Gecache location coordinates - Improve eTrex position reporting accuracy

[Guest bobhogan]

orpscon" distributed for free by the Corps of Engineers (http://crunch.tec.army.mil/software/corpscon/corpscon.html) that converts from Lat/Long to, among other things, state plane (X,Y) coordinates.

 

I entered the Lat/Long for each of my waypoint readings and had the software generate the corresponding X,Y of the point. Now, rather than degrees, I had units of feet to look at. And here is what I found:

 

Depending on the latitude and longitude of your geocache you will have different values, but for our area in the northeastern part of the USA, a change of 0.001 of a minute in latitude results in a positional shift of about six (6) feet in the Y direction and a change of 0.001 of a minute in longitude results in a positional shift of about five (5) feet in the X direction. Not too bad and it narrows finding the geocache down quite well.

 

I started pondering the internal accuracy of the eTrex. Given that simple calculators store many decimal places internally and are only limited to the result they can display by the dimension of their output screen, I believed a higher accuracy could be retrieved from my eTrex.

 

I downloaded the freeware program "GPS Utility" (http://www.gpsu.co.uk/download.html) and used it to transfer all of the waypoints stored in the eTrex to my PC. They all transferred with more decimal places (higher accuracy) than had been reported on my eTrex display, just like a simple calculator.

 

I then took the same points listed above and evaluated them with the higher accuracy data observed through the GPS Utility software and found that at the most, I was varying by no more than 0.0004 of a minute in lat or long which translates to positional accuracies of 1 to 3 feet as opposed to 5 or 6 feet.

 

The bottom line of all this is: if you are placing a geocache and have the ability to improve your statistical average position by downloading the more accurate data, take the extra time and do so. Those seeking your geocache will appreciate it.

 

If you've read all the way to here, thanks for your attention and good geocaching!

 

-Bob

[Guest jeremy]

First, thanks for your extremely informative note. I always wondered what a variations in coordinates affect the actual placement in meters/feet.

 

So my question is, what format on the eTrex would be the most exact, without actually downloading it to your PC? The cord can get a bit pricey so for those without one I'd like to make sure I request the best possible coordinates you can read on the unit.

 

I've done many different recalculations when I don't get the coordinates in the format we use on the site, so I'm open to any form of LONG/LAT coordinates.

 

Jeremy

[Guest Bob Hallett]

Bob,

This is a good point. At my house in Ayer, 0.001 minutes in longitude translates to 4.5 ft. and in latitude to 6 feet. If you consider both together, you would move in a 45 deg direction. This equates to 7.5 ft. If this error is introduced by both the stasher and the stashee, it could mean 15 ft. At least the stasher could remove some of it.

 

Bob Hallett

GIS Specialist

[Guest Mike_Teague]

The most accurate format for most receivers is UTM... Its got 1 meter of resolution, even on the cheap GPS's...

 

certainly better than the system itself can acheive..

[Guest Bob Hallett]

Mike,

I'm not sure of the level of knowledge our readers have of coordinate systems, so could you explain this a little more.

 

Bob H.

[Guest Mike_Teague]

UTM divides the world up into 60 6 degree zones of longitude... the coordinates are simply how many meters to the east or or west of the "central meridian" of that zone, and how many meters north or south of the equator you are..

 

IE. 10 T 0525219 5056970

 

I am in zone 10 T , 25219 meters east ( of the zone 10 central meridian, 5056970 meters north of the equator..

 

try this http://joe.mehaffey.com/utm-faq.txt

 

or

http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.html

 

should explain it a bit better

 

Also, none of this means that UTM is going to make your GPS more accurate.. It's still gonna give the same accuracy... The resolution of UTM is basically too good for un-augmented GPS..

 

[This message has been edited by Mike_Teague (edited 12-21-2000).]

[Guest Mike_Teague]

UTM divides the world up into 60 6 degree zones of longitude... the coordinates are simply how many meters to the east or or west of the "central meridian" of that zone, and how many meters north or south of the equator you are..

 

IE. 10 T 0525219 5056970

 

I am in zone 10 T , 25219 meters east ( of the zone 10 central meridian, 5056970 meters north of the equator..

 

try this http://joe.mehaffey.com/utm-faq.txt

 

or

http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.html

 

should explain it a bit better

 

Also, none of this means that UTM is going to make your GPS more accurate.. It's still gonna give the same accuracy... The resolution of UTM is basically too good for un-augmented GPS..

 

[This message has been edited by Mike_Teague (edited 12-21-2000).]

