[mapserver-users] How to improve raster imagery display ?

[Rainer Kalliany]

Dear all,

Yet for some months, my wife (who is the far more experienced programmer,
and me (rather responsible for collecting/re-formatting/providing data)
are developing a MapServer-based webGIS. Your community already has
helped us several times a lot with your experience, and we are most
grateful for that great support.

Now, with our project almost finished, the so far pretty poor performance
in displaying raster images (aerial orthophotos at a ground resolution of
0.25 metres) is becoming a critical issue. In particular when several
users want to work at the same time, what happened last week :-(

So far we had our orthophotos organized in a seamless grid of ECW-images,
10000 by 10000 pixel each (corresponding to 2500 by 2500 metres on ground).

 From that dataset our MapServer produces the "orthophoto" layer on demand,
at scales of 1 by 1000, 2000, 5000 or 10000. With the standard settings
of 72 dpI, that renders pixels of 0.353, 0.706, 1.764 or 3.528 metres.

I am well aware that at least for the 10000 scale I should introduce
a layer with reduced resolution. I already have prepared such a dataset.

But our main concern are the scales 2000 and 1000, where the displaying
process is producing many dozens of tiles, each 256 x 256 pixel .png-files.
Along with the other layers (where the vector-data also are displayed by
rasterized tiles which at least are much smaller), the composition of one
screen sometimes takes up to 3 minutes !

NOW MY IDEA is to produce all those .PNGs (with their weird directory
structure and naming, like \t-86016\l3072\t-84992l4352.png, which how-
ever I already have understood) in advance and myself. For the conversion
from ECW to PNG I might use GDAL, but without any resampling necessary.

I also consider raising tilewidth/tileheight from the standard value 256
to 512 or even 1024 pixels.

Now my question is, if that is a good idea or not.

If you disagree with me: Do you have some alternative proposal ?

If you think my planned approach basically is a good one, do you have
any supporting advices, on issues I possibly did not consider ?

THis is becoming a dcisive isuue for the success of our whole project,
and therefore we will be most grateful for any hint !

With kind regards,

Rainer & Susanne Kalliany

2008-09-13, 23:25 UTC
-- 
DI. Rainer Kalliany - rainer at kalliany.at

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