[Taxacom] odometer georeferencing
dyanega at ucr.edu
Mon Feb 7 11:34:24 CST 2011
Fred Schueler wrote:
>Like many who antedate 1995, I've got a huge body of observations
>"georeferenced" by odometer readings along highways.
>Kari Gunson - http://www.eco-kare.com/ - has proposed to GIS these by
>listing the geographic co-ordinates of every 10m of the highways they're
>along, so I'll just be able to select reference points from web maps,
>calculate the distance from the reference points, and then assign the
>observation the co-ordinates that match these distances.
>I wonder if this method has been used by others, and if there's any
>place where such tables are archived? I haven't tried to search for such
>accounts on line, since I don't know what the method would be called.
As someone who has a full-time technician hired just to do
georeferencing, I had not heard of any such attempts at mapping
"reference points" along highways, but it sounds very promising -
right now, this sort of procedure is done manually with Google
Earth's path-tracing tool. That being said, one observation MUST be
kept in mind:
There is a difference between specimen labels that refer to highway
markers versus labels that refer to odometer readings.
A label based on highway markers *should*, in principle, explicitly
state "marker #" somewhere, and if it does not, then making the
assumption that the label refers to a marker can lead to error, given
that mile/kilometer numbers don't necessarily start at the edge of a
town (which is where most people will zero their odometer reading).
Accordingly, there can be up to a several-mile discrepancy between
the two methods; we have enough cases of known localities with
various collectors' mileage readings to be quite confident that one
*cannot* start measuring distance from the center of a town when
georeferencing. At this point, for cases without highway markers,
there really is no good substitute for a human being visualizing
where he/she would zero the odometer, using a Google Earth satellite
photo. Even the best automated lookup tools (i.e., those with error
radius, like Biogeomancer) are frequently *woefully* far off simply
because they use the center of a town as the starting point (e.g.,
just imagine what happens with automated lookup of a specimen from "1
mi E" of a city that is 5 miles in radius).
Highway markers should also, in theory, be quite stable once
georeferenced, where the edge of a town is a subjective quantity, and
subject to *wide* interpretation - especially when dealing with
legacy material 50 or more years old. At least for US material, our
experience is that the vast majority of labels use odometer readings,
and thus are more subject to error. I have little doubt that this is
correlated with the inconsistency across the US in the presence and
visibility of highway markers.
It would definitely be interesting if there were variations between
countries in how highway markers were numbered (e.g., if any of them
started from the edge of a town/city, and what happens if that edge
shifts over time).
At any rate, Gunson's proposal would create a very nice and useful
tool, certainly, but like any tool, one has to be careful to use it
for the proper job. But Eco-Kare is a commercial enterprise - would
these data be free, or for sale?
Doug Yanega Dept. of Entomology Entomology Research Museum
Univ. of California, Riverside, CA 92521-0314 skype: dyanega
phone: (951) 827-4315 (standard disclaimer: opinions are mine, not UCR's)
"There are some enterprises in which a careful disorderliness
is the true method" - Herman Melville, Moby Dick, Chap. 82
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