> At this photo, the background doesn't fit at all (due to projection) > > http://picasaweb.google.com/lh/photo/GuC2cacBVQVfFD13LGLmYaPDFtHHugbL4UR4b9mel2I?feat=directlink >
Mmm... That shouldn't happen. What steps did you take to have this image? > > Here it does fit, but the buildings aren't perpendicular > > http://picasaweb.google.com/lh/photo/NnAsPY4cGoSRe9uKt_fWTKPDFtHHugbL4UR4b9mel2I?feat=directlink > > Are they then "normal" perpendicular in the osm map, if I draw them like > these buildings in the second photo? > You don't have a clear idea of what a projection is, do you? I suggest you have a look at the wikipedia article ( http://en.wikipedia.org/wiki/Map_projection). To summarize: There is no "normal" projection. The osm slippy map uses a specific projection called "mercator" but there are hundreds more. Each of these projection serve a specific purpose for a specific place, mostly having proper length measuring both in longitude and latitude (think of a photo right over the top of your house). In the specific mercator projection, the measurements are only perfectly right at the equator. At every other latitude, the image is distorted so that a North-South length might seem up to infinitely longer than it really is (the greater absolute value of latitude, the more distortion). That's why on most paper world maps (which are also in mercator projection), Antartica is completely out-of-proportion (and, by the way, the poles cannot be shown, as the min/max latitude are ~-85°/85° in mercator). Each country has its "own" projections as to properly represent geographical features on its territory. As an example, France currently has 9. So, to see perfectly "square" buildings in this specific part of Italy, you'd have to use the specific projection designed by the geographical institutions of Italy for this specific region. OSM is only recording the lat/lon of the nodes. So anyone needing to see square buildings there would need to take the osm data and paint the map using this specific projection. Merkaartor can use any projection. So you'd just have to find this specific projection, enter it in the projection editor and voilà, the osm data will be drawn "square". This specific WMS only providing EPSG:4326 projection (= lat/lon), you probably won't be able to use the background image reliably, though, as Merkaartor is only able to reproject images in "square" projections like mercator and EPSG:4326, probably not the Italian one which is probably conic. To summarize the summary: Even if the nodes are perfectly placed for a square building, they won't appear square on the osm map because of the mercator projection. Regards - Chris -
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