Hello For a long time I wanted to hear opinion on the topic of topology rules.
By "topology rules" here I mean just simple rules such as: * polygon X should not overlap polygon Y * polygon X should always be above polygon Y * point X should be not further from line Y than D etc. There are no such rules in OSM (or I have not found them). I do not see them being mandatory in the nearest future for various reasons. But having them described somewhere would add some benefits (or so I think). For example: * it would introduce general agreement which in theory should make it easier for people to understand how polygons should be mapped. Do we use type X or type Y - depending on what is or isn't below/above. * some errors could be found (say addr:street should be not further than 5km from a way with the name tag having exactly the same name). * it would be easier for cartographers or the like. They would not have to guess which polygons should be above which ones in drawing order, for example when creating garmin maps it is not always possible to control the order of layers, so we have a question which goes first: forest or water? People often report problems that water or island is not visible and the problem is usually because of incorrect objects in OSM: multipolygon not created so the same area is actually covered by both water and landuse polygons. (Garmin maps is not the only area with such a problem, this is just an example) * general resulting/exported "GIS" datasets (shapes, geodatabases etc.) would be more "correct" in GIS sense. * different "area calculations" would be more correct. Say we want to calculate area in a region covered by land, water, farmland etc. As it stands now we could get a total area larger than area of a region in question. This has been tested in Lithuania for years now and looks fine. We do not introduce questionable rules, just obvious ones: no overlap between forest, water, wetland, meadow, residential, industrial, commercial etc. wood only above residential/commercial/industrial. The last one gives advantage that for large scales we can ignore wood objects as they represent detail as compared to forests which represent large scale stuff. Note this is just an example, I'm not saying this must be the global standard. The process could be: 1. We introduce VERY simple rules which are mostly a de facto standard anyway: no overlap between forest and water, addr:street is "near" the street, building must not overlap other building. 2. Check, fix, look for objections/problems. 3. Fix/adjust existing rules. 4. Go to step 1. What do you think in general? P.S. Some of these rules are already implemented as QA rules in say osmose, just not described as "topology rules". -- Tomas _______________________________________________ talk mailing list talk@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk
