[Tilecache] Problem with spherical mercator at one zoom level

[Bjørn Sandvik]

Hi,

These are the tile bounds calculated by TileCache when pre-generating
map tiles for zoom level 0 and 1 (world map, spherical mercator):

C:\Python25\python.exe tilecache_seed.py country_labels2 0 2 -f

###### (0, 0, 0), (1, 1, 0)

Cache miss: -20037508.34,-20037508.34,20037508.3384,20037508.3384,
Tile: x: 0, y : 0, z: 0, time: 0.177000045776
00 (000000, 000000) = (-20037508.3400 -20037508.3400 20037508.3384
20037508.3384) [0.1810s : 5.461/s] 1/1

Cache miss: 20037508.3384,-20037508.34,60112525.0168,20037508.3384,
Tile: x: 1, y: 0, z: 0, time: 0.0149998664856
00 (000001, 000000) = (20037508.3384 -20037508.3400 60112525.0168
20037508.3384) [0.0160s : 9.896/s] 2/1

Cache miss: -20037508.34,20037508.3384,20037508.3384,60112525.0168,
Tile: x: 0, y: 1, z: 0, time: 0.0150001049042
00 (000000, 000001) = (-20037508.3400 20037508.3384 20037508.3384
60112525.0168) [0.0170s : 13.447/s] 3/1

Cache miss: 20037508.3384,20037508.3384,60112525.0168,60112525.0168,
Tile: x: 1, y: 1, z: 0, time: 0.0130000114441
00 (000001, 000001) = (20037508.3384 20037508.3384 60112525.0168
60112525.0168) [0.0150s : 16.591/s] 4/1

###### (0, 0, 1), (2, 2, 1)

Cache miss: -20037508.34,-20037508.34,-0.000799998641014,-0.000799998641014,
Tile: x: 0, y: 0, z: 1, time: 0.0699999332428
01 (000000, 000000) = (-20037508.3400 -20037508.3400 -0.0008 -0.0008)
[0.0710s : 15.918/s] 1/4

Cache miss: -0.000799998641014,-20037508.34,20037508.3384,-0.000799998641014,
Tile: x: 1, y: 0, z: 1, time: 0.0680000782013
01 (000001, 000000) = (-0.0008 -20037508.3400 20037508.3384 -0.0008)
[0.0700s : 15.580/s] 2/4

Cache miss: 20037508.3384,-20037508.34,40075016.6776,-0.000799998641014,
Tile: x: 2, y: 0, z: 1, time: 0.00799989700317
01 (000002, 000000) = (20037508.3384 -20037508.3400 40075016.6776
-0.0008) [0.0090s : 17.717/s] 3/4

Cache miss: -20037508.34,-0.000799998641014,-0.000799998641014,20037508.3384,
Tile: x: 0, y: 1, z: 1, time: 0.0639998912811
01 (000000, 000001) = (-20037508.3400 -0.0008 -0.0008 20037508.3384)
[0.0660s : 17.275/s] 4/4

Cache miss: -0.000799998641014,-0.000799998641014,20037508.3384,20037508.3384,
Tile: x: 1, y: 1, z: 1, time: 0.0739998817444
01 (000001, 000001) = (-0.0008 -0.0008 20037508.3384 20037508.3384)
[0.0750s : 16.664/s] 5/4

Cache miss: 20037508.3384,-0.000799998641014,40075016.6776,20037508.3384,
Tile: x: 2, y: 1, z: 1, time: 0.00800013542175
01 (000002, 000001) = (20037508.3384 -0.0008 40075016.6776
20037508.3384) [0.0100s : 18.113/s] 6/4

Cache miss: -20037508.34,20037508.3384,-0.000799998641014,40075016.6776,
Tile: x: 0, y: 2, z: 1, time: 0.00899982452393
01 (000000, 000002) = (-20037508.3400 20037508.3384 -0.0008
40075016.6776) [0.0100s : 19.500/s] 7/4

Cache miss: -0.000799998641014,20037508.3384,20037508.3384,40075016.6776,
Tile:x: 1, y: 2, z: 1, time: 0.00799989700317
01 (000001, 000002) = (-0.0008 20037508.3384 20037508.3384
40075016.6776) [0.0100s : 20.794/s] 8/4

Cache miss: 20037508.3384,20037508.3384,40075016.6776,40075016.6776,
Tile: x: 2, y: 2, z: 1, time: 0.00999999046326
01 (000002, 000002) = (20037508.3384 20037508.3384 40075016.6776
40075016.6776) [0.0110s : 22.030/s] 9/4

----------------------------------------

Why are several of these tiles out of bounds defined by spherical
mercator (-20037508.34, -20037508.34, 20037508.34, 20037508.34). I've
made a Google Spreadsheet showing the mismatch:
http://spreadsheets.google.com/pub?key=tBo1PLS1uldW01SgSv3se8A&single=true&gid=0&output=html

[country_labels2]
type=Mapnik
mapfile=C:\EarthAtlas\maps\merc_country_labels2.xml
spherical_mercator=true

Cheers,
Bjørn

http://blog.thematicmapping.org



2010/1/16 Bjørn Sandvik <bjorn at thematicmapping.org>:
> Cross-posting from Mapnik-users:
> https://lists.berlios.de/pipermail/mapnik-users/2010-January/002824.html
>
> I'm experiencing a strange error with TileCache rendering labels with
> Mapnik and a point shapefile. For zoom level 1 only labels on the
> southern hemisphere (the two bottom tiles) are rendered - see the
> image attachment on the link above. The other zoom levels are rendered
> correctly. It works when using standard latitude/longitide projection
> (EPSG:4326), but not "spherical mercator".
>
> Is this an issue with TileCache?
>
> Cheers,
> Bjørn
>
> http://blog.thematicmapping.org
>