Funny that this topic came up.
I applied to a company for a job last month that deals in gps systems
for surveying. They are accurate enough to mm, that's millimeters they
can grade a 1 degree slope in a highway. They use 24 different gps
satellites (US) along with 11 GONASS satellites (Russian). No doubt
these cost a lot more than a skycaddie, but i think in gps systems you
get what you pay for.
FYI the website is http://www.topcongps.com/tech/index.html
besides who among us amateurs can hit their 7 iron within 3 meters
consistently anyhow. I know I can't.
sean weijand
On 27-Oct-05, at 9:00 PM, Ed Reeder 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
