[mapserver-users] Image Speed Questions---Generic

[Sam Paske]

I tangled with this one when I was putting together a page just to get
familiar with mapserver
 http://www.kapurgis.com/agsmaps/t-cdrbrg/zoning_init.html ).

What you see on the page referred to above is a township in Wisconsin, USA.
There are nine orthorectified photos tiled to produce an aerial image of the
entire township. Each one is a 30 meg tif. I tried it the simple way first,
just to get it working. I just put each tif in the map file as a separate
layer. Seems to work well, but on my 450mhz mapserving machine (x86), it
slows down quite a bit as it processes 270 mb of image data. If the viewer
is on a slow modem the difference is not so noticable, but on a lan it is
significant. One thing I did notice is that as one zooms into the map, the
draw time will decrease as the server processes fewer of the images. At
least that is what appears to be happening.

The performance when zoomed way in is pretty good, so my next step was to
create a single, lower resolution image of the 9. My plan was to use this
image at greater viewing extents, and switch to the 9 greater detail images
as the user zooms in. After a bit of experimenting, I ended up with a 60mb
image (a composite of the nine) for zoomed out viewing and the nine
origionals for zoomed in viewing, making a total of ten layers in my map
file necesary.

I was pretty happy with the result, so I didn't go on to try any other
methods. However, my example only references a single township. If I were
working with an entire county (or a state) of 30mb images, I might vary my
approach.

Hope this helps,
Sam Paske
Kapur AGS

-----Original Message-----
From: owner-mapserver-users at lists.gis.umn.edu
[mailto:owner-mapserver-users at lists.gis.umn.edu]On Behalf Of
Ballard,Lowell
Sent: Thursday, July 26, 2001 12:15 PM
To: 'mapserver-users at lists.gis.umn.edu'
Subject: [mapserver-users] Image Speed Questions---Generic


Say, for example, you needed to display/serve high-res imagery (on the
Internet) for a large spatial extent (e.g., a state or region).  There would
be several ways to get this done.

For example:

1. You could use multiple images and reference each as an individual layer
(administrative nightmare).

2. You could use multiple images and get at them through an image catalog.

3. You could mosaic them into one large image (could get REALLY big
fast--probably not feasible)

4.  If pyramid layers/aux/MrSid were supported you use them to display
moderate sized mosaics or original imagery (greatly reduce storage footprint
but uncomressing hammers CPU cycles).

5. You could resample imagery at different resolutions (1m pixel resolution;
5m; 10m) and reference each collection (e.g., 5m) with a different image
catalog depending on viewing scale (i.e., viewing at county-extent use 10m
catalog; city-level use 5m; subdivision use 1m).

6.  You store them all in SDE (~2:1 compression lossless).

7. About any combination of the above (e.g., resample imagery to 1m, 5m etc
and create pyramids for those).

8. .......

I'm curious how others would accomplish/approach this task.  I can post a
summary.

Thanks,

Lowell Ballard