[Gdal-dev] [off topic] Cyclic projection and Error propagation

[Matt Wilkie]

Hello Listers,

I've posted these question to a variety of discussion forums, and have 
received disappointingly few responses. Please indulge me while I reach 
a little further for some learned thoughts; do you have some to share?

thanks,

-matt

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( http://yukongis.ca/bin/view/Main/CyclicProjection )

  Repeated Projection and Error Propagation

     * How much error is involved in projection?
     * How many projection cycles are "safe"?
     * Which softwares have the lowest error propagation rate?
     * Are the answers different for raster and vector data?

For example, one of our data providers' native storage format is UTM. 
However they only distribute data in geographic decimal degrees. So when 
they send data to us they project it first. Now of course when we 
receive the data the first thing we do is reproject it into Albers to 
match the rest of our data.

So before we do any analysis at all the data has been projected twice to 
three different coordinate systems. Chances are that when our upstream 
provider was compiling the data, they started with data from an existing 
source, which was likely in a different coordinate system.

We do something with the data which results in a new theme. Other people 
are interested in the data so we project to a popular coordinate system 
for distribution, which is probably decimal degrees again. The 
downstream users' first item on the agenda will likely be to project to 
match their favourite projection. And on we go.

Over the course of time this cycle will get repeated many times by many 
people with various software. At what point, if ever, should we be 
concerned?


Other places this discussion might be happening:

     * ESRI user forum on Projection Engine good info here!
     * ESRI Customer support (no response yet)
     * SAFE Software tech support (no response yet)
     * news:comp.infosystems.gis, Google Groups gateway (no response yet)
     * Proj4 mailing list (appears to have been spammed into oblivion :( )


-- 
matt wilkie
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Geographic Information,
Information Management and Technology,
Yukon Department of Environment
10 Burns Road * Whitehorse, Yukon * Y1A 4Y9
867-667-8133 Tel * 867-393-7003 Fax
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