25 ], [ 76.5625, -45.25 ], [ 82.1875, -45.75 ], [ 89.6875, -45.75 ], [ 101.5625, -45.25 ] ] ] } } }) : WriteError({ "index" : 0, "code" : 16755, "errmsg" : "Can't extract geo keys: { _id: \"busted\", geolocation: { type: \"Polygon\", coordinates: [ [ [ 101.5625, -45.25 ], [ 76.5625, -45.25 ], [ 82.1875, -45.75 ], [ 89.6875, -45.75 ], [ 101.5625, -45.25 ] ] ] } } Loop is not valid: [ [ 101.5625, -45.25 ], [ 76.5625, -45.25 ], [ 82.1875, -45.75 ], [ 89.6875, -45.75 ], [ 101.5625, -45.25 ] ] Edges 0 and 2 cross. Edge locations in degrees: [-45.2500000, 101.5625000]-[-45.2500000, 76.5625000] and [-45.7500000, 82.1875000]-[-45.7500000, 89.6875000]", "op" : { "_id" : "busted", "geolocation" : { "type" : "Polygon", "coordinates" : [ [ [ 101.5625, -45.25 ], [ 76.5625, -45.25 ], [ 82.1875, -45.75 ], [ 89.6875, -45.75 ], [ 101.5625, -45.25 ] ] ] } } }) WriteError@src/mongo/shell/bulk_api.js:465:48 mergeBatchResults@src/mongo/shell/bulk_api.js:871:49 executeBatch@src/mongo/shell/bulk_api.js:940:13 Bulk/this.execute@src/mongo/shell/bulk_api.js:1182:21 DBCollection.prototype.insertOne@src/mongo/shell/crud_api.js:264:9 @(shell):1:1