Jump to content
Sign in to follow this  
Followers 1
Rhialto

±2m precision!

Recommended Posts

Today while searching for a place to hide a new cache my Vista HCx was showing ±2m precision on the sats screen! I thought the minimum was software blocked to be ±3m.

 

Anyone else? I don't have a screen capture to show you but next time maybe I'll take a picture.

 

I wonder if it's because of the new firmware or new sats launched a few months ago?

 

And now the question is: can it also display ±1m precision? :smile:

 

EDIT: well it seems it's common now. Got too excited for not much! :-) Maybe I just forgot the last time I've seen ±2m...

Edited by Rhialto

Share this post


Link to post
Today while searching for a place to hide a new cache my Vista HCx was showing ±2m precision on the sats screen! I thought the minimum was software blocked to be ±3m.

 

Anyone else? I don't have a screen capture to show you but next time maybe I'll take a picture.

 

I wonder if it's because of the new firmware or new sats launched a few months ago?

 

And now the question is: can it also display ±1m precision? :smile:

 

EDIT: well it seems it's common now. Got too excited for not much! :-) Maybe I just forgot the last time I've seen ±2m...

 

The "accuracy" display and the "EPE" estimated position error numbers don't really give a true indication of your GPSr's accuracy. Ex. You and a friend may have identical GPSs, holding them side by side and yours shows +/-2m and your friends shows +/-4m when in reality both may have the same precision/accuracy even though the "accuracy" field indicates otherwise. The number is not intended to be any more than a reference number... meaning the lower the number the better the satellite signal as compared to other times when you don't have as good of a signal. It is possible that occassionally under the most ideal conditions you could actually have an accuracy of +/- 2m but very unlikely to get this type of precision very often. Under good conditions a more realistic expectation of accuracy/precision would be in the range of +/-3m-6m. After all, we are not paying high dollars for super accurate units.

 

The true test to test the accuracy of your unit is to find and go to a known benchmark that has had the coordinates "adjusted" not "scaled" and compare your GPS's coordinates to the benchmark. There are alot of old posts covering this same subject. http://forums.Groundspeak.com/GC/index.php...racy++benchmark

Share this post


Link to post

My question is: is that accuracy correct? Are you really within 2 meters when it displays this?

From Garmins website re. waas

3 meters: Typical WAAS position accuracy.

This does no mean it will always be 3 meters

You must also factor in that the person that hid the cache may of been using an older GPS that does not suport waas. In that case you could be off by 20 feet.

It is a good practice to start looking for what would make a good place to hide a cache when you are about 20 feet away. After a while you will develop goesenses and the finding will become easier

Share this post


Link to post

From Garmins website re. waas

3 meters: Typical WAAS position accuracy.

This does no mean it will always be 3 meters

You must also factor in that the person that hid the cache may of been using an older GPS that does not suport waas. In that case you could be off by 20 feet.

It is a good practice to start looking for what would make a good place to hide a cache when you are about 20 feet away. After a while you will develop goesenses and the finding will become easier

I'm starting to wonder whether I've just developed an incredible case of beginner's luck here.

 

I picked up a TT720 for road navigation at Christmas, and after hanging around a geocacher on a regular basis, figured it would be fun to try this unit in that application. It's a SiRF III chipset, and that's all I know about its abilities on the front end. It provides either snapped-to-road coordinate data (not good for geocaching) or, if you go to the right screen, the raw GPS information in your choice of formats -- I use the dd mm.mmm for this purpose since it seems to be the geocaching-preferred notation. It's the screen that shows the satellite orbit positions and individual signal strengths.

 

Granted, I've done a whopping total of 5 caches, and all have been in good weather conditions with clear air above. I know that helps, but ...

 

After allowing the numbers to settle for about 15 seconds, the 720 has put me within no more than 4 feet of the actual location of the hides in each of the 5 cases. That would seem to indicate that not just my unit, but those of the people placing the caches as well, have some serious accuracy going for them. AFAIK, the TT720 doesn't even incorporate WAAS.

 

So should I thank my lucky stars, or has the technology with the advent of the SiRF III really gotten this good?

