[MS4W-Users] Strategies for dealing with large area Mr SID files

[Jeff McKenna]

On 10-10-04 5:16 PM, Bill Harbour wrote:
> I have eight Mr SID images that cover my County (1,500 square miles). I
> used gdaltindex to build a tile index of the Mr SID’s and created a
> layer for that tile index in the map file. t takes mapserver about 10 or
> more seconds to display the raster. Do any of you guys have
> suggestions-strategies for improving the display times of the images??
>

Hi Bill,

I agree that 10 or more seconds to display your images in a mapfile is 
unacceptable.  (by the way, for testing mapfile speeds I always 
recommend using the shp2img utility, with the "-all_debug 5" switch, as 
noted at: 
http://www.mapserver.org/optimization/debugging.html#step-5-test-your-mapfile)

What I have done in the past, when using a large area of images, is to 
use a generalized image of the area when zoomed out (in GeoTIFF format), 
and then when users zoom in switch to the actual tileindexed MrSIDs.  Is 
that a possibility for you?

Also, do your MrSID files have overviews embedded in them?  I believe 
GDAL should be able to use these overviews for faster drawing.

Of course, for speed in MapServer, I must tell you that the optimal 
configuration is to have all of your rasters in GeoTIFF format (possibly 
converted with gdal_translate http://www.gdal.org/gdal_translate.html), 
with overviews for each GeoTIFF generated by gdaladdo 
(http://www.gdal.org/gdaladdo.html), and then indexed with the 
gdaltindex utility.  But I understand the GeoTIFFs might not be an 
option for you.

Hopefully one of the points above is useful for your situation.

-jeff


-- 
Jeff McKenna
MapServer Consulting and Training Services
http://www.gatewaygeomatics.com/