Error Measures:
1 sigma,  2 sigma,  3 sigma,  50% CEP, 2DRMS, 95% confidence >  What does this all mean?

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.

But then:  What does EPE mean on my GPS?  If EPE reads 10 feet,  then I am within 10 feet of the actual  lon/lat position readout on the GPS- Right???

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