Question on accuracy in heavy tree cover

[John Walters]

I just started geocaching and have a concern that my GPS isn't necessarily as accurate as I had hoped it was. I am using a Megellan Meridian Platinum. I've only used this unit in relatively heavy tree cover a few times, so I'm not sure what I should reasonably expect from the unit.

The first cache we navigated to was in relatively thin tree cover and the unit seemed stable and very accurate.......I walked right up to the coordinates and sure enough, I was at the right spot. The next couple of coordinates I navigated to were in heavy cover the unit seemed stable, but if I stood still for 1-2 minutes the coordinates would change, or if I did an about-face the coordinates would change, or if I moved 2'-3' they would change far more than they should have. When this would happen I would go to the screen showing the satellites and I always had 5 or more locked in with excellent signal.

Is this normal for use in heavy cover? I have the unit set to read degrees/minutes/decimal minutes.

 

[Andrew Kalinowski]

There may be several factors at play here.
First of all; heavy canopy severely affects GPS signal. There is not only attenuation but also a good chance for reflections. The signal that does get through is often only from satellites that are from one sector of the sky (where canopy is lighter) and there is hardly anything from other sectors so, DOP can go way up resulting in poor accuracy. To top it off, Meridian Platinum (and many other Magellans) is known for heavy smoothing / filtering and it also takes it's sweet time to alert to signal loss. IIRC, it can take 30 seconds or more before that receiver tells you that signal isn't all that good...
Smoothing can play havoc with position if you are moving very slowly - it can assume that you stopped and start auto averaging and ignore slow change in position until the change becomes substantial or more rapid. All in all, the position (under heavy cover) could easily be, at any time, off by 20 yards or so and sometimes even more.
That's the bad news but, there is also not so bad news - you can take couple extra minutes and deal with it fairly easily.
Once you get close to the position that you seek, stop but, just before you stop glance at the screen if the receiver senses motion. If it doesn't, take few faster steps or wave the receiver around to make sure that it didn't go into auto-averaging mode before you actually stopped. Than you stop. Hold the receiver above your body for a minute and than consult the screen as to how far and in what direction your target is. At this point, use magnetic compass and step counting to get to the target - it's going to be more accurate than your receiver and will put you, most of the time, right on top of the target co-ordinates.
Notice that I wrote "target-coordinates", not "cache". I did so because, there is always a chance that the record of the cache co-ordinates isn't very accurate. After all, the person who placed the cache had to deal with heavy cover as well.

 

[John Walters]

Hey Andrew,

Thanks for the info. That certainly makes sense and what you describe is dead on with how it's behaving. I was staring at the coordinates and not noticing if it was averaging. Generally I'm walking fairly briskly if I'm on a trail, but I do slow significantly once I get close.

Any suggestions on a newer model that may perform a bit better, or am I going to see similar response from other units?

John

 

[Andrew Kalinowski]

Virtually all Lowrance as well as new Garmins with SIRF chipset are pleasure to use under canopy.

 

[John Walters]

Thanks for the info Andrew. I may just spring for a newer unit and see how I make out.

 

[John Walters]

Do any of the eTrex models have the SIRF chipset, or only the 60CSx & 76CSx models?

 

[Andrew Kalinowski]

AFAIK eTrex The "H" models (HC, HCx) have good receivers. I haven't tried any of these yet but, I think that they should work relatively well under cover.

 

[The Fixer]

It depends on your unit. I have been GEO Caching for over 2 years now and started with the Garimin E-trex - Legend. This unit used to loose signal all the time. I then bought a Garmin 60CSX over a year ago, and I cannot believe the difference. It even gets a signal in the basement of the house. I upgraded the 64 meg memory chip with a 2 gig chip and I now have half of the United Stated road map on it and the other half has the TOPO maps on it. I have been very impressed with this unit.