[OSM-talk] Need Help Bulk Uploading some POIs

[David Earl]

On 12/12/2007 15:23, Bone Killian wrote:
> David Earl wrote:
>>
>> Are you sure the data has the correct datum for the lat/lon values? A 
>> different datum could account for offsets like this.
>>
>> David
>>
> Pardon my newbishness, but what's a datum?
> 
> I know the file in question was described as being intended for import 
> into Garmin / Magellan GPS units.  Could they be using a different datum 
> than we are?

It is possible, though I'd be a bit surprised.

Lat/lon numbers have no intrinsic meaning. They are just the spherical 
coordinates of a point on an ellipse that is used to approximate the 
Earth. There are many possible choices of ellipse (including both its 
radii - ie. approximation of sea level - and its centre) and it is this 
ellipse that is the datum. The reason for using different datums is that 
they can give a better local approximation to the true surface in some 
particular region. (Actually, I suppose there's nothing that says it has 
to be an ellipse either, but in practice they all are).

For example, the widely used worldwide approximation is called WGS84 
(which is an almost spherical ellipse, squished at the poles), and I'd 
expect your info to be expressed in this. gpx files are supposed to be 
by definition (but there's nothing to stop someone failing to realise 
this and putting lat/lon info in a gpx file which has some other datum). 
There are then European and national ones this side of the Atlantic, 
e.g. OSGB36 While the ones used by the UK's Ordnance Survey give a good 
approximation here, the other side of the ellipse is buried half way to 
the centre of the Earth over the other side.

http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/index.html
is an excellent paper on the subject, though inevitably anglocentric.

David