Can An Etrex Legend Need "tuning"?

[+GeoDoozer]

I 've used my eTrex Legend for about 3 months and I feel that I know it very well. I understand it's limitations with tree-cover, loss of direction when moving slowly, etc. - but I seem to always be off by 30-60 feet SouthWest of the actual location. At first I thought this might be because of spotty satellite reception at the time or poor terrain; or possibly bad coordinates by the cache-placer. But I've noticed this trend is always present. I've even gone to NGS BenchMarks and been off. And I consistently notice that when I visit caches, and the closest reading I can get is 45 feet, then a few hours later another log appears saying they navigated to within a few feet - that hurts.

 

I've learned to work with this inaccuracy, by trusting my "where would I hide a cache" skills, and only using my GPSr to get to the general location. But I would sure like to post a log saying, "The coordinates were right on."

 

Is it possible that my legend needs some kind of Garmin maintenance. I saw somewhere on their site that when a new unit is manufactored it is tuned and tested. Any ideas?

[+GeckoGeek]

Are you sure you've set the unit to the right map datum setting? Garmin has a large number of datum to select from, many just a little bit off from the others. WGS 84 is the standard for most GPS use. NAD83 is close enough not to worry about it. But anything else like NAD27 can through quite a bit of error into your readings.

[Neo_Geo]

Have you tried a Firmware Update?

[+EScout]

GeoDoozer:

Are you going to the super-accurate "adjusted" Benchmarks? If so, and you rounded the seconds/ minutes and set this as a waypoint, the waypoint should be within a couple of feet of the benchmark. The previous suggestion to check your datum is critical.

When I do this with my Legend, especially with WAAS on and received, the distance shown on screen to the waypoint, when standing over the spot where it should be, is usually only a few feet(3 to 7). With WAAS off, it usually shows me to be within 10 feet.

[+GeckoGeek]

Just to add to what EScout said, the location of some NGS benchmarks are "scaled". That means that someone read the location off a map and the error could be as much as 600 feet or so. The NGS marks with an "adjusted" location will be more accurate then your unit is capable of displaying.

[+GeoDoozer]

Wow! Thanks for the fast replys.

 

Yes, I am set to WGS84

Yes, I am at 3.60 firmware

How do I find an Adjusted Benchmark? I am at 33.28.535/82.2.099

 

One more question.....after I post a new topic, is there anyway to set it to send me email notifications...if I forget to do it when I first post it?

[+RobRee]

a handful of the benchmarks in your area look to be good. maybe look for one that has been logged by a fellow geocacher.

 

robbie

Edited January 5, 2004 by robree

[+GeoDoozer]

How can I determine which benchmarks are most accurate. I did a search and most seem to have Control Data notes that say either:

 

1)The horizontal coordinates were scaled from a topographic map and have an estimated accuracy of +/- 6 seconds.

 

or

 

2)The horizontal coordinates were established by classical geodetic methods and adjusted by the National Geodetic Survey in September 1997.

 

I found one that says: The horizontal coordinates were established by GPS observations and adjusted by the National Geodetic Survey in September 1999.

 

But it is on an airport runway and therefore inaccessible. Of the three types of control data, which would be best?

[+Sparky-Watts]

 

One more question.....after I post a new topic, is there anyway to set it to send me email notifications...if I forget to do it when I first post it?

Yes there is. Just below the reponse box where you type your message is a check-box that says "Enable email notification of replies?"

[+Team GPSaxophone]

 

One more question.....after I post a new topic, is there anyway to set it to send me email notifications...if I forget to do it when I first post it?

Yes there is. Just below the reponse box where you type your message is a check-box that says "Enable email notification of replies?"

So that's how how follow me around the forums...

 

I seem to always be off by 30-60 feet SouthWest of the actual location.

Have you tried finding caches placed by different people? Some GPSr's only have 2 digits of accuracy in the decimal-minutes field. If you're trying to get a fix on that with a more acurate unit you should be within 50-60 feet +/- your GPSr's accuracy of 10-20 feet.

[+Sparky-Watts]

Yes there is. Just below the reponse box where you type your message is a check-box that says "Enable email notification of replies?"

So that's how how follow me around the forums...

 

No, I'm actually hacked directly into your computer and ghost-writing replies as you type!

[+GeckoGeek]

How can I determine which benchmarks are most accurate. I did a search and most seem to have Control Data notes that say either:

 

1)The horizontal coordinates were scaled from a topographic map and have an estimated accuracy of +/- 6 seconds.

 

or

 

2)The horizontal coordinates were established by classical geodetic methods and adjusted by the National Geodetic Survey in September 1997.

See the emphasis in my quote back. You want adjusted.

 

When using CG's search page for your location, look for a "triangulation station" or "survey disk".

 

Ideally, you want to find one that the original datasheets shows as "suitable for GPS observation", but I wouldn't ignore one that didn't have that phrase. However, I'd probably pass one one that said "not suitable for GPS observation".

[+EScout]

For the super-accurate benchmarks:

NGS Datasheets

 

Go to this page, and select your state.

On the next page, select your county and check GPS sites only, any stability.

Sort by lat or long

Choose the benchmark disks, not CORS.

They show lat and long to one-hundred thousandths of a second. You will have to round off and imput this into your GPSr as a waypoint. You can calculate the how far this waypoint is from the benchmark by knowing the distance of a thousandth of a minute (about 6 feet in lat, about 4 to 5 feet in long in N. America) So if the adjustment puts you 1 foot east of the benchmark and 2 feet south, make a vector to that point. When you stand there and read your distance to the waypoint, you will know how accurate your GPSr is on that day at that site.

[+GeckoGeek]

For the super-accurate benchmarks:

True, but even the normal adjusted marks have accuracy far beyond what the GPS can even display.

 

True, but even the normal adjusted marks have accuracy far beyond what the GPS can even display. A bigger issue would be to check and see if the mark is using WGS84 or NAD83. There is a slight difference, but it's usually less then a handheld GPS accuracy anyway.

[+EScout]

Gecko Geek:

The second half of my post explains how to deal with the high precision of the benchmark. The waypoint you enter in your GPSr is the rounded off position of the benchmark to the precision of the GPSr. If you are using ddd mm.mmm, rounding will put the waypoint within 3 feet north or south and 2.5 feet east or west of the benchmark, here in So Cal.

For practical purposes, if you stand over the benchmark and it shows it to be within 10 to 15 feet away, you know your GPSr is doing a good job. At 5 local "adjusted" benchmarks, I have found my GPSrs (Legend and Meridian) to be "accurate" to within 2 to 10 feet typically; at the low end when receiving WAAS.

[0800peter]

... you will know how accurate your GPSr is on that day at that site.

and that sattelite constitution and this weather condition

 

well in case you come to germany one time vist the

Tobias mayer reference cache where my geko101 got 0m deviation

 

http://www.geocaching.com/seek/log.aspx?lu...1-7DCD20C09DF1}mylog

peter

[+GeckoGeek]

Gecko Geek:

The second half of my post explains how to deal with the high precision of the benchmark.

What I was reacting to is that you were limiting the selection to only GPS sites. In my experience, there aren't that many to choose from. Your milage my vary.

 

When I replied, I thought that selecting only GPS sites would be overly restrictive.

For checking a consumer GPS any old triangulation station will do as long as it has a good view of the sky. On the plus side, any GPS site will have to have a very good view of the sky. I wasn't really disagreeing withyou, just trying to give him a better selection of marks to choose from so he wouldn't have to go so far out of his way.