Georeferencing of collecting localities

[Fewless, Gary]

I can speak to the difference between NAD27 and NAD83 in the Great Lakes area of North America (roughly 45 N latitude).  Our experience has been that the longitude is similar for the two, but Latitude is off by about 210 meters--I can't recall which direction at the moment.

Gary Fewless


-----Original Message-----
From: Mary Barkworth [mailto:Mary at BIOLOGY.USU.EDU]
Sent: Thursday, April 10, 2003 5:40 PM
To: TAXACOM at USOBI.ORG
Subject: Re: Georeferencing of collecting localities


Out of great curiosity - and even greater ignorance - how much
difference does use of different datums create? Are we talking
centimeters, meters, kilometres? Does it vary with latitude and/or
longitude? 




-----Original Message-----
From: Doug Yanega [mailto:dyanega at POP.UCR.EDU]
Sent: Thursday, April 10, 2003 4:24 PM
To: TAXACOM at USOBI.ORG
Subject: Re: Georeferencing of collecting localities


Bob Kriegel wrote:

>What many people don't realize is that GPS instruments allow you to
switch
>among several different grid systems -- called datums.  A fixed
position on
>the ground has different longitude/latitude or UTM coordinates in
NAD27,
>NAD83 or WGS84 datums.  So, if you are recording georeferences on your
>specimen labels or in a database -- do you also document what datum the
>point was recorded in?

How does one tell? I think I could eventually figure out what datum
my Garmin is using (the default, I'm sure), but what about the
70-year old USGS or Forest Service topo maps I'm sometimes using? For
that matter, what about the websites I use to do look-ups? I don't
see anything on the USGS or NIMA websites which indicate which datum
they use. Also, isn't WGS84 the most common in present use? The
DeLorme topos we use are WGS84, and I'd suspect that the Garmin
default is, as well. If the other datums have more restricted use,
perhaps context-specific, might it not be possible to infer which
system a given record was based on, at least in terms of probability?
--

Doug Yanega        Dept. of Entomology         Entomology Research
Museum
Univ. of California - Riverside, Riverside, CA 92521
phone: (909) 787-4315 (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