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<h1><a name="Repeated_Projection_and_Error_Pr"> </a> <a name="Repeated_Projection_and_Error_Pr"> Repeated Projection and Error Propagation </a></h1>
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<li> How much error is involved in projection?
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<li> How many projection cycles are "safe"?
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<li> Which softwares have the lowest error propagation rate?
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<li> Are the answers different for raster and vector data?
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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.
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So before we do any analysis at all the data has been projected twice to three different coordinate systems. Chances are that when <strong>our</strong> upstream provider was compiling the data, they started with data from an existing source, which was likely in a different coordinate system.
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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.
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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?
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<em>I posted this message to a variety of discussion forums. The results will be summarised and posted here. Should you wish to throw your 2 bits in, you can do so in any of those forums, or here on this page (see the 'Edit' link at the bottom of the page). -- <a href="/bin/view/Main/MattWilkie">MattWilkie</a> - 19 Mar 2004</em>
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<em>Other places this discussion might be happening:</em>
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<li> <a href="news:comp.infosystems.gis" target="_top">news:comp.infosystems.gis</a>, <a href="http://tinyurl.com/2lcbx" target="_top">Google Groups gateway</a>
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<li> ESRI user forum on <a href="http://forums.esri.com/Thread.asp?c=93&f=984&t=120601" target="_top">Projection Engine</a>
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<li> ESRI Customer support (no response yet)
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<li> SAFE Software tech support (no response yet)
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<li> <a href="http://remotesensing.org/proj/" target="_top">Proj4</a> mailing list (appears to have been spammed into oblivion)
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</ul>
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