[OSM-talk] Wrong scale in slippy map
lars at aronsson.se
Thu Apr 23 00:51:36 BST 2009
Looking at the area where I live,
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
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