A demo of Generating Google Tiles With mapserver

[Jon Saints]

I have implemented a demo using python mapscript and
mapserver that gives very basic functionality simillar
to google maps.

I do not pre-render maps at the various scales.
mapscale are rendered on the fly and the indiviual
tile images are sent directly to the user - never
saved on disk.  I know this will cause problems for
labeling, but there may be a way around this with some
DHTML tricks.

Please remember that this demo is running off of my
desktop computer over a wifi connection. performance
may be very slow and my poor desktop might crash with
too many hits. I offer only a proof of concept and
welcome coments from the list.

Right now the demo only works in Mozilla Firefox (on
Win, Linux and OsX).  If you zoom in far enough on the
washington DC map you will see aerial photos.


Click the "START HERE" button to start the app.
http://216.15.56.185:8080/jons/indon/client/test.xul

Thanks
JOn







--- Stephen Woodbridge <woodbri at SWOODBRIDGE.COM>
wrote:
> Hi all,
>
> I posted about generating google-like tiles using
> mapserver earlier. I
> took it to the next step of writing a perl/mapscript
> script to compute
> the cellsize for each zoom scale and used that to
> compute the number of
> tiles I would need to generate to cover my map
> extents. I thought I
> would share the results with you. You can probably
> save maybe 20-30% on
> the number of tiles if you can recognize all water
> only tiles and
> eliminate them, but this leaves you with 70-80% of
> what looks like
> infinity for most of us.
>
>         Scale   Num Tiles
>    ---------- -----------
>      50000000         256
>       4000000        3072
>       1000000       40960
>        500000      150784
>        150000     1634816
>         50000    14586880
>         15000   161796096
>         10000   363410944
>          5000  1453643776
>          2700  4984920576
>    ---------- -----------
>   Total Tiles  6980188160
>   Approx. Disk Space (assume 3K per tile) =
> 21,443,138,027,520
>
> SOoooo, it requires about 7 Billion 128x128 tiles
> and assuming that the
> average tile size is about 3K (the smallest tile
> that I have seen google
> return) it requires 21 Tera bytes of disk space. It
> gets worse!
>
> If you assume you can generate 1000 tiles minute
> (16+/sec) then:
>
> 6,980,188,160 / 1000 tiles/min / 60 min/hr / 24
> hr/day / 365 day/yr =
> 13.3 yrs
>
>    1,000 tiles/min = 13.3 yr =  4847 days
>   10,000 tiles/min = 1.33 yr =   485 days
> 100,000 tiles/min = 0.13 yr =    49 days
>
> The only way to do the higher generation rates is in
> a highly
> distributed environment or you run into I/O bottle
> necks both reading
> and writing data.
>
> Oh yeah, I only used 10 zoom scales above, Google
> has 15, so you do the
> math :)
>
> An alternative to pre generating the tiles, is
> generating them on the
> fly as they are need and caching them, then removing
> the least used ones
> over time. This is a much more complicated scenario
> and requires a lot
> of additional code to manage them. Also you can not
> just generate a
> single tile as needed because of the way mapserver
> handles dynamic
> placement of labels. You would need to generate a
> "super" tile of say
> 8x8 or 16x16 tiles and chop it into the standard
> tiles. I looked at
> generating a 16x16 super tile at 2048x2048 pixels
> that would get chopped
> into 256 tiles for pre-generation, but 6x6 tiles
> (768x768 pixels) might
> be better for dynamic generation. You would have to
> find the sweet spot
> where the super tile is large enough to minimize
> labeling issues and
> small enough the it generates and chops quickly.
>
> -Steve W.
>