[OSM-talk] Wrong scale in slippy map

[Lars Aronsson]

Looking at the area where I live, 
http://www.openstreetmap.org/?lat=58.407&lon=15.600&zoom=18&layers=B000FTF

these buildings are 11 metres wide and not 22 metres as the scale 
indicates. The difference is explained by the latitude 58.4 
degrees and cosine(60°) = 0.5.

Maybe one year back, I reported exactly this bug in JOSM and it 
was fixed.  Now I find it in the slippy map.  How long has it been 
there?  Is it an OpenStreetMap bug or a OpenLayers bug?

The metre was once defined as one ten-millionth of the distance 
from the equator to the north pole.  Each latitude degree (of 
which there are 90) is thus 111 km long, everywhere.  At the 
equator, each longitude degree is also 111 km, but at the latitude 
of 60° (Oslo-Stockholm-Helsinki-St. Petersburg), each longitude 
degree is only 56 km.  At each zoom level of the slippy map, the 
longitudes (meridians) run vertical at a constant pixel width, 
meaning that the scale (metres on ground to pixels on screen) 
changes as you pan north or south.  The scale is different at the 
top and bottom of the screen, very much so at the low zoom 
numbers, but insignificantly at the higher (deeper) zoom levels.


-- 
  Lars Aronsson (lars at aronsson.se)
  Aronsson Datateknik - http://aronsson.se