[Gdal-dev] A Question of Coordinate Storage

[David M. Baker]

Frank,

Thanks for the quick reply.

We are not proposing to store the data in NAD27; it makes me uneasy too, but
a majority of our data vendors report coordinates in this system. I would
opt for WGS84. The fact of the matter is that for most of the data we deal
with we do not know provenance of the original coordinates, so my guess is
we are well within the decimal precision of any system we may use.

I see your point about UTM only be local east-west.  I had not considered
that!  Accept for the 2D and 3D seismic data that we collect (the bins of
which are surveyed in x,y's on the ground) most of our data has been
converted from one system to another before it is delivered (for the most
part NAD27).  Example: A well location is reported in NAD27 that would fall
in the in the very west extreme of UTM13 is converted to UTM14 for storage.
Would there not be significant error in the location in stored.  Also, if we
had a 3D survey that was surveyed in local coordinates of UTM13, would there
not be a difference?  Also, is more error introduced if a geologist then
re-projects this same well to a state plain system?  Would we have been
better off storing in WGS84?

Thanks,
David



-----Original Message-----
From: Frank Warmerdam [mailto:warmerdam at pobox.com] 
Sent: Sunday, October 07, 2007 11:55 AM
To: David M. Baker
Cc: gdal-dev at lists.maptools.org
Subject: Re: [Gdal-dev] A Question of Coordinate Storage

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
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org