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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | Re: GISList: info on projections needed |
| Date: |
11/22/2002 07:09:50 PM |
| From: |
Robert Heitzman |
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>Indeed, we are making the satellite mosaic of the Earth, and would like to
>implement said projections. Can someone in the list help me?
One of the primary purposes of projections is to allow for more accurate
measurements (distance or area) in an area of interest - usually a Country
possible split into zones. Perfecting the projection for local use makes it
unusable on a global scale. So local projections should not really be of
concern for a global mosaic.
I think you will find most other services that do this sort of thing use
geographic (lat/long) projection - aka unprojected data. There is still a
lot of processing effort that goes into processing the raw images into an
"unprojected" format. when the whole data set is viewed there is sever
distortion at the poles.
There are a variety of projections that split up the global data, each with
there own tradeoffs. However, none of these are used as local projections
(I'm sure there are exceptions - like the artic regions).
The case where you may want to use local projections is if you are serving
up very small pieces of the global image and you want to do the user a favor
by letting them choose the projection for export. Unless you limit the
choices before hand you are looking at reprojecting data as it is requested
- something that will make your computer vendor pretty happy! In general
most of us expect global images in Lat/Long and re-project our local layers
to work with it, or we make the effort to re-projected the image to the
local projection.
You might want to take a look at the NASA Blue Marble site.
Do a search for 'demis server' and take a look at their global map server.
They were playing with adding the Blue Marble data to the main Demis demo
server - it might be there now.
Check this out for more info:
http://exchange.manifold.net/manifold/manuals/5_userman/start_projections.htm
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