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
