[Gdal-dev] A Question of Coordinate Storage

[David M. Baker]

Frank,

The info Dean presented was quite good!  My point to GIS has been just that,
it is a lot easier to maintain our data in WGS84 and reproject.  Most of our
data is reported in lat/lon in decimal degrees to 7 decimal places, -/+ a
1/10 of a foot, an precision that is much higher then the real accuracy of
the data.

Thanks for the replies, this has been a great help!

David

-----Original Message-----
From: Frank Warmerdam [mailto:warmerdam at pobox.com] 
Sent: Sunday, October 07, 2007 2:23 PM
To: David M. Baker
Cc: gdal-dev at lists.maptools.org
Subject: Re: [Gdal-dev] A Question of Coordinate Storage

David M. Baker wrote:
> 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.  

David,

As Dean pointed out typical algorithms to evaluate transverse mercator will
have increasing error as you move out from the central meridian - that is
a "round trip" from WGS84 -> UTM -> WGS84 will incur some error.  But I
don't
think it is significant near the zone bounds.  In my experience you need to
be quite a ways from the central meridian before this sort of error
accumulates
significantly (perhaps 9 degrees - one whole zone over).

 > 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?

In my experience the reprojection error for point data in areas where
projections are well defined/implemented is not all that significant,
though there is always a bit.

But, it really sounds like the accuracy of your source data isn't great
anyways so some of these precision arguments are likely not very relavent.
For feature (vector/point/etc) data there is something to be said for the
simplicity of keeping everything in WGS84, and reprojecting to other
coordinate systems as needed.

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