Re: WAAS, waypoints and waiting
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Sun, 25 Jun 2006 22:59:28 GMT
Pieter Litchfield wrote:
"Eric" <egrumling1@xxxxxxxxxxx> wrote in message news:f8j8925sa9e4ftc75nmvortmv9rucfeivs@xxxxxxxxxx
On Thu, 15 Jun 2006 13:01:19 -0500, "Peter H. Coffin"
<hellsop@xxxxxxxxxxxxx> wrote:
On Wed, 14 Jun 2006 22:52:54 -0600, Eric wrote:
Today I marked a few waypoints for a mapping/design project I'm doing
for work. This does not require high precisition. When I was setting
waypoints I then set the GPS to navagate to the just set waypoint to
get an idea of how far I was between landmarks (conduit for utility
demarcs). Basically, I just waited for the displayed distance from
the prior waypoint to settle down before I marked the next one.
That'll be good enough for someone knowing what to look for to find a
panel box by getting to a "panel box here" waypoint.
I had WAAS turned on. I checked the Satelite page a few times and it
did show some correction data for a few satelites, but not all. I
could only get a decent lock on 4-5 sats at a time. I am using a
Garmin Etrex Vista.
How much "faith" should I place in these waypoints? Compared to a
measurment wheel (on soft dirt road), would this be considered
more/less/as reliable a measurement method (keeping in mind I didn't
have a compass with me, but I really only need distances from one
conduit to the next))
We can talk about how good GPSs are as much as you like. How good is
your measure wheel?
Would I have gotten any better readings by waiting for a minute (or
longer) before storing a waypoint?
You'd get better readings marking a waypoint every fifteen minutes for a
day or two and finding the center of the scatter. But that's probably
more work than its worth. You'll probably find all the waypoints within
a few tens of feet of each other, and thus within a few tens of feet of
whatever you're trying to mark.
Thanks for the response. After plotting known waypoints on a map, I
was happy to see them within 2 feet of the map location. It gives me a
lot of confidence (which I already had) in using a standard GPS for
rough measurements.
Has anyone done a side by side comparison of WAAS -vs- DGPS to compare
acuracy? How about off the shelf consumer/surveyor equipment? It seems
to me that from what I've read, the DGPS would still be more accurate,
but how much is enough? Yes, I know I'm opening a can of worms here,
but in a world were up until recently, deeds listed boundries as "the
corner is 50 yards from a large oak tree," something that will give
even 3 meter accuaracy should be considered good enough. Unless
there's a gold mine involved...
Actually I think WAAS is more accurate. I used to own a Garmin 12xl with a Garmin DGPS receiver mounted in a backpack. There is a DGPS ground station within 20 miles of my home. It worked well and seemed accurate. However, I don't know of a definitive experiment.
I believe DGPS is more accurate closer to the broadcast station since it deals in local conditions. The claimed accuracy was, I believe, <3 meters horizontal 95% of the time. My current WAAS equipment claims exactly the same. But I believe that WAAS is less sensitive to errors resulting from the distance to the ground station that show up in DGPS (as in the old Coast Guard of NDGPS system).
If you are navigating harbors along the coasts of the U.S. and wish to employ DGPS instead of WAAS, I'll guess that there is no significant difference in performance. If you want to wander around remote parts of the US, there may be.
Sorry for the unrelated comment--Pieter, I'm excited about your new
Celestron c6-sgt telescope. Of the newsgroups I participate in,
sci.geo.satellite-nav is by far closest to what a newsgroup ought to
be--the sharing of news, information and solutions to user problems.
Best regards to all.
-Sam
.
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