Wild Sanctuary wrote:
>
> Yet another problem: On a recent soundscape data gathering project
> for the National Park Service, we took GPS readings at each site we
> recorded with two seperate units. We could never get them to provide
> exactly the same reading given the built in distortion for most
> commonly available units. Sometimes they deviated by as much as 100
> meters or more and altitude measurements were off by hundreds of
> feet. As a matter of fact, each time we took readings, the same
> device would give a different outputs. Since we knew the altitude and
> precise coordinates (because the sites had been previously mapped) we
> were able to note the metadata with some accuracy.
>
> Unless one is using something like a calibrated Trimble (about
> $15,000USD), accurate GPS readings, forest or no, will be unlikely
> until the distortion chips are removed.
We are well and truly spoiled. Naturalists of not very far back would
laugh at you. And line up to buy any GPS they could get.
President Clinton had all that removed, you are way behind. You get full
accuracy now. There were never distortion chips, it was done by the
settings of the system. It was almost like flipping a switch on the
system for the fix.
Even 100 meters is far far better than the alternatives available to
most and hugely better than the average museum record. Believe me, I've
read a large number of museum records. If you insist on using more than
one unit, average the results. In fact, you will get far better results
if you use the built in averaging of a GPS. Nearly all have the facility
to average their readings over time. Plain handheld Garmins can get
within less than 10 meter error with 5 or 10 minutes averaging. And that
was before the removal. I checked the 12XL against survey benchmarks
numerous times to find that out. GPS that also pick up WAAS are
available now for a relatively cheap price. Which will provide you a
more accurate instant reading. Since the removal, the manufacturers are
improving their GPS units in minor ways that always help.
But you have to realize that any method has a uncertainty. Note that the
fact that the sites were previously mapped is no indication you have a
accurate location. Maps are frequently off, even full blown surveys make
errors. GPS is probably more accurate than most maps, and in some cases
that includes parts of USGS topos. Every method of doing this has
errors. But, if I have to find the spot, and I get only one choice of
info, I'll choose GPS. Preferably with a few notes about the locality.
GPS, like all the rest of our equipment must be used with some knowledge
of it's operation. A little common sense goes a long way too. You don't
need any $15,000 piece of equipment to do it. Most scientists using such
equipment don't even need it. And, yes, if you have a more accurate
survey of the point, by all means put that down. Let's see, just how
often does that occur? That can be depended on probably only right next
to a benchmark.
So use the GPS and quit whining. What you going to do instead, things
like car odometer readings? Or maybe so many miles NNW of some town?
Estimated off a highway map? Or even a Topo? GPS is the most reliable
method we have short of getting out the survey crew. And few of us can
afford that, especially as the animals move around.
As for under trees, it is a problem. As has been mentioned, a external
antenna helps. And the newer models are better in this regard than older
ones. I'm sure you can find situations where GPS won't work, recording
bats in caves comes to mind. But for most they will work even if you
have to take a reading at a nearby point and estimate the final distance
that's still better than alternatives.
Walt
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