<HTML dir=ltr><HEAD><TITLE>Re: [OSM-talk] Oh look, a scale bar. But...</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>Since I said it is wrong, I think I should provide the way to solve it :</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>- We can use the latitude of the center of the view to compute the scale, though this will be inaccurate when we are in zooms 0 to 5 approx. Let L be the latitude of the center of the view.</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>- In zoom z each parallel is covered by 2^z tiles.</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>- The length in meters of the paralel is 40,000,000 * cos(L)</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>- So the width/height of each tile can be assumed to be:</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>40,000,000 * cos(L) / 2^z</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>In the case of Linkoping:</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>L = 58.4<BR>z = 18</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>tile w/h = 80 m.</FONT></DIV>
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<DIV>Cheers,</DIV>
<DIV>Lucas<BR></DIV>
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<DIV><FONT face=Tahoma size=2><B>De:</B> Juan Lucas Dominguez Rubio<BR><B>Enviado el:</B> jue 17/04/2008 1:37<BR><B>Para:</B> talk@openstreetmap.org<BR><B>Asunto:</B> Wrong scale bar :(<BR></FONT><BR></DIV></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>Reusing Lars Aronsson's message, we can see that the new scale bar is wrong. The buildings in this screenshot should be 32 x 11 m (attached example.jpg)</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>Cheers,<BR>Lucas</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>-------------</FONT></DIV>
<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>[...]<BR>I've never cared too much about that scale, but now I know that <BR>these apartment buildings along Syrengatan are 11 x 32 metres and <BR>not 22 x 64 metres as JOSM would lead you to believe, <BR><A href="http://www.openstreetmap.org/?lat=58.407&lon=15.600&zoom=17&layers=0BFT">http://www.openstreetmap.org/?lat=58.407&lon=15.600&zoom=17&layers=0BFT</A><BR>[...]</FONT></DIV>
<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3>-- </FONT></DIV>
<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=3><BR> Lars Aronsson (lars at aronsson.se)<BR> Aronsson Datateknik - <A href="http://aronsson.se/">http://aronsson.se</A></FONT></DIV>
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