Share this post


Link to post

ecanderson,

 

I hear ya, I too have had my GPSr literally put me right on top the cache as long as I let it settle; I just thought this was normal. I just started geo caching (12 chaches total found no DNFs yet!) since I bought a Vista HCx three weeks ago. However, I have been using a GPSr for the past 14 years while out hunting to so I could find the truck quickly when I was done. I first started out with a 8 channel Magellan (200 something) back in 93 then migrated to a 12 channel Magellan 310 but finding a truck doesn't take any great skill. :smile:

Share this post


Link to post

Today while searching for a place to hide a new cache my Vista HCx was showing ±2m precision on the sats screen! I thought the minimum was software blocked to be ±3m.

 

Anyone else? I don't have a screen capture to show you but next time maybe I'll take a picture.

 

I wonder if it's because of the new firmware or new sats launched a few months ago?

I noticed that too, after the most recent chipset software update for the Vista HCx. But I'm convinced it's a (newly introduced) display error. I have the EPE set up as a display field on the compass page, and when it's showing 2 m on the sat page, it shows 3 m on the compass page. Further, if I change the units to Statute, it shows +/- 9 or 10 ft on the sat page. That's bigger than 2 m, obviously.

Share this post


Link to post

The question is, did the person who placed the cache you are looking for have good accuracy?

Were they even very careful, or did they just quickly press the mark button?

On my own caches, I usually take readings every thirty seconds over an 8 or 10 minute period, then average the decimal results.

However, I've looked for caches that were consistently reported as being 40 or 100 feet off. I even looked for one that was a half mile (800 meters) off. It could have gotten me and my daughter killed.

 

Parsa

Share this post


Link to post

Today while searching for a place to hide a new cache my Vista HCx was showing ±2m precision on the sats screen! I thought the minimum was software blocked to be ±3m.

 

Anyone else? I don't have a screen capture to show you but next time maybe I'll take a picture.

 

I wonder if it's because of the new firmware or new sats launched a few months ago?

I noticed that too, after the most recent chipset software update for the Vista HCx. But I'm convinced it's a (newly introduced) display error. I have the EPE set up as a display field on the compass page, and when it's showing 2 m on the sat page, it shows 3 m on the compass page. Further, if I change the units to Statute, it shows +/- 9 or 10 ft on the sat page. That's bigger than 2 m, obviously.

ok thanks, that's what I was looking for... I *think* I've never seen ±2m before today and maybe it has something to do with the latest firmware. But again maybe my memory serves me bad but I'm pretty sure it was the 1st time.

Share this post


Link to post

If you want more of a challenge, look up and old cache that is under bunch of trees. I have a feeling you will have to expand the search to an area of about 20 feet.

I have seen improved accuracy over the past two years

Share this post


Link to post

Today while searching for a place to hide a new cache my Vista HCx was showing ±2m precision on the sats screen! I thought the minimum was software blocked to be ±3m.

 

Anyone else? I don't have a screen capture to show you but next time maybe I'll take a picture.

 

I wonder if it's because of the new firmware or new sats launched a few months ago?

 

And now the question is: can it also display ±1m precision? :lol:

 

EDIT: well it seems it's common now. Got too excited for not much! :-) Maybe I just forgot the last time I've seen ±2m...

I've gotten a 7ft reading on my old (non "H") Vista before. And that's just 7ft, not ±7ft, which makes no sense.

 

How do you have a precision of negative 2 meters? :huh:

Edited by Prime Suspect

Share this post


Link to post

Folks, those accuracy or EPE numbers are a VERY ROUGH GUIDE only. Don't make too much of them. They are not real accuracy numbers. Very large grain of salt required. As was said above, find an adjusted benchmark, then look at how far off you are. THAT number will be real.

 

[For a credibility check here, I sell military versions of GPS receivers, and do some writing about GPS technical matters].

Share this post


Link to post

I've gotten a 7ft reading on my old (non "H") Vista before. And that's just 7ft, not ±7ft, which makes no sense.

 

How do you have a precision of negative 2 meters? :huh:

A quoted accuracy of "±6 metres" (say) simply means that the unit ESTIMATES the accuracy to be within 6 metres (give or take). It simply means that you are estimated to be within a radius of approximately 6 metres of the indicated position - you could be 6 metres north, south, east or west - and you could be further than that!

