Hi Ed,
There is a better than 50-50 chance (68-32 chance) that you will be within
6-meters. Keep in mind that the probability that the error is exactly
1.2-meters is zero. You can only talk about the probability of it being
less (or greater) than some number. It is actually a little bit better
than that (I think, I haven't touched probability theory in awhile) because
the probability is that both errors aren't additive.
I don't think they have anything like WAAS at golf courses. The need for
that level of accuracy isn't that great (at least until the PGA starts
using it). At Poppy Ridge they came out and surveyed the course really
well, and, yes, the major selling point is what it does for the golf course
management. Because the computer knows where all the carts are on the
course it monitors pace of play and they no longer have roving marshals, so
they cut back on marshal staff (saves money to help pay for the gps). The
jury is still out on that one. They have the system set up to disable
'forward' if someone drives the cart into the rough - they can only back
out. It is working pretty well at Poppy Ridge, but they bought the same
system for Poppy Hills (the other NCGA course) and all the trees make
signal receipt worse and the carts sometimes shut down on the cart
paths. Sigh.
Later. Bedtime.
Alan
At 09:00 PM 10/27/2005 -0700, you wrote:
Alan,
Thanks! You almost answered the question I had. I wasn't clear. In my
example, points A AND B were determined by MY GPS's readings. I'm
thinking of cases where the course you play is not in a database. However,
you make the critical measurements yourself.
If I measure point A (the edge of a fairway bunker) I know it can be 3
meters off. If later in another round of golf I wonder how far it is to
point A from my current location (point B), how much will it be off? I
can see it COULD be six meters. What would it LIKELY be, I have no
idea. However, I think you do :-)
You also touched on another question I had, cart-based GPS systems. I
looked at a few sites that provide those systems and they mainly talked
about benefits to the course. However, nowhere could I find anything that
said how accurate they were. I was thinking that PERHAPS they used some
local, non-satellite signals (similar to how I think WAAS works) to make
them more accurate. Any ideas?
Thx,
/Ed
Alan Brooks wrote:
Hi Ed,
It doesn't quite mean that. If you take a whole bunch of readings and
plot the number of times you get each value you will likely get what is
called a 'normal' distribution - your standard 'bell' curve. If you
calculate the 'standard deviation' of the curve you will get values where
68% of the data is inside those values - and 32% is outside those
values. If you take two standard deviations 95% is inside those values
and 5% outside. Three standard deviations 98% is inside, 2% outside. I
believe the 3-meter accuracy on the gps is one standard deviation, so if
you take a gps measurement on a known location 68% of the time you will
get a reading within 3-meters of the known location.
This is the best accuracy you can reliably get. That accuracy can be
degraded by several factors, including a poor signal caused by
interference from trees, buildings, powerlines, etc, or a weak
receiver. That is the measurement of a single location. Measuring a
distance (as we do on a golf course) you can also have errors associated
with how accurately the course locations were measured. If the center of
the green location in the database is off by, say, 5-meters, even if the
gps measures your current location exactly you can still have a distance
measurement off by up to 5-meters.
Keep in mind that for a gps system to determine location it takes the
signal from three satellites. Not all of the satellites are always
working so sometimes the signal you receive is not optimum. With my
SkyCaddie you can see the distance improve as the hand held unit locks
onto better signals and refines the location measurement. It often takes
tens of seconds for the distance to stabilize. Often you are comparing
the distance you are measuring with the gps to markers on the golf course
- that may or may not be accurate themselves. On a course I play
regularly (Poppy Ridge) the course markers and my gps usually agree
within a few meters. On other courses I have played I have seen very
poor agreement - often in excess of 10-meters. If I trust the gps I
usually have better results.
All in all, I have been reasonably pleased with the SkyCaddie. It would
be nice to get the kind of rapid update on location we get from the cart
mounted gps units at Poppy Ridge, but hey, the SkyCaddie costs about
1/20th what the cart units cost. It'll get better.
Regards,
Alan Brooks
At 01:53 PM 10/27/2005 -0700, you wrote:
This site has some good information regarding golf GPS and virtually
anything to do with GPS
http://www.gps-practice-and-fun.com/golf-gps.html
One other piece of information, the Magellan Companion GPS and the
SkyGolf unit have an accuracy of +/- 3 meters. This would mean that the
WORST case error would be 6 meters between two GPS-located points and
the BEST case it would be right on the money. Does this imply that on
average you'd be 3 meters off? I have no idea.
/Ed
