[Xastir] Announcing tile support for OSM

[Arnie Shore]

Folks, a bit of overlap between what I'm doing OSM-wise, so I thought I'd
intrude with an undirected ramble and share some experience.

I'm the lead programmer on a free open source computer-aided-dispatch
package that's based on GMaps, and I've started looking at what it wd take
to get off GMaps in favor of OSM.

Rationale is pretty obvious: The RACES community and others have been
beating on me (gently!) for operation sans an internet connection.  So,
since the application is browser-based  the plan is for tile storage at the
local server, with OpenLayers JS also stored locally.

Question #1 for me was re the hd storage requirements for a typical
county-sized area.  And, related, what's the zoom impact on storage needed.
Most of the users I'm aware of seem to run about four tiles on-screen.

To answer that, I picked my local AA County,  Md as a test case, wrote a PHP
script to download and store the tiles into a directory structure that OL
works naturally with, and started the download.  (I used zooms 9 through 17
as the  range, with a 1-second sleep between files.  Anyone interested in
the script let me know.)

Mebbe 60 hours later, I had me a directory with over 150k files and a
storage requirement of over 800MB.  With a fairly simple OL/PHP script and
directory structure created by the download script, I now have me a PHP demo
slippy-map application.  (I've promised myself to avoid using
'proof-of-concept' terminology, but it applies.)

A number of thoughts fall out:

1.  That 1-second sleep is a major contributor to download time, given the
surprising - to me - number of files.  (Predictable, but ... .)
2.  Zoom as tight as 17 is prbly too much of a good thing.  Prbly 14/15 cd
work OK, but I see it as a user choice.
3.  Can prbly use every other zoom between the extremes.  Mebbe worthwhile,
mebbe not.

FYI,
Arnie Shore
Annapolis, MD