I'm using mercator-projected overlays from a crowdsourced website where volunteers georectify images of maps by hand. Unfortunately, the slippy map that serves as the site's interface wraps and allows users to place points outside [-180, 180]/[90,-90].

I have a bunch of features with bad coordinates, like the following:

var feature = {"geometry": {"type": "Polygon", "coordinates": [[[-231.304241727798, 1682.74422719708], [825.474710540096, 1682.74422719708], [825.474710540096, -272.6456886681], [-231.304241727798, -272.6456886681], [-231.304241727798, 1682.74422719708]]]}, "type": "Feature", "properties": {"id": "7014"}}

Here's some relevant data from the WMS file:

<LatLonBoundingBox minx="-180" miny="-90" maxx="180" maxy="90" />
<BoundingBox SRS="EPSG:4326"
            minx="-180" miny="-90" maxx="180" maxy="90" />
<Layer queryable="0" opaque="0" cascaded="0">
    <LatLonBoundingBox minx="-231.304" miny="-272.646" maxx="825.475" maxy="1682.74" />
    <BoundingBox SRS="EPSG:4326"
                minx="-231.304" miny="-272.646" maxx="825.475" maxy="1682.74" />

I've tried to wrap the coordinates back into something that a map library can read, but to no avail, first using this:

lng = ((x + 180) % 360) - 180), lat = ((y + 90) % 180) - 90)

then after figuring out problems with mod in Javascript:

lng = ((((x + 180) % 360) + 360) % 360) - 180
lat = ((((y + 90) % 180) + 180) % 180) - 90

but I still get incorrect coordinates. How do I convert these out-of-bounds mercator coordinates back to something that I can put on a map?

1 Answer 1


It looks like that feature geometry is corrupted, as the coordinates are just the min/max bounding box of the SRID as set by the WMS. (Plus or minus a few significant digits)

The problem seems to stem from EPSG:900913 being defined incorrectly. 900913 is no longer in use, it has since been changed to 3857. The bounding box should be +/- 20 million X and +/- 10 million Y. Note that the mercator XY extent is different than the lat/long extent.

  • Sorry, I didn't make this clear-- I generated that GeoJSON feature from a KML file containing the metadata / bounding box for an image. That's when I noticed that the coordinates were incorrect and discovered a lot of other georectified images with similar problems.
    – mike
    Apr 1, 2015 at 0:19
  • Do you have an example of bad coordinates and what they should be? That might help to determine what's going on.
    – Mintx
    Apr 1, 2015 at 21:53
  • Pastebin. The problem is that the slippy map wraps latitude and longitude into a never-ending map and users are scrolling outside of normal lat/long boundaries to place points. It's a bug in the mapping library, and I'm not able to figure out the math to correct these coordinates after the fact.
    – mike
    Apr 3, 2015 at 16:45

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