[Tilecache] threshold

[Brent Pedersen]

hi, first thanks to the developers for tilecache--and the help on irc.
i was having some problems with tiles not showing up. at first i
thought it was because of the metaTiling going out of the designated
extents. that was not the case.

i was able to fix this by digging into Layer.py and setting
exact=False in Layer.getCell().
however, i think the real problem is with threshold.
it is set to .001 which seems fine for lat/lon, but for a scale in
meters, .001 is a millimeter (i think)...

anyhoo, a quick fix is to set threshold in __init__ as something like:

self.threshold = math.abs(bbox[2]/10e4)
or
(bbox[2] - bbox[0])/10e4

havent done enough testing to be sure this is the case, but it seems
to have fixed the problems i was seeing.... and it makes sense.

is there a trac for tilecache?
-brent