Slightly Concerned About Accuracy

[+FatGaz]

Took my new Garmin Etrex out for a bit of a test run, and to get the 'feel' for it. Went to a local park, parked the car and set 1st waypoint by the car. Then I followed various footpaths marking waypoints at junctions.

 

After about 10 waypoints I decided to work my way back to the car. A small part of my route was under light tree cover. GPS seemed to struggle a little.

 

GPS stated '20ft accurate', yet was often 50ft / 60ft 'out' at junctions, even in the open.

 

When I got back to the car it said that I had still got 80ft to walk!

 

Will I still get these inaccuracies when I download routes / waypoints from my PC? Suppose it will make the whole thing more 'fun'.

 

Is this a common 'fault'? Do other people experience this often?

 

Overall an 'interesting' experience - just proves to me that you still need a map, and the GPS is a useful additional tool.

 

Cheers

[+Red90]

How long was it turned on before marking the first point? It is important to have the unit on with a good view of the sky for 10 to 15 minutes first so that it can lock on as many satelittles as possible before marking a waypoint. In open areas, you should be accurate to 20 feet or less almost all of the time.

[planewood]

Do you remember what the height of the bars were on the satellite page at the time?

 

In comparing the readings from my cx against a surveyed location, I'm finding I get best accuracy when at least 4-5 solid bars are maxed out. There have been several times when I had 7 or 8 solid bars at the 50% height level and I was only getting 20-30 feet accuracy. So, I think I'm getting best accuracy when I have at least 4 bars showing at their maximum height. In such case, the readings have been within 5' - 8' of the surveyed target. The stated accuracy on the cx seems to be a fairly reliable number.

 

If you can find a surveyed location close by (like a bench mark or a triangulation station) I would suggest taking a dozen or so readings there over several days. This will help you better understand the accuracy of the readings you get later.

[+Car54]

Took my new Garmin Etrex out for a bit of a test run, and to get the 'feel' for it. Went to a local park, parked the car and set 1st waypoint by the car. Then I followed various footpaths marking waypoints at junctions.

 

After about 10 waypoints I decided to work my way back to the car. A small part of my route was under light tree cover. GPS seemed to struggle a little.

 

GPS stated '20ft accurate', yet was often 50ft / 60ft 'out' at junctions, even in the open.

 

When I got back to the car it said that I had still got 80ft to walk!

 

Will I still get these inaccuracies when I download routes / waypoints from my PC? Suppose it will make the whole thing more 'fun'.

 

Is this a common 'fault'? Do other people experience this often?

 

Overall an 'interesting' experience - just proves to me that you still need a map, and the GPS is a useful additional tool.

 

Cheers

 

A couple of things to keep in mind, assuming 20' accuracy throughout, when you mark your waypoint in can be up to 20' off in any direction. When you retrace your steps, it can still be 20' off. So, you have a max error of 40'. In high brush, this can mean you don't even see your previous path.

 

yes. The person marking the waypoint did not have the one infallible gps, so you will get a compounding of his error with yours. If you are really lucky, they will cancel out and you will be right on target.

 

[c] all GPS' have errors (it's built into the system). If you have very good satellite coverage with no obstructions, you can get your error down to 8' if you're lucky. That still doesn't mean that the original readings taken by the hider were that accurate, so you will still have to search for things.

 

Yes, most GPS' will allow some map data to be downloaded and this is invaluable if you want to exactly retrace your steps. As you noticed, the GPS won't guide you exactly. However, it can head you in the right direction to cross a real trail or road.

 

joe

Edited March 21, 2006 by Car54

[+Greymane]

[c] all GPS' have errors (it's built into the system). If you have very good satellite coverage with no obstructions, you can get your error down to 8' if you're lucky. That still doesn't mean that the original readings taken by the hider were that accurate, so you will still have to search for things.

 

"it's built into the system"??? Not anymore. If you have a good receiver (SiRF III) and a good view of the sky, you should be within 10 feet "most" of the time. When I am marking things on the property, I am often amazed at how accurate this lines up with the aerial photos I use. The Selective Availability (built-in error) has not been present since 2000.

[+AndrewRJ]

[c] all GPS' have errors (it's built into the system). If you have very good satellite coverage with no obstructions, you can get your error down to 8' if you're lucky. That still doesn't mean that the original readings taken by the hider were that accurate, so you will still have to search for things.

 

"it's built into the system"??? Not anymore. If you have a good receiver (SiRF III) and a good view of the sky, you should be within 10 feet "most" of the time. When I am marking things on the property, I am often amazed at how accurate this lines up with the aerial photos I use. The Selective Availability (built-in error) has not been present since 2000.

 

Since the OP was talking about his etrex a 20 foot accuracy is about as good as you can expect at almost any time. In light tree cover I regularly would only get a 30 to 40 foot accuracy, a lot depends on how you hold the unit. Sounds like you aren't seing anything I haven't experienced before.

[+Silny Jako Bek]

I had a similar problem recently with my new Legend Cx.

 

Similar Thread

 

I returned mine and received another that actually works correctly.

[+Car54]

[c] all GPS' have errors (it's built into the system). If you have very good satellite coverage with no obstructions, you can get your error down to 8' if you're lucky. That still doesn't mean that the original readings taken by the hider were that accurate, so you will still have to search for things.

 

"it's built into the system"??? Not anymore. If you have a good receiver (SiRF III) and a good view of the sky, you should be within 10 feet "most" of the time. When I am marking things on the property, I am often amazed at how accurate this lines up with the aerial photos I use. The Selective Availability (built-in error) has not been present since 2000.

 

I wasn't actually referring to Selective Availability, but rather the accuracy of the instruments involved. I can see how it might sound like I was referring to selective availability though.

 

joe

[Team Pike]

well i've noticed this with my yellow etrex, but thats fine as i dont really expect it to be anymore accurate.

 

When i use british grid waypoints i can get within about 15ft of a cache, but when i use WGS84 hddd.mm.mmm waypoints for multi caches, i can't get under 180ft!!! Then it spins around and tells me 300ft another direction! When i did finally find a multi cache (thanks to the clue not the gps!) my etrex was registering over 300ft away.

 

Any ideas??