<HTML dir=ltr><HEAD><TITLE>[OSM-talk] Wrong scale in slippy map</TITLE>
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<P>> Looking at the area where I live,<BR>> <A href="http://www.openstreetmap.org/?lat=58.407&lon=15.600&zoom=18&layers=B000FTF">http://www.openstreetmap.org/?lat=58.407&lon=15.600&zoom=18&layers=B000FTF</A><BR>> <BR>> these buildings are 11 metres wide and not 22 metres as the scale<BR>> indicates. The difference is explained by the latitude 58.4<BR>> degrees and cosine(60°) = 0.5.<BR>> <BR>> Maybe one year back, I reported exactly this bug in JOSM and it<BR>> was fixed. Now I find it in the slippy map. How long has it been<BR>> there? Is it an OpenStreetMap bug or a OpenLayers bug?<BR>> <BR>> The metre was once defined as one ten-millionth of the distance<BR>> from the equator to the north pole. Each latitude degree (of<BR>> which there are 90) is thus 111 km long, everywhere. At the<BR>> equator, each longitude degree is also 111 km, but at the latitude<BR>> of 60° (Oslo-Stockholm-Helsinki-St. Petersburg), each longitude<BR>> degree is only 56 km. At each zoom level of the slippy map, the<BR>> longitudes (meridians) run vertical at a constant pixel width,<BR>> meaning that the scale (metres on ground to pixels on screen)<BR>> changes as you pan north or south. The scale is different at the<BR>> top and bottom of the screen, very much so at the low zoom<BR>> numbers, but insignificantly at the higher (deeper) zoom levels.<BR>> <BR>> <BR>> --<BR>> Lars Aronsson (<A href="mailto:lars@aronsson.se">lars@aronsson.se</A>)<BR>> Aronsson Datateknik - <A href="http://aronsson.se">http://aronsson.se</A><BR>> </P>
<P>======================<BR>Hello. I have changed my mind lately regarding this issue. I think there are two ways to approach this, and both are legitimate:<BR><BR>- Strictly: every coordinate system has has pros and cons when drawing a world map. If you have a map in EPSG:4326, Greenland is awfully stretched, etc, but that's the way it is using that ref. system. If you use the British national grid for all of Europe, many countries will look ridiculous, but, whether we like it or not, those are the coordinates of those countries in that projection. That is happening in Scandinavia with Spherical Mercator: Sweden is ridiculously large, but sorry, that's the way it is using that projection, so the scale bar is right. The problem is that the projection is not good for that part of the world.<BR><BR>- Using common sense: As you say, the top and bottom of the screen have about the same scale if you zoom in a bit, so it is possible to forget about the coordinates provided by the projection and create a locally sensible scale bar.<BR><BR>I have seen the first option used on OSM's front page and the second option in Cloudmade's website.<BR><BR>Regards,<BR>Juan Lucas</P></BODY></HTML>