[Az-Geocaching] How accurate is the GPS?

[Atherton, Bill (AZ15)]

Here is a good writeup on accuracy and how WAAS helps.  
Bill - The Watcher

http://www.kentuckyhydrografx.com/gpserror.htm
GPS Accuracy
The good news is that the GPS units everyone purchased are now accurate to
better than 10 meters 95% of the time. That's only about two boat lengths.
I don't know about you, but I can return to individual stumps, brush piles,
and points in no time if I know I am 30 feet or less from it. 

UPDATE!!!! 
Now what about the "WAAS Capable" thing I see in the ads for GPS??
All error has not been removed from the gps signals. Only the "intentional"
error by the military has been removed. 
Other sources of error include the slight deflection of the satellite
signals as they come through a portion (ionosphere) of the atmosphere. Local
satellite signal reflections (multi-path) occur around trees, buildings, and
other stuctural features. Your own body can even reduce signal strength and
quality when using a handheld gps.
The good news is that the ionospheric error can be corrected for by WAAS or
DGPS enabled receivers. If a gps unit receives the timing adjustments along
with the regular satellite signals, it can then use that information to
compute a more accurate position with less error. 
The WAAS signals are in the same frequency range of the GPS satellite
signals, which means your 12 channel receiver will use 1 of it's channels to
receive the WAAS corrections. A DGPS receiver is a separate device that has
to be purchased and hooked to your existing GPS.
WAAS is an error correction system designed for use by the aviation industry
to reduce the atmospheric error potential in gps calculations. A series of
land based stations calculate and provide timing corrections to counteract
the atmospheric signal delays. These land based receivers relay their
information to satellites in geo-synchronous orbit (that means they are in
the same place all the time relative to the earth -- unlike the moon and
unlike normal GPS satellites ). 
Land based DGPS is basically the same idea and has been around for a long
time. DGPS corrections have been available through coast guard beacons
distributed near most major rivers and lakes where barge traffic is common.
In our area, there are beacons in St. Louis, Memphis, and Louisville. DGPS
is a low frequency radio signal that requires a separate receiver that then
hooks to your gps to provide correction information. 
>From what I can tell, there is only 1 WAAS satellite that we can receive the
corrections from. That's OK, since it only takes 1 for the information we
need. The interesting thing is that the WAAS satellites are located directly
above the equator and relatively low on the horizon. 
The WAAS satellite location relative to Kentucky is at an azimuth of
approximately 130 degrees (roughly southeast) and 35 degrees above the
horizon. That means if you are fishing a reservoir with fairly tall ridges,
the WAAS signal may be unavailable since it is blocked by hills. The signal
has to be "line of sight" or you won't get it. Folks in the western US may
not benefit much at all since it will be only 10-15 degrees above the
horizon there. WAAS is still in the development stage and we will hopefully
see additional satellites come online in the future. A satellite directly
south of the US would increase the height above the horizon significantly.
We should be fortunate here in western kentucky, since we are in rather
"rolling" terrain, the hills shouldn't be too much of a problem. 
GPS accuracy with WAAS corrections is specified to be < 3 meters 99% of the
time. From some of the tests I have seen on consumer grade receivers, the
specifications are accurate.
You will literally be within "spitting distance" of a spot when < 3 meters
away.
If you have noticed, commercial GPS manufacturers are really starting to
promote "marine" receivers. This is partly because of the WAAS availability
plus they are adding moving map capabilities with lake information. They
will not likely push the WAAS thing as hard for gps units designed for land
use since the WAAS signal is more likely to be obstructed by trees or a
variety of other structures. 
Just to summarize the accuracy you might expect to get with a common gps
receiver.
GPS with no correction capability -- <10 meters 95% of the time
GPS with DGPS receiver -- 3 - 5 meters 95% of the time
GPS with WAAS corrections -- < 3 meters 99% of the time

FYI there are survey-grade receivers and equipment available that can get
accuracy in the sub-centimeter range. That kind of stuff usually sells in
the neighborhood of 40 - 50,000 dollars.