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Wed, 14 Apr 2004

Terraserver: Tiles Vs OpenGIS Web Map Server

Starting with TerraServer tile:

  • X=204
  • Y=2932
  • Z=19N
  • S=13 (8m)
Using this python snippet:
TILE_PX = 200
def terra2utm(x, y, z, s):
    x = x * TILE_PX * math.pow(2, s-10)
    y = y * TILE_PX * math.pow(2, s-10)
    return x, y
The extent of the tile can be computed:
uL = terra2utm(204, 2933, "19N", 13)
lR = terra2utm(205, 2932, "19N", 13)
Which gives:
  • uL = 326400, 4692800
  • lR = 328000, 4691200
That then converts to (in WGS-84):
  • uL = 42.36810, -71.10832
  • lR = 42.35405, -71.08843
Now we can compare the tile to the WMS version:
TileWMS

Close, but a bit distorted. Bringing the WMS image up in the Terra-Map Server web interface shows that we ended up with the wrong resolution. (Width Resolution (m/px): 8.3850 Height Resolution (m/px): 7.5952)

Where did I go wrong?

 







Last modified: Fri, Mar 20 02:18:23 2009 GMT