[Guest Betty]

lculated as 6.074 (ft / 0.001 min at equator) * 0.7431 (cosine of 42 deg) = 4.51 (ft / 0.001 min at 42 deg). At some other latitude where you may be, the number of feet per thousandths of a minute longitude equals 6.07 times the cosine of your latitude.

[Guest jxs2151]

Yeah, that is *exactly* what I was thinking, yeah. Dang, that is technical!

 

>Figure the numbers of feet per 1/1000 minute of latitude and longitude as follows. The number [snip] longitude equals 6.07 times the cosine of your latitude.

[Guest PneumaticDeath]

quote:

---

Originally posted by Mike_Teague:

The most accurate format for most receivers is UTM... Its got 1 meter of resolution, even on the cheap GPS's...

---

 

When I was a Physics TA, we used to drill into the heads of first year students the difference between "Accuracy" and "Precision". It may seem pedantic, but this is important now. UTM may be more precise, but probably not more accurate.

 

Here's an example. The statement that I'm over 30 years old is accurate, but not terribly precise. The statement that I'm 39 years, 2 months, 15 days, 4 hours and 52 minutes old is very precise, but not accuate, since I'm actually 32 years old.

 

Your assertion that even cheap GPS receivers get 1m resolution seems a little suspect. On my eTrex legend, in average conditions (6-8 sats), I get between 15 and 20 ft accuracy (5-6 m). With 10 good sats, and WAAS, I get 3-5 (1-1.5 m).

Expressing a coordinate to 1/2 m precision when my accuracy is +/- 5 meters seems a little excessive. And even when your GPS says that the accuracy is +/- 5 feet, there's only a 60% chance that it's actually withing 5 feet, since I think it expresses one standard deviation.

 

This is why physics and chemisty teachers are always going on about significant digits. Just because your calculator (or GPS) gives you 10 digits of precision, doesn't mean that you can trust all those digits.

 

[This message has been edited by PneumaticDeath (edited 11 July 2001).]

[Guest Scout]

quote:

---

Originally posted by Mike_Teague:

none of this means that UTM is going to make your GPS more accurate.. It's still gonna give the same accuracy... The resolution of UTM is basically too good for un-augmented GPS..

---

 

Isn't this basically the bottom line in all these discussions? The datum and formats used to communicate position all have more precision than the system's accuracy.

 

The system itself is accurate to about 10 m. That's more than 30 feet. Sources of error include ionosphere error, satellite clock error, multi-path error, receiver error, geometric effects, user error, etc. Receiver error plays less and less a role with newer models.

 

Anecdotes of finding a cache within 3 feet of its posted coordinates are heartwarming, but don't count on it. You can do everything right and still be off by 30 feet. Posting the coordinates with more and more numbers after the decimal point won't help at all if the source data can't be trusted within 10 m or so.

 

http://www.cnde.iastate.edu/staff/swormley/gps/check_accuracy.html

 

[This message has been edited by Scout (edited 11 July 2001).]

[Guest Scout]

quote:

---

Originally posted by PneumaticDeath:

Your assertion that even cheap GPS receivers get 1m resolution seems a little suspect.

---

 

What could be meant by this is that the receiver's contribution to the total estimated error is only 1m. With the latest receivers, this is plausible. But you still have all the other sources of error (ionosphere, satellite clock error, multipath error, geometric error, etc.) to worry about. Combined, the total error is back to about 10m. And most of that cannot be improved by using a different model of GPS.

 

http://www.ngs.noaa.gov/FGCS/info/sans_SA/

 

 

[This message has been edited by Scout (edited 11 July 2001).]

[Guest SteveL]

11 July 2001).]

[Guest Mike_Teague]

quote:

---

Originally posted by Scout:

What could be meant by this is that the receiver's contribution to the total estimated error is only 1m.

 

 

---

 

that's not what I meant.. All I originally said is that the UTM system as implemented in GPS receivers shows a resolution down to the meter..

 

I plainly also said that just because it will display a number that represents one-meter-resolution doesn't mean the thing is actually that accurate (precise?)

 

After all, they all also have ONE FOOT RESOLUTION on the altitude.. And we all know how that can be trusted.

[Guest SteveL]

Does anyone have Delorme's PostPro? I have an Earthmate and a laptop - maybe this is the way to get a more accurate fix when hiding a cache. Sound perfect!

 

Also, Professor Antonio Tabernero has information on generating RINEX files from Garmen units.

