accurate hiding w/etrex vista cx

[+Team Swamp]

I have hidden caches with a yellow etrex and got the coordinates right on. I hid my last one with a Vista cx and recieved several replies that I was 20' off. What is the best way to get the coordinates correct. I tried coming at the spot from several angles, but was still off. When I find a cache, the Vista is always dead on.

[Motorcycle_Mama]

20 feet is within the accept accuracy range of consumer grade GPS units.

 

However, for increased accuracy you can take several reading and average the results.

 

It may not have been your receiver that was off.

[jmundinger]

I have hidden caches with a yellow etrex and got the coordinates right on. I hid my last one with a Vista cx and recieved several replies that I was 20' off. What is the best way to get the coordinates correct. I tried coming at the spot from several angles, but was still off. When I find a cache, the Vista is always dead on.

 

When I have hidden caches, I plotted three waypoints and for each of those waypoints, I used the averaging feature. Then, before leaving the site, I checked each of the three waypoints to make an initial assessment of each. Before writing up the cache, I plotted the three points in USAPhotoMaps and then chose the plot that I thought was best.

[+Team Swamp]

How do I use the averaging feature

[+oisact]

Create a new waypoint. At the bottom left of the screen you'll see a button "AVG", which will put the GPSr in averaging mode. Hold it steady for a while and let it do its thing.

 

Dan

[jmundinger]

How do I use the averaging feature

 

There are three "bottons" on the bottom of the Mark Waypoint page. "AVG", "MAP", and "OK".

 

Select "AVG". The screen will change to the Average Waypoint page and the unit will start averaging the waypoint for the unit's current location. When I do this, I usually set the unit down and then let it run for a minute or two. On the Average Waypoint page, there is a button for "SAVE", which you can use to stop the averaging and the screen will then revert to the Mark Waypoint page. At that point, select OK and the averaged waypoint will be saved.

[+Team Swamp]

Thank you, I've been leary to hide anymore until I can get the location accurate.

[+oisact]

BTW, if you want to average an existing waypoint, the technique is a little different. Select the waypoint and pull up the detail screen. Hit Menu, then you'll see an option "Average Location".

 

Dan

[+Miragee]

I have hidden caches with a yellow etrex and got the coordinates right on. I hid my last one with a Vista cx and recieved several replies that I was 20' off. What is the best way to get the coordinates correct. I tried coming at the spot from several angles, but was still off. When I find a cache, the Vista is always dead on.

It is interesting that you mention this because I looked for a few caches placed by someone who also has a Vista Cx. We DNF'd one, but after finding the others and noticing they were all about 16' off our GZ, to the NE, we went back, started searching to the NE of our GZ and found the cache.

 

Since I was using a Vista C, I was surprised at the consistent difference in the coordinates . . .

 

I don't have an explanation . . .

[passdump]

Check this out for info on GPSr accuracy here. It might be slightly on the technical side but I suppose your question requires a slightly technical answer! Some of the links on that page have good info too.

 

For a short answer GPSr accuracy varies with conditions, time of day, signal quality etc. Also, different GPSrs will vary from each other. Just because someone says your measurement was off doesn't necessarily mean it was off. Their GPSr may be at fault.

 

One thing you can check on your unit, make sure you are using WGS84 as your map datum.

Edited September 25, 2007 by passdump

[+hogrod]

I would check what unit firmware you are using. I along with a few others had some issues with accuracy with 2.70. I haven't noticed any issues with the 2.80 firmware update, you can get the updated firmware here.

 

To check what unit firmware you are currently running: goto main menu, setup, system, hit the menu/find button(lower left) and select software version.

[+Team Swamp]

I was still running 2.60. I heard bad things about 2.70, so I didn't update it. Hopefully this will be better. How do you change the map datum?

[+hogrod]

How do you change the map datum?

 

Main menu, setup, units. its the second option down, it should be set to WGS 84.

