[Gdal-dev] A Question of Coordinate Storage

[Frank Warmerdam]

David M. Baker wrote:
> The question: is it wise to store coordinates that cover such a wide 
> area in a single projected coordinate system, or would it be better to 
> store them in some geographic coordinate system, e.g., NAD83 or WGS84?  
> And if this not the best way (UTM14), why?  What are the pitfalls?  Is 
> it really more accurate, or precise to store data in a Cartesian 
> coordinates and suffer the conversion to a different projection as needed?

David,

I will make a few observations.

1) You mentioned NAD27 earlier.  All else being equal, I would be very
queasy about keeping spatial data in the NAD27 datum as the NAD27 datum is
often poorly defined, and there is some cost and error in shifting between
it and more regular datums like WGS84 since it is accomplished with approximate
grid shift files.

2) For coordinate data, like vector features, there is some difference in
precision between coordinate systems with a local origin as opposed to a
global origin.  That is, the further the coordinate system origin is from
the data, the greater the loss of precision.  Storing stuff in WGS84 lat/long
uses a global origin and so would lose a few bits of precision in double
precision storage compared to data stored in a coordinate system with a
local origin such as state plane for instance.  UTM however is really only
local in the east-west direction (relative to the central meridian of that
zone offset by the false easting).  Because UTM uses the equator as it's
Y origin precision of northing values is generally no better than working
with latitudes in degrees.

3) For raster data every resampling step results in spatial and radiometric
damage.  So it is best to keep the data in the coordinate system it was
collected in, or that it will be used in, if possible.

I don't think any of these 3 points will definatively settle the discussion,
but hopefully it will give slightly more concrete precision and accuracy
issues to consider.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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