Georreferencing of collecting localties
Roger J. Burkhalter
rjb at OU.EDU
Fri Apr 11 09:24:04 CDT 2003
Lurk mode off.....
The differences between NAD27, NAD83 or WGS84 can be several hundred meters.
It is important to plot these using the correct datum. Topo maps (for the US)
made between circa 1929 and 1985 use the NAD27 datum, some made from 1984
into the mid 1990's use both the 27 and NAD83 reference points.
Photorevisions may or may not change the datum. Topos for the US made prior
to 1927 will often use GLO markers or other surveys (railroad, etc.) for
datum. On top of this, we all have UTM coordinates.
Check your GPS (if used), it will tell you either in the manual or as a menu
choice what datum is used. USGS topos have the datum listed at the bottom
center below to countour interval. Some computer or online programs (i.e.
topozone, terraserver, mapserver, etc.) may alow you to choose the datum
used. You can then change the datum reference on these programs so GIS plots
are uniform. Typically, if given, I'll list the datum for a location but plot
with UTM coordinates.
I'll also sometimes have multiple localities for a given geographic "spot".
With fossils, most of the time you know what formation the specimen is from,
and, typically, you know where it outcrops. On older collections, Collctor A
gives a locality within a quarter section (1/4 square mile); Coll. B only
gives a section (1 sq. mile), while Coll. C gives 1/4 1/4 1/4 section and
lists the horizon; Coll. D uses a GPS and has it within 5 meters. I wasn't
their and cannot state that these items all came from the same spot, so I
list 4 localities. Point is, localities are a guide listing occurences and
are an interpretation of a point on earth. Two people using two brands of GPS
can stand with thier backs to each other and get two different readings. Try
taking your GPS to a USGS benchmark and taking readings at different times of
the day (with different satellite coverage), you'll get different readings
with the same GPS. Its all relative.
Lurk mode on....
Sam Noble Oklahoma Museum of Natural History
More information about the Taxacom