[postgis-users] ST_Transform troubles

[Frans Knibbe]

Hi Mike,

Thanks for your response.

About the order of coordinates: I see that PostGIS uses EPSG as the 
authority that defines coordinate reference systems. If you look up the 
definition of EPSG:4326 (for example at http://www.epsg-registry.org/, 
use 'retrieve by code'), you can see that it explicitly says that the 
axes are latitude, longitude. So it seems the standard that is used in 
PostGIS specifies (latitude, longitude), not (longitude, latitude).

About accuracy/precision: The original coordinates in my example were 
(253328, 593188). Those values are in meters. So the measurement is 
accurate on the level of a meter. Now look at the number 
6.86264236062518 (longitude). The unit of measurement in this case is 
degrees. One degree of latitude is 1852 meters. The number 
6.86264236062518 implies that it has an accuracy of about 
0.00000000000001 degree, which is 0.00000000001852 meter. So suddenly, 
after transforming, the coordinates seem to have been measured at a 
submicroscopic level!

I agree that a high precision is a good thing, but only if it is 
combined with a high accuracy. The precision in this case is clearly far 
too large. Another point that can be made here is that the large number 
of insignificant digits in this case cause bloating of data, which is a 
nuisance if these data are serialised.

Regards,
Frans

On 2011-06-23 16:05, Mike Toews wrote:
> On 24 June 2011 01:19, Frans Knibbe<frans.knibbe at geodan.nl>  wrote:
>> POINT(6.86264236062518 53.3160795502069)
>> There are two things wrong with this result:
>> 1) The coordinates are in the wrong order (EPSG:4326 uses latitude,
>> longitude).
> They are in the correct order. Standards say "X, Y" which are "long,
> lat". This convention is commonly confused, as "lat, long" is very
> common.
>
>> 2) There are too much significant numbers in the result (the implied
>> accuracy was increased by ST_Transform).
> It's "precision" (not "accuracy") that was increased. This is
> generally a good thing, and is required to represent global positions
> within fractions of a millimeter. The "significant digits" method of
> determining precision does not work here as the actual re-projection
> calculations are not simple.
>
>> I would have expected a result like
>> POINT(53.31608 6.86264)
> You can format geometry any way you like, e.g. for reporting as
> "53.31608N 6.86264E". But if you are passing data for applications,
> keep to standard WKT and high precision if you can. The distance
> between the high-precision and 5-decimal precision is about 16.5 cm,
> which can be significant to many users.
>
> -Mike
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