[mapserver-users] TIFF tiles and indexes

[Richard Marsden]

I'm making some progress with MapServer, and I now have it using the 
satellite tile that I wish to use.
What I've done so far:

The tile was originally a 24 bit JPEG, 5517 x 4889 pixels.
MapServer's inbuilt conversion to 8 bit colour was inadequate and the 
image did not have any alpha encoding (not sure if the JPEG format 
supports alpha?). The image has black vignettes which should be transparent.

Locally on my PC I converted the image to GIF with black set to 
transparent. The conversion program's TIFF mode did not support 
transparency.

That worked much better in MapServer. Transparency was detected 
properly, and 8 bit palette colour produced a perfectly acceptable 
image. I was concerned about efficiency so I looked at tiles. I used 
gdal_translate to convert the GIF into a tiled TIFF. This was followed 
by gdaladdo to produce overviews of 8x and 64x decimation.

The result still works and is probably a little bit quicker - nothing 
really obvious mind you.

The TIFF file is now 29.3Meg.  This compares to the original JPEG of 
4.2Meg and the GIF of 14.2Meg.
Okay, I would expect the JPEG to be the smallest and the TIFF to be the 
largest.
Also from what I've read, MapServer is meant to be faster with TIFFs 
because decoding is easy.

The area of greatest interest that most users will be looking at is 
actually quite small. Ie. it will be viewed at high zoom levels.
So, is it more efficient to use lots of individual TIFFs (indexed with 
gdaltindex) or just one big tiled TIFF?
My intuition would suggest the individual files are faster, but is also 
clear that there are quite a few factors and a single file might be quicker?
If I choose individual files, what is a good rule of thumb for optimum 
tile size?

Are there any good utilities for making raster sub-tiles?   Unless I 
"can't see for looking", I can't see one in the GDAL utility list?


Richard

-- 
Richard Marsden
Winwaed Software Technology LLC
http://www.winwaed.com
Tools and Add-ins for MapPoint - http://www.mapping-tools.com