 

(Note: These are only "estimates" - not "guarantees"!!)

Share this post


Link to post

It's a crazy thing isn't it.

I believe the accuracy is just an estimate. But a very, very close estimate.

I have an older Magellan Meridian. Love it. Had it for years.

I routinely get 7 foot and 10 foot accuracy readings. So one day I wanted to put it to the test.

I went out to search for a survey marker. Found the marker and while standing directly over it

my Meridian was reading 7 feet. That works for me.

 

-Jeff :huh:

Share this post


Link to post

A quoted accuracy of "±6 metres" (say) simply means that the unit ESTIMATES the accuracy to be within 6 metres (give or take).

No, it doesn't. The uncertainty (the give or take) is not contained in the distance, but in the percentage. Most units use 95% as the limiter. It's saying that 95% of the time, you will be within X distance of the actual coordinates. There is no "plus or minus" a distance. That wouldn't make sense. Imagine dropping a rock in the middle of a field, and asking someone to stand a negative 7 meters from the rock. You will only get a puzzle stare.

 

It seems you're just trying to justify your misuse of the ± symbol. Again.

 

Very simply, unless a value can go negative, the use of the ± symbol is incorrect. The distance value is always expressed as a positive number.

Share this post


Link to post

Very simply, unless a value can go negative, the use of the ± symbol is incorrect. The distance value is always expressed as a positive number.

 

A coordinate error can be either + or - and will create a error in either one direction or the other of a specific distance. The idea of considering distance as +/- in reference to a coordinate system doesn't seem all that peculiar since there is a datum in mind to which the motion is associated (moving north in the northern hemisphere being plus, and moving south in the northern hemisphere being a minus).

 

That a coordinate error is being described as a "possible actual distance from target center" error makes perfect sense to me -- it has more meaning than specifying the error in 0.001 minute increments, especially as many people don't really know what that represents in terms of distance.

 

If your latitudinal error is, for example, +/- 0.002 minutes, that'd give about a +/- 12 foot error radius from center along the N/S axis, and if the longitudinal error is also +/- 0.002 minutes, that'd give about a +/- 9 foot error radius from center (up where I live) along the E/W axis.

Share this post


Link to post

A quoted accuracy of "±6 metres" (say) simply means that the unit ESTIMATES the accuracy to be within 6 metres (give or take).

No, it doesn't. The uncertainty (the give or take) is not contained in the distance, but in the percentage. Most units use 95% as the limiter. It's saying that 95% of the time, you will be within X distance of the actual coordinates. There is no "plus or minus" a distance. That wouldn't make sense. Imagine dropping a rock in the middle of a field, and asking someone to stand a negative 7 meters from the rock. You will only get a puzzle stare.

 

It seems you're just trying to justify your misuse of the ± symbol. Again.

 

Very simply, unless a value can go negative, the use of the ± symbol is incorrect. The distance value is always expressed as a positive number.

 

Maybe you had better contact Garmin and let them know that THEY are misusing the ± symbol - because my Summit HC tells me my location to an estimated accuracy of "± 6 metres" (or whatever). :ph34r:

 

True - your distance is always a positive figure, but your location coordinates can be larger or smaller than the estimated location - so ± is in fact the correct approach. In proper mathematical notation, to NOT use the ± would imply that the actual distance lies within the range of quoted distance (as a minimum) to quoted distance plus the error estimate (as a maximum). This is not the case - the actual distance is estimated to fall within the range of quoted distance PLUS OR MINUS the error estimate.

 

If I tell you my location (whether in Latitude / Longitude, or UTM Eastings / Northings doesn't matter), and an estimated ± precision, all I am saying is that my ACTUAL Easting or Northing is ESTIMATED to be the value I have specified, PLUS OR MINUS my estimated precision.

 

That is, a quoted Easting of 704 395 ± 10 metres simply means my actual Easting is LIKELY to be within the range of 704 385 to 704 405 metres (with 95% confidence, IF you believe the accuracy estimate that your GPSr - which I always take with a whole handful of salt!)

Share this post


Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  
Followers 1

×
×
  • Create New...