Comments on: Determining GPS Circular Error Of Probability (CEP)

By: Leszek Pawlowicz

Leszek Pawlowicz — Sun, 16 Aug 2009 02:42:31 +0000

Tried it again with the Garmin at a random point, with the antenna vertical:

50% = 0.49
90% = 0.79
95% = 0.91
98% = 1.40

Results are better, but there was still a NW-SE axis to the data. If I get more time and two Garmins, I’ll try to compare horizontal and vertical at the same time.

By: Bill01

Bill01 — Sat, 08 Aug 2009 12:23:45 +0000

Standing the Garmin up will make quite a difference as opposed to laying it down, having considerable (daily) time with GPS60, 60CSX, Trimble GeoXH (2008 Series)& GeoExplorer3 w/wo Hurricane & Zephyr antenna setups, Magellan Triton 500 and 1500, DeLorme PN-20 & PN-40, and Ashtech (Thales/Magellan) ProMark 2 & Promark 3 units, over the last 5 years. The Garmin was meant to be used pointed upward for best acqusition (quadrifillar-helix) versus Holux glorified patch (like the Trimbles) face-up orientation for best acquisition. Try it again.

By: Nick Hopton

Nick Hopton — Thu, 30 Jul 2009 09:36:39 +0000

Remember that you’re dealing with GPS data downloaded as WGS84 by DNR Garmin (DNRG) and projected to a local coordinate reference system. DNRG uses Proj4 to do the projecting and I don’t know how accurate this is likely to be in your part of the world. In my neck of the woods, southern England, I get an error of about 1.5 metres when projecting from WGS84 to OSGB using DNRG. It might be worth checking what your conversion errors are.

Regards, N.

By: Terry

Terry — Wed, 29 Jul 2009 17:38:26 +0000

I don’t think it really make a difference if the benchmark was off. Looking back at your earlier post, I’m inclined to believe that the answer lies in the position plots. While the Garmin’s plots seem to be widely scattered, they’re widely scattered in pretty much all directions. The Holux’s points, while having a much tighter grouping, seem to concentrate themselves in three principle directions (generally, East, West and Southwest). The Holux plots are notably absent Northward and Southeastward. It just stands to reason that the Garmin would average closer to the center, regardless of where that center actually is.

By: PMarc

PMarc — Wed, 29 Jul 2009 13:30:28 +0000

I would serioulsy survey that point with a DGPS.

Not that I would doubt the benchmark position, except when known or when the BM has been placed at an obviusly bad place (I’ve seen one put over a landfill). In any case, ask the NGS as to when the BM was last resurveyed.

By: Peter Guth

Peter Guth — Wed, 29 Jul 2009 13:06:31 +0000

I would seriously consider the accuracy of the benchmark. I’ve had students do a lab with cheap old Garmin ETREX units to get the location of a tidal benchmark, and we always get differences from the NOAA position about what you’re getting. I also know that the bench mark in question was physically moved when the pier was refurbished, and cannot get any word from NOAA that they resurveyed it, and the station history does not reflect the move.

The other thing that I would look at is quantization of the positions–with our units you can only get values that are about 1 m apart, and nothing in between (it’s actually 0.01 minute, I think). I don’t know if newer units change this, or if the method affects the results–we download the tracks. For most uses the level of accuracy is good enough, and the quantization makes a good student learning point.