[Mapserver-users] Large Raster files

[Norman Vine]

Jan Hartmann writes:
> 
> Martin Weinelt wrote:
> 
> Yes, both preprocessing tasks (projecting and converting RGB to indexed 
> colors) are almost mandatory for production servers

:-)
 
> > Everything is done by GDAL and the utilities. I decided to have tiles about 
> > the size of the map, without knowing exactly why this would be better then 
> > 2000px (Jan). 
> 
> Nothing holy about the number. It's just that multiplying and dividing 
> by 2 is so easy with a computer :-). I chose 2000, because a single 
> request should be as much confined to one tile, but it should not have 
> to resample the image too much. Most map images are about 500 by 500 
> pixels, so there is a good chance that it is completely within one tile.

I suggest keeping the base tiles a power of 2 ( i.e. 2048 is close to 2k )
However if you are serving individual maps that you just use one 'overviewed' 
tiled geotiff image.  

Of course if you are making seamless maps of large areas then there is always 
a practical limit on file sizes that are 'easily' handled.  

Note I use 'base' images of 100's of megabytes routinely and am very
happy with response times from overviewed and tiled geotiffs with mapserver
 
> > I also think that compressing the rasters is even contraproductive, because 
> > 'in memory' there is no compression and you add an extra job (decoding) to 
> > the application.
> 
> That's what I thought too, but serving compressed TIF files is not much 
> faster than uncompressed ones, at least in my experience. Some of my 
> rasters have a very high rate of compression (5 to 20 times), so I would 
> run into disk problems with this procedure. Reason is (I guess) that 
> regions can be extracted from compressed tifs without decompressing the 
> whole file, so the actual time lost to decompressing the region is 
> marginal.

FWIW - I have found that tile based compression is almost always a win as
IO and not processor time is usually the bottleneck

Is anyone familiar with this application ?
http://www.zoomify.com/default.asp

Cheers

Norman