[+wsgaskins]

One other thing that affects both hiders (at the time of the hide) and finders (at the time of their find) is the actual position of the satellites that they have locked onto.

 

You should get better and more repeatable results if you have locked onto satellites that are "far apart" in the sky, which means that best case would be to have locked onto satellites that are near each horizon and some directly overhead as well. I've added the satellite signal strength page to my default page sequence on my Garmin eTrex. This way, right before I try to take a waypoint, I can check to see if my strong satellites are all directly overhead (often the case in tall or dense forests for me) -- which indicates that I'm gonna get very bad position measurements, or are they kinda spread out over the display -- which indicates that I'm gonna get somewhat OK measurements (and I can now actually believe the 'EPE' measurement that is shown on the average waypoint screen.)

[passdump]

You should get better and more repeatable results if you have locked onto satellites that are "far apart" in the sky....I can check to see if my strong satellites are all directly overhead -- which indicates that I'm gonna get very bad position measurements, or are they kinda spread out over the display -- which indicates that I'm gonna get somewhat OK measurements.

 

I never heard that before

[+wsgaskins]

It has to do with "DOP" - Dilution of Precision. It's easier to more accurately determine your actual position if you have range data from things far apart. There are lots of types of DOP - see here. And interestingly, since the satellite's orbits are known, you can predict when a good configuration of satellites will be available at any given location. There's software to do that here.

 

Think of it kinda like this - if you wanted to know where you were on a plane (flat) surface, and you could measure the distance to two things straight ahead of you but those two things were only a few degrees apart, how good would you know your position? If you could measure the distance to something straight ahead and something straight to your left (90 degrees apart), how good would your position calculation be?

[passdump]

It has to do with "DOP" - Dilution of Precision. ....

 

Very interesting. I never knew that. Thanks for the links, those programs for predicting the positions of satellites at any given location in the future look great. I am going to get them. Thanks.

[passdump]

... those programs for predicting the positions of satellites at any given location in the future look great. I am going to get them.

 

I got the Trimble Planning software (didn't bother with the Magellan) and on a first look its great. Easy enough to use too. Thanks again for the link (& info on DOP)!

[jmundinger]

It has to do with "DOP" - Dilution of Precision. It's easier to more accurately determine your actual position if you have range data from things far apart. There are lots of types of DOP - see here. And interestingly, since the satellite's orbits are known, you can predict when a good configuration of satellites will be available at any given location. There's software to do that here.

 

Think of it kinda like this - if you wanted to know where you were on a plane (flat) surface, and you could measure the distance to two things straight ahead of you but those two things were only a few degrees apart, how good would you know your position? If you could measure the distance to something straight ahead and something straight to your left (90 degrees apart), how good would your position calculation be?

 

Thanks for the explanation and for the links. This has been interesting/helpful.

 

In looking at the links that you posted, I assume that the software can be used to determine when the most accurate results are possible. But, the software does not have the capability to actually calculate corrections. Are those assumptions correct?

[+wsgaskins]

All this misc. knowledge is a side effect of having setup a puzzle cache series called "GPS Quiz".

[+wsgaskins]

Hmmm.... interesting thing that I came across when I was going back over my links of GPS information.

 

The NavCen - the US Military operators of the GPS satellites - have a prediction report for measurement accuracy.

http://gps.afspc.af.mil/gpsoc/performance_reports.htm

 

With the current PDOP prediction plot for the CONUS (Continental US) three days from now is shown here:

http://www.navcen.uscg.gov/gps/pdop.htm

 

The colors on the map represent the 95% chance that your measurement error will be that many meters or less. So now you can check that out and plan when is the best day to go and take new measurements!

[+wsgaskins]

In looking at the links that you posted, I assume that the software can be used to determine when the most accurate results are possible. But, the software does not have the capability to actually calculate corrections. Are those assumptions correct?

You are correct. The actual corrections are highly variable - they depend very much on the environment you are taking measurements in. Could be multipath reflections, clouds, interference, etc, etc,etc.