**These terms tell us the PROBABILITY that a particular measurement
(GPS Measurements in the present examples) is MORE ACCURATE than some particular
value. For instance, If you are told that your GPS position
measurement is accurate to 10 meters CEP, this means that there is
a 50% probability that your measurement lies INSIDE a circle with a radius
of 10 meters. This also means that there is a 50% probability that
your measurement lies OUTSIDE the 10 meter radius circle!**

**Should you be told that your GPS measurement is accurate to
within 25 meters (95% confidence), This means that you can be 95%
sure that your measurement is somewhere within a 25 meter error circle
and there is a 5% chance that the error is LARGER than 25 meters.**

**Relationships between some common measurement notations (from Sam
Wormley)**

**sqr(alpha) Probability Notation**
**-----------------------------------------------------------------------**
** 1.00
39.4% 1-sigma or standard ellipse**
** 1.18
50.0% Circular Error Probable
(CEP)**
** 1.414
63.2% Distance RMS (DRMS)**
** 2.00
86.5% 2 sigma ellipse**
** 2.45
95.0% 95% confidence level**
** 2.818
98.2% 2DRMS**
** 3.00
98.9% 3 sigma ellipse**
**------------------------------------------------------------------------**

**What if you want a perfectly precise GPS measurement? From
an engineering standpoint, there is no such thing. There is
always a "probability of error" in -any- measurement. With present
GPS technology and expensive survey grade GPS instruments and by post processing
field measurements, it is possible to achieve SUB-CENTIMETER measurement
accuracy. This kind of accuracy is not possible with "inexpensive
consumer GPS receivers".**

**You can find information about the achievable position and altitude
accuracy of a "common consumer GPS receiver" with and without DGPS now
that SA is turned OFF with reference to David Wilson's article CLICK
HERE.**

**DEFINITELY NOT!! EPE is generally an ESTIMATE OF POSITION ERROR
and not a GUARANTEE of maximum position error. In fact, Garmin's
EPE readout is generally accepted to indicate that there is an EQUAL probability
that the error is GREATER or LESS THAN the indicated EPE. This is
the 50% CEP value given above. As shown above, to be 95% confident
that your measurement is within a circle of a fixed radius, you would
have to multiply Garmin's EPE value by two. To be 98.9% sure that
your measurement is within a circle of a fixed radius, you would
have to multiply Garmin's EPE value by about 2.55. Magellan's
EPE numbers appear to be even more optimistic (maybe the 1 sigma value
or even lower) while Lowrance seems to be someplace between the RMS
and 2 sigma values.**

**In short. Your GPS's EPE readout is just a "figure of merit".
It is NOT an indication that the given position readout is within "EPE"
feet of absolute perfection.**

**Joe Mehaffey**