# Pdop Vs. Epe

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Both PDOP and EPE give an accuracy rating in different ways, but they appear to be effectively the same thing expressed differently. I ask because my Merigold only displays EPE while my HAiCOM shows PDOP.

Are there any diffences other than in the way in which they are expressed?

Edited by Xangxa

What is the acronym?

If it is nearly the same info, what is the interest in it?

What is the acronym?

PDOP is "Percent Dilution of Position" or "Position Dilution of Precision"

EPE is "Estimated Position Error".

Edited by reidster

My interest is to confirm if there are differences or if they are the same. And if there are differences, what they might be so I can make an intelligent use of their values.

EPE is Estimated Position Error in feet

PDOP is Position Dilution of Precision

I was able to find the formula EPE (2drms) = 2 * HDOP * SQRT [uRE^2 + UEE^2] so there is a definite relationship. I'm curious how URE (User Range Error) is determined. I'm not sure what 2drms stands for (maybe a unit of measure?)

I was able to find the formula EPE (2drms) = 2 * HDOP * SQRT [uRE^2 + UEE^2] so there is a definite relationship. I'm curious how URE (User Range Error) is determined. I'm not sure what 2drms stands for (maybe a unit of measure?)

The URE value is transmitted by the GPS satellites and is an estimate of the range error in meters based on uncorrected atmospheric effects, satellite clock errors, ephemeris uncertainty, and also used to include the effects of Selective Availability. I think it's typically about 4m now that SA is off.

2drms means 2-D (two dimensional) Root-Mean-Square, so it's the value you'd get if you took lots of measurements, determined how far off they were horizontally from the true position, squared these position differences, averaged them, and then took the square root of that average.

Are there any diffences other than in the way in which they are expressed?

The calculation of PDOP is based upon the geometry or position of the satellites. The more spread out and lower on the horizon they are, the lower or better the PDOP value is. Having more satellites visible also improves this value.

Calculating EPE uses PDOP, signal quality, WAAS correction, etc. PDOP calculation is pretty much standardized, where as EPE is well basically… cr*p for being used as a standard. Each manufacturer calculates it differently; even models within one brand seem to differ. I use EPE strictly and only as a reference for quality of accuracy. To base comparison of models using EPE is pure lunacy.

Trimble provides free software to calculate and graph PDOD

Trimple Planning software

That's good to know. Now I know that I'm comparing apples to oranges. I was planning on taking both GPS's into the field and was going to give more credence to the one with lower errors. But now that I know they're not comparable, I despise both of them equally

So the PDOP doesn't account for how accurate your GPS really at any moment in time? It sounds like all it does is tally up potential errors where EPE tries to quantify the error.

Now everybody knows where I got my "handle"

From Error Measures: on GPSInformation.net

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

I know my old GPS12 had a secret menu which displayed DOP so it's not that recreational GPSr's can't caulculate it but that the manufactureres choose dumb it down and use EPE which can be biased to suit their marketing needs.

Edited by PDOP's

PDOP,

Thanks!!! You posted some great information. Now I know why my iFinder's EPE is usually twice that of a Legend in side-by-side comparisons. It helps confirm my notion that statements using EPE to compare models are useless.

reidster.

Edited by reidster

Wow, this is really good info.

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

Anyone have inside information for a Magellan Meridian Gold like this? It doesn't have to be accurate, but a rough calculation. E.g., multiply EPE by X to convert lie to truth

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

Anyone have inside information for a Magellan Meridian Gold like this? It doesn't have to be accurate

Unfortunately the quoted statement doesn't come close to being true for Garmin or any other GPS models. It's derived from the earlier statement that the EPE represents the 50% CEP and then making the further assumption that the error distribution is Gaussian. A Gaussian distribution has a rapid decrease in the probability in the tails where it falls off exponentially. So if the one-sigma error is say 5m we expect to see 68% of results within that, 95% within two sigma (10m), and over 99.5% within three sigma (15m). So with a Gaussian distribution you can be essentially certain that you'll never see errors that are more than about four times sigma.

That's the type of distribution you get if there are many independent sources that contribute to the error. As shown by various previous discussions on these forums, GPS receivers are usually quite accurate but sometimes give us results that are way out in left field (or out over the Pacific while we're actually in the midwest), and they sometimes indicate that we're moving at thousands of mph when we're just out for a stroll. Although these large errors are rare, they aren't anywhere near as rare as they would be if the errors were truly Gaussian. Errors by GPS receivers do tend to follow the shape of the central part of the Gaussian "bell curve", so it's reasonable to calculate the size of the circle needed for the one and two-sigma (68% and 95% confidence) levels of accuracy, but extrapolating much beyond that (as above to 99%) runs into problems with the statistics.

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