[OSM-talk] Datums

[Andrew Rowbottom]

On 1/15/07, Andy Robinson <Andy_J_Robinson at blueyonder.co.uk> wrote:

> OSM takes latitudes and longitudes and therefore is working in WGS84,
> however I don't think it generally matters what your GPS is set to use for
> its displays, My GARMIN still dumps out files in the WGS84 lat/lon format
> regardless of what I'm doing or which datum I'm using for the display.

Just to clear things up a little.

Unfortunately lat/lon is not quite as simple you might expect. The
main reason for this is because the world is not a sphere, though
there are other smaller differences on where exactly the center of the
earth is defined. The surface of the earth is usually approximated by
an ellipsoid which then has a knock on effect on the lat/lon reported
for a point on the surface. Many countries have their own definition
of the ellipsoid that is carefully tweaked so the "surface" is a
passable fit for the surface of the earth in the specific country,
though it may be terrible in other countries (I had experience of this
using a GPS configured for Japan that seemed to give lat/long coords
about 500m away from where I was standing).

I don't know about Garmin units, but downloading using GPSBabel from
my old Magellan GPS 315 dumps out tracks in the lat/long coordinates
for the datum that was in use when the track was laid down. I've
personally verified this by tracking the same short route twice with
different datums. The two routes differed by a fairly constant
significant amount. Once I did a datum conversion on one the
difference between the two tracks was much more in line with my
expectations of GPS (20meters or so).

If you want to see this difference you can try using an online gpx plotter tool.


It's worth noting here that GPSBabel does not (or didn't) do datum
conversions, at least for my Magellan unit, it just assumes WGS84.

I'm OK myself to set the Datum on my GPS to WGS84 (and have done so),
but I'm worried (OK only slightly) that many users are using country
specific datums which will give a drift to all their tracks.

Andrew Rowbottom