[Mapserver-users] Tileindex questions.
ed at topozone.com
Fri Mar 26 17:07:18 EST 2004
No, creating seamless edgematched tiles does not take care of that
aspect. It doesn't address the issue of how MapServer determines
*which* of the 528 files needs to be used to produce a requested map.
If you're requesting an image that requires most of the source images, a
TILEINDEX won't help much. But you'll have other problems - if you
think that's going to happen (in your case you say it won't) you should
have resampled lower-resolution images available to serve those
lower-resolution requests. Otherwise you're going to be reading a
zillion pixels from disk only to throw away most of them.
President and Chief Mapmaker
From: Bob Basques [mailto:bob.basques at ci.stpaul.mn.us]
Sent: Friday, March 26, 2004 4:59 PM
To: Ed McNierney
Subject: Re: [Mapserver-users] Tileindex questions.
Ed McNierney wrote:
>I'm not quite sure what you mean by "composite layer" as opposed to
>TILEINDEX, but you should certainly use a TILEINDEX.
A composite LAYER would be a single set of data for the whole of the
LAYER coverage, vs seperate TILEs of data (500+) for the same coverage
>A TILEINDEX is a very efficient way for MapServer to determine which
>source images overlap the requested output area.
For discuassion purposes, I have already taken care of that aspect, the
tiles are seamless and edge matched in the generation process.
> In your example, any
>given map request *ignores* 97% of the files. Instead of making
>MapServer open every file to see whether it's interesting, the
>TILEINDEX will very quickly determine WHICH 16 files you need for a
>given request, without even touching the other 512.
>It is in just such cases that a TILEINDEX is very helpful. If you were
>going to end up using all 528 images in the output anyway, there
>wouldn't be much point in quickly figuring out which ones you needed.
So the closer one gets to the whole coverage are for a request, the less
effective the Tile Index will be. That's what I was looking for as a
>When a small number of tiles from a large dataset are to be retrieved,
>a TILEINDEX is the way to go.
Got it. Thanks Ed. That clears that up.
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