[postgis-users] Rounding off coordinates

[Gerry Creager N5JXS]

<SOAPBOX>
You've identified a problem that is pervasive in the GIS community.

Entirely too many people attempt to use all the precision offered by 
their application, GPS device, or survey, without understanding the 
limits of accuracy of the data.

While I'm a strong proponent of recording spatial coordinates to the 
extents of their accuracy, I'm similarly a proponent of appropriate 
management of the error terms to not infer extended accuracy in cases 
where we have no basis.

What does your metadata say about the limits of the data?  If you're 
getting cm. horizontal accuracies from your GPS data (easily done with 
survey techniques) report the data to that degree.  If you're using a 
consumer-grade receiver, I'd be leary of reporting data past the 6-m 
accuracy limits routinely demonstrated for autonomous position.

If you're working off cadastral data derived from conventional survey 
data, the extent of accuracy should be recorded on each platte.

The list goes on.

Error management is taught in first-year chemistry and physics in the 
universities here, but is frequently lost on the students.  We need to 
be mroe diligent.

</SOAPBOX>

Sorry.  We return you to your regular programming.

Gerry
Gino Lucrezi wrote:
> Hi all!
> 
> Since all coordinates are stored as double precision floating point numbers, they carry a great number of digits.
> 
> I know it is good to store them, because they help mitigate error propagation in many cases.
> 
> However, when displaying results, it is not really useful to show decimals up into the nanometer range. I'm sure noone's GIS data is SO precise :-)
> 
> SQL has the TRUNC and ROUND functions for scalar data. Is there any similar function for geometric data?
> 
> Gino Lucrezi
> Penta Consulting Services Srl
> 
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
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