I am currently mapping a large university (amenity=university). JOSM detects errors (overlapped area) when wooded areas (natural=wood) overlap the university polygon. Only wood/university overlaps are shown as errors, there is no error over the segments of the wooded area that is outside the university. To avoid these errors, I see three possibilities.

  • To exclude these elements from the polygon, even if they are actually owned by the university.
  • To split these elements at polygon’s boundary, using relations (multipolygons)
  • To do nothing.

Which one is expected according to OSM best practices? Thanks,

asked 20 Sep, 16:44

jfd553's gravatar image

accept rate: 0%

edited 20 Sep, 19:55


Is it possible to link to the area so that people can have a look at exactly what the error is?

It sounds like a false positive, and if that's the case I'd definitely report it on JOSM's trac. For info, I've always found the JOSM devs to be extremely helpful and responsive to things raised with them in this way.

(20 Sep, 20:06) SomeoneElse ♦

Just to be clear, is the validator reporting these as errors or warnings? I don't think I've seen overlapping areas reported as errors in the past. If they are now, it could be a bug.

(21 Sep, 17:44) alester

Validator reports the errors as "Other-Overlapping Areas". So it may just be a "Have a look" message.

(29 Sep, 20:24) jfd553

My basic answer is 3) "do nothing", such cases are quite commmon in OSM and therefore need to be considered by data consumers.

Longer answer below.

There is no need in OSM mapping for a single polygon reflecting landuse/landcover in any given area. The reasons for this are many:

  • Woods, lakes, meadows, residential areas all exist in university, school and hospital campuses. These are either not, or unlikely to be, errors. I would regard the JOSM validation rules to be incorrect in these situations.
  • Individual polygons will be created by different mappers at different times with different levels of knowledge of OSM, time available to add data, expectations of how the data will be used, quality of imagery at the time the polygon was mapped (which may be vastly different now).
  • Small polygons with a different class within a larger polygon are very common and entirely reasonable way of mapping the data. The main OSM map produced using CartoCSS makes a number of assumptions based on area and tag to decide which ones should be rendered on top of others. These algorithms work very well. (One exception is in the area you mention, where a small wood overlaps a larger feature, see https://github.com/gravitystorm/openstreetmap-carto/issues/2641 for discussion).

If you want a tesselated layer consisting of a contiguous set of polygons you need to post-process OSM data. You may need something like the CartoCSS algorithm, although in practice I have found a simple hierarchy of tags can work well (slide 16 here). My approach (slide 24 in previous link) involved progressive spatial unions and clipping according to the hierarchy I used. The main complication was dealing with invalid geometries at all stages of the process.

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answered 21 Sep, 14:39

SK53's gravatar image

SK53 ♦
accept rate: 20%

Not sure why amenity would conflict with woodlands and bodies of water, so I assume the issue is nested landuse tagging. Which is one reason that I avoid using landuse for things like wooded areas and bodies of water. I prefer using natural tags. Actually, I prefer landcover tagging (are those trees there "naturally" or because of some human reason? Don't know, don't care, but they are there thus landcover rather than natural). But that is poorly supported so I generally dual tag for natural and landcover.

If something is really a land use, like maybe a reservoir, in the middle of another land use, then make the larger one a multi-polygon if it isn't already. Then make the smaller contained land use an inner polygon.

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answered 20 Sep, 18:16

stf's gravatar image

accept rate: 18%

Wooded areas are labelled as natural=wood, water as natural=water. Only sections of wooded areas that overlap the amenity=university are identified as errors.

(20 Sep, 19:26) jfd553

Are you sure you haven't accidentally added a wood tag to the university?

(20 Sep, 23:55) InsertUser
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question asked: 20 Sep, 16:44

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last updated: 29 Sep, 20:24

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