 

SteveL

[Guest rmpgis]

I've done quite a bit of GPS data collection using Trimble GeoExplorers (mapping grade GPS, US$3500)with a published accuracy of 3-5m. Testing at known places, geodetic monuments, I often get within a meter or two. This done by collecting at least 180 points, postprocessing (code) with base station, and averaging. The longer you can take data, the better. Survey grade GPS (US$10k) using carrier processing and multiple base stations can get cm accuracy.

 

Recreational grade GPS has an accuracy of about 10m. Often it's better but you have no way of knowing unless you're at a known location. The best method is to average over as long a time as possible, multiple visits will help too. Throw out any obviously wrong data and average the rest.

 

Try testing at a USGS monument, http://www.ospl.state.nc.us/geodetic/ngsdb.html You can look them up on a topo and then search for the data sheet or just pick one and navigate to it with your GPS. Think of it as a giant collection of virtual caches (Monument Confluence Project?)

 

John

GIS Specialist

 

quote:

---

Originally posted by SteveL:

What can be done? Averaging is a good first step ? but averaging for 20 minutes during a particularly bad GPS day might not do any good. Better to do long term averaging (24 hours?), and find where the data set settles. How about using a Trimble Differential GPS station to place the cache? I don?t have any experience with them, but I suspect the accuracy has got to be a little better than our hand held units. Post processing of GPS data? Does anyone have experience with this? Any surveyors out there? Take a look at http://www.ngs.noaa.gov/ for more. How about using existing survey monuments? Is there one near your cache? Can you perform an offset measurement from it?

 

SteveL

 

---

[Guest Betty]

feet per 0.001 minute of longitude at the degrees and minutes of latitude tabulated (covers USA)

 

deg/min 0 5 10 15 20 25 30 35 40 45 50 55

26 5.46 5.46 5.45 5.45 5.44 5.44 5.44 5.43 5.43 5.42 5.42 5.42

27 5.41 5.41 5.40 5.40 5.40 5.39 5.39 5.38 5.38 5.38 5.37 5.37

28 5.36 5.36 5.35 5.35 5.35 5.34 5.34 5.33 5.33 5.33 5.32 5.32

29 5.31 5.31 5.30 5.30 5.30 5.29 5.29 5.28 5.28 5.27 5.27 5.26

30 5.26 5.26 5.25 5.25 5.24 5.24 5.23 5.23 5.22 5.22 5.22 5.21

31 5.21 5.20 5.20 5.19 5.19 5.18 5.18 5.17 5.17 5.17 5.16 5.16

32 5.15 5.15 5.14 5.14 5.13 5.13 5.12 5.12 5.11 5.11 5.10 5.10

33 5.09 5.09 5.08 5.08 5.07 5.07 5.07 5.06 5.06 5.05 5.05 5.04

34 5.04 5.03 5.03 5.02 5.02 5.01 5.01 5.00 5.00 4.99 4.99 4.98

35 4.98 4.97 4.97 4.96 4.96 4.95 4.94 4.94 4.93 4.93 4.92 4.92

36 4.91 4.91 4.90 4.90 4.89 4.89 4.88 4.88 4.87 4.87 4.86 4.86

37 4.85 4.85 4.84 4.83 4.83 4.82 4.82 4.81 4.81 4.80 4.80 4.79

38 4.79 4.78 4.78 4.77 4.76 4.76 4.75 4.75 4.74 4.74 4.73 4.73

39 4.72 4.71 4.71 4.70 4.70 4.69 4.69 4.68 4.68 4.67 4.66 4.66

40 4.65 4.65 4.64 4.64 4.63 4.62 4.62 4.61 4.61 4.60 4.60 4.59

41 4.58 4.58 4.57 4.57 4.56 4.56 4.55 4.54 4.54 4.53 4.53 4.52

42 4.51 4.51 4.50 4.50 4.49 4.48 4.48 4.47 4.47 4.46 4.45 4.45

43 4.44 4.44 4.43 4.42 4.42 4.41 4.41 4.40 4.39 4.39 4.38 4.38

44 4.37 4.36 4.36 4.35 4.34 4.34 4.33 4.33 4.32 4.31 4.31 4.30

45 4.29 4.29 4.28 4.28 4.27 4.26 4.26 4.25 4.24 4.24 4.23 4.23

46 4.22 4.21 4.21 4.20 4.19 4.19 4.18 4.17 4.17 4.16 4.16 4.15

47 4.14 4.14 4.13 4.12 4.12 4.11 4.10 4.10 4.09 4.08 4.08 4.07

48 4.06 4.06 4.05 4.04 4.04 4.03 4.02 4.02 4.01 4.00 4.00 3.99

49 3.98 3.98 3.97 3.96 3.96 3.95 3.94 3.94 3.93 3.92 3.92 3.91