[Taxacom] Impossible geocodes

mesibov at southcom.com.au mesibov at southcom.com.au
Fri Jan 30 03:43:49 CST 2015


Alex, I agree with you that DD is a more useful lat/lon format, although I
don't agree with you that DMS is weird, there's a good reason for it. And
DMS is a little bit better than DD for implying an uncertainty.

If I write a latitude in the form 43d 16m 23s S, you might guess the
latitude is known to the nearest second, or roughly +/- 11 m at that
latitude. If I write 43d 16m S, you might guess that the latitude is only
known to the nearest minute, or roughly +/- 670 m.

In DD, the two latitudes to 4 decimal places are

-43.2731
-43.2667

in both cases implying an uncertainty of +/- 4 m. You can't tell from
looking at these DDs in uniform format (say, to 4 decimal places) that one
had a much bigger uncertainty than the other, although you might if the
number of places given reflected the uncertainty, say -43.27 for the
second one (roughly +/- 400 m at that latitude).

By an interesting coincidence, the implied uncertainty in
DMS-to-the-nearest-second is about the same as the horizontal uncertainty
in a handheld GPS reading (unless conditions are exceptionally good). The
DD-to-4-places uncertainty is optimistic for handheld GPS, and the
DD-to-5-places you often see in published papers is +/- half a metre and
evidences a religious faith in numbers. DMS is definitely handy for
GPS-determined locations.

I would back DD 100% if it was always accompanied by a specified
uncertainty, like the Darwin Core one: 'The horizontal distance (in
meters) from the given decimalLatitude and decimalLongitude describing the
smallest circle containing the whole of the location'. Unfortunately,
that's not the case. Very few of the museum collection databases I've
looked at even have an uncertainty field for location data, and if they do
it's largely unfilled.

Until collectors and data compilers and georeferencers routinely add an
uncertainty to their lat/lons, I prefer to see 43d 16m 23s S and 43d 16m
S, because I know for sure there's a difference.

(Sending this from my old email as Taxacom doesn't seem to like my new
one, robert.mesibov at gmail.com)




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