Is there an OSM map that shows the original/source geometry without manipulation, for instance in the highest zoom/scale?
Testing some 20-25 publicly available maps I could not find one. Many geometries look just fine/perfect but actually they are wrong. Consequently, many anomalies remain not corrected in a long time (maybe for ever) because they look fine in maps the (local) mappers are looking at. Let me illustrate the issue with some line-work objects, for instance roundabouts. Looking at examples here or here we may get an impression that these geometries are correct and nice. However, in the source data these geometries look badly like here or here There are some 10K similar roundabout ussies (not counting error cases) some of them being really serious anomalies like here . The roundabout looks just perfect but, essentially, in the source data this roundabout does not exist at all.

asked 25 Apr '17, 11:03

sanser's gravatar image

accept rate: 5%

Most maps generated from OSM data are intended to be maps and the rendering process will tend to smooth out small irregularities and (for example if Bezier curves are used) might even show geometries that do not exist in that form in the original data. This is not specific to OSM btw. The data layer on shows the original geometry as do most editors.

Naturally from a pedantic pov the roundabouts themselves should really be cleaned up and it would seem to be a good subject for a maproulette challenge if you can generate a list.

Note: your last example may simply be a case of wrong tagging (not everything that looks circular is legally a roundabout).

Edit: see my comment below DaveF answer: what is likely happening is that whatever you are using to extract OSM data is loosing precision on export, as the geometries you have in your illustrations have nothing to do with what is in the data.

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answered 25 Apr '17, 11:39

SimonPoole's gravatar image

SimonPoole ♦
accept rate: 19%

edited 25 Apr '17, 14:25

Simon, thanks for your time. I fully agree about the map-makers data preparation strategies. Implicitly you are saying that you don't no for an OSM map that has an option to show the source (not manipulated) data in some higher scale. I know this is not a place for arguing but for clarity I have to add two more comments.
The "edit" text is confusing and could be valid in one and only one case - if the OSM regular dump is wrong (essentially not a "dump"). Otherwise, the examples perfectly reflect the data from the dump with no precision loss and the image format here is irrelevant.
The "note" related to the last example is also confusing. The problem/error is more in geometry than in tagging. In the roundabout construct ( ) there are 4 poly-lines all tagged roundabout. So, there is much higher probability that the circular shape in the middle is a roundabout compared to your suggestion (in brackets). Though, there is no even a segment of that shape in the source data.

(26 Apr '17, 22:24) sanser

Hi Where are you getting your source data/images from?

This is the first roundabout in Wireframe mode in Potlatch 2. Looks pretty round to me:

Could your Jpeg creator be set to low detail?

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answered 25 Apr '17, 13:41

DaveF's gravatar image

accept rate: 6%

It would not be a JPEG artefact, if it is actually on output only it looks like (too) limited precision in the output coordinates as you can clearly see a point raster.

(25 Apr '17, 14:20) SimonPoole ♦

The data is from the OSM dump, XML format, from some weeks ago. The images are JPG format of the screen dump from a vector map/rendering, and perfectly reflect the vectors/poly-lines in 1:1 scale from the source and there is no precision loss. The objects are the same in years and that is my point. In maps they are rendered manipulated and just fine, so there is no anomaly visualisation that could trigger the local mappers attention.

(26 Apr '17, 21:03) sanser

I just loaded way 158898111 into JOSM and the nodes in the source data are arranged in a perfect circle. Whatever's causing the loss of precision is happening somewhere in your toolchain, which apparently isn't the perfect 1:1 reflection that you thought. What software are you using that displays the "drunken" roundabouts?

(27 Apr '17, 00:16) alester

@sanser Provide your source XML for this one:

(27 Apr '17, 13:08) DaveF
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question asked: 25 Apr '17, 11:03

question was seen: 1,886 times

last updated: 27 Apr '17, 13:08

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