Gps Units

[gm100guy]

I had the pleasure of using a few gps units when teaching my gps course.

 

We had:

 

Magellan

Sportrack

Meridan

 

Garmin

Etrex

Etrex Venture

 

Lowrance

gm100

 

Eagle

Expedition2

 

These are all 12 channel receivers and we did not use waas at anytime.

 

I found that they all worked about the same to find any waypoints that I programed into them and were all within a few meters of each other.

 

I have found that most gps receivers are the same and that the person using and understanding the gps makes the biggest difference on being able to zero in and get to any waypoint in the gps.

 

The test that I did that I think showed how accurate each unit was when we were out using the units I recorded and downloaded each track file. When I put these track files in my Softmap program and overlaid them, it did not matter what unit it came from they all matched the area we were walking around and matched the route each gps unit took.

 

But when you look at differant features each gps offers I do have some thoughts on that and will not get into them.

 

So in closing I think that any gps that is 12 channel will work for geocaching and if users have problems, it mostly the human factor and not the gps.

 

In closing to test a gps I think what you need to do is go out and record a track and then over lay it in a mapping program. Then look and see what errors you have and not just rely on finding a set of waypoints. Also to get a good test is get a waypoint off a map that is accurate and not from this web site and program each unit and see how close you can get to the waypoint.

 

If you want a good test to see how well your gps works do this. Go to a mapping program and select a waypoint and record it. Then using utm mark another waypoint 100 meters away in any direction. On the ground mark each point and count the paces between and this will give the number of paces to go 100m. I had 5 gps and had them try this, only 2 teams found the 2 waypoints. When I checked the others it came down to operator error.

 

[Goldguru]

PLZ tell us what units the 2 were using.... ..........Gg

[+Hard Oiler]

Also to get a good test is get a waypoint off a map that is accurate and not from this web site and program each unit and see how close you can get to the waypoint.

Interesting observations and I agree that it's more the operator than the unit that effects GPS accuracy although satellite position and particularly local reception also has a lot to do with it. And you do need to be aware of the quirks of your particular unit - the Meridian "slingshot" effect for example.

 

Map accuracy is another problem. I've found both the 1 in 50,000 (as used in Softmap) and the 1 in 20,000 topos round here not to be particularly reliable - buildings and roads not in the right spot, creeks and streams moved - although main street intersections are generally reliable.

 

GPS Visualizer is very useful for checking track and waypoint accuracy and you can superimpose on Canadian maps or satellite images (or both together). Also there are several caches around located at primary reference benchmarks such as Calibration Cache and Test your GPS.. Interesting how all the logs get very close to the correct coordinates. If you know the right answer it's much more probable that you'll get the answer right

[+Maraudeur]

Hey GM100GUY

 

I have an Ifinder and I also have softmaps but i can't find a way to convert **.USR files from my Ifinder into in to *.SVS for softmap and or the other way around!

 

Do you know of a software out there?

 

Maraudeur

[+PDOP's]

Map accuracy is another problem.  I've found both the 1 in 50,000 (as used in Softmap) and the 1 in 20,000 topos round here not to be particularly reliable - buildings and roads not in the right spot, creeks and streams moved - although main street intersections are generally reliable.

 

Using a survey benchmark is the way to go. Topographic mapping is not as accurate as you might think.

 

The following is from the NRC website

Planimetric accuracy should then fall into one of five categories:

    * Stereodigitized NTDB data set at the 1:50,000 scale that has a resolution of 10 m for 90% of all data (generally covers urban areas) (around 400 files).

    * Scanned NTDB data set at the 1:50,000 scale that has a resolution of 25 to 100 m for 90% of all data (generally covers suburban areas).

    * Scanned NTDB data set at the 1:250,000 scale that has a resolution of 125 to 500 m for 90% of all data (complete coverage of the Canadian landmass).

    * Unknown accuracy at the 1:50,000 scale: more than 100 m.

    * Unknown accuracy at the 1:250,000 scale: more than 500 m.

 

You can check the accuracy of individual maps sheets here. The horizontal accuracy of the sheet with my home town is 100metres. No wonder that some of my waypoints seem to be off more than a little.

Edited March 8, 2004 by PDOP's

[gm100guy]

Thanks for the replies and thoughts.

 

I will be busy most of this week but if I get a chance I will test the units at my cache Test your GPS and see how accurate each unit is and let you know.

 

I did know that the maps could be off, but most times I find them accurate enough for me. I can only think of one time when I took a waypoint of Softmap and it was not on the right spot.

 

What is the slingshot effect? If I think what it is, could you not use pinning and not have the position jump all over the place?

 

topos round here not to be particularly reliable

And I wll not get into how bad the base maps are in some units are. I like when I drive along hwy 118 near Port Carling here in Ontario with my base map and it shows my car in Lake Muskoka.

 

I also have to go to North Bay this weekend and if I can get the wife to drive I will hook up each unit to palm with mapoplis running and see if they all match up on a moving map program.

 

Edited March 8, 2004 by gm100guy

[+Hard Oiler]

What is the slingshot effect? If I think what it is, could you not use pinning and not have the position jump all over the place?

It's the lag in position updating that Meridians have as a result of their position averaging feature. I suspect that they also attempt to predict where you will be before you get there. In open country this isn't a problem however when the signals are patchy it has the effect of causing you to overshoot the waypoint. I noticed it a lot at first but after a while you develop techniques to approach caches such that it doesn't become noticeable. Slowing down, holding the GPS up to get a better signal are a couple of techniques that work well.

 

Curiously my old Magellan 315 which also averages doesn't show the effect to anything like the same extent and (herecy!) is probably better for finding caches. Of course it doesn't have the maps, the memory, the screen size and in any case my Meridian cost much more so it must be better

[gm100guy]

cost much more so it must be better 

 

I will try and use that line when I want to buy someting on my wife.

 

Next time I am out with a Meridian I will see what that averaging feature is doingand let you know what I think of it.

 

[phern47]

 

I also have to go to North Bay this weekend and if I can get the wife to drive I will hook up each unit to palm with mapoplis running and see if they all match up on a moving map program.

 

Sounds like a fun project. Please give us an update when your experiment is over.

[+Renegade Knight]

I wonder which GPS OS is more likely to cause Operator Error.

 

For example the classic Magellan overshoot issue and masked signal loss issues.