My New Gps Was Way Off

[+dmanc53]

I'm new to geocaching and own a Garmin Trex Legend. I've used this unit for a few vacation trips, but this is the first time I've been able to challenge it's accuracy.

 

I placed a cache in my yard, marked the waypoint, then had my neighbor's kids help me find the cache. We were standing right over the cache and it said we were off course by 92 feet.

 

tried a new waypoint with the same results. No wonder I couldnt find a cache I was looking for in a park a few days earlier.

 

What could the problem be? do I need to reset the GPS? I live in a cove/valley and there are many dead zones, but we tried the last waypoint in a better reception area. do weak signals affect accuracy?

 

Thanks for the info

 

DM

[+Stunod]

I'm new to geocaching and own a Garmin Trex Legend. I've used this unit for a few vacation trips, but this is the first time I've been able to challenge it's accuracy.

 

I placed a cache in my yard, marked the waypoint, then had my neighbor's kids help me find the cache. We were standing right over the cache and it said we were off course by 92 feet.

 

tried a new waypoint with the same results. No wonder I couldnt find a cache I was looking for in a park a few days earlier.

 

What could the problem be? do I need to reset the GPS? I live in a cove/valley and there are many dead zones, but we tried the last waypoint in a better reception area. do weak signals affect accuracy?

 

Thanks for the info

 

DM

Let your GPS sit on the cache for a few minutes before marking the waypoint. You cannot just walk up and mark it...give it time to settle down.

[+Boot Group]

We agree, slow down and let your GPS catch up. Also, did you check how many satelites you were catching at the time? We don't have your same GPS, but the nature of the tool should only put you off 20 feet or so when you have enough satelites working for you.

[+EraSeek]

Most likely NOT your GPS. There can be many factors involved. Rain and clouds NOT one of them (a common myth). Satellite geometry, can big a big factor, multi-pathing (signal bounce and timing delay), also YES you need to have your GPS up and recieving for at least 12 minutes before marking a waypoint, also an active ionosphere can throw you off (WAAS corrects this), and signal interference. Not to worry. It happens sometimes. Accuracy is within "such and such" feet 90 percent of the time....but then there is the other 10 percent of the time.

[+SixDogTeam]

You are wise to check out your GPSr and practice with it before caching...

[+dmanc53]

I'm going to try marking the waypoint slower. Also will try in a open field. My location is even difficult for a cell phone, many hills and heaviely wooded.

 

I'm glad I'm learning this in the safety of my yard and not hunting for my supper!

[+tirediron]

Did you double-check that your GPSr is set to WGS84 Datum and not something else?

[+GeoCyclist]

The eTrex line will tell you the GPS accuracy. Like the others said let you GPS settle down for a few minutes and check the accuracy (and number of satelites) before marking a way point.

[+welch]

Did you double-check that your GPSr is set to WGS84 Datum and not something else?

? Well, unless they changed something between the time when they marked it and came back to try and find it, I wouldn't think that could matter

 

Try repeating the test in an open area, but sit your etrex on top of whatever your marking and wait a few minutes as suggested. If its still off 90ft, try shuting off the gps and waiting a minute and turning it back on, and see what it says.

[+Seamus]

I live in a cove/valley and there are many dead zones, but we tried the last waypoint in a better reception area.  do weak signals affect accuracy?

The bit about being in a cove/valley may be the biggest culprit here. Depending on the slope of the sides, the depth, and your proximity to them, as well as how much of the sky is visible, you may be suffering from bad multipath interference.

 

Essentially, your GPSr may be receiving not only the direct signal from the satellites, but secondary "echoes" reflecting off of the walls of your valley. Multiple incidences may be confusing the receiver, causing your accuracy to drop off significantly. I've had problems with multipath at several caches, all of which were close to steep, rocky slopes ot other similar features. At one location in particular, we were able to find at least four individual points within a seventy-foot radius where two different GPS units said we were less than five feet away from the target point.

 

Another thing to think about is the geometry of your satellite spread. You should be able to see which satellites you have locks on from the satellite screen on your unit. If the only ones locked are all clustered together, or in a line, you will be dealing with very small angles, and there will be very little difference in the signals that your receiver has to sort out. In such situations, your accuracy will suffer considerably. The GPS satellites are in 24-hour orbits, so they will be in approximately the same locations at the same times day after day. A few years ago in Maryland, I found that there was a period in the afternoon, usually around 2:00 to 2:30 or so, where most of the "birds" would fall into a line with very little spread from side to side. For about half an hour, our accuracy would fall off to the point that we would be lucky to get to within 75 feet of a given point.

 

Just remember that you're dealing with very weak signals, beamed down to earth from a number of orbiting satellites, and that those signals must contend with the unpredictabilities of the ionosphere, atmosphere, and geographic conditions before they reach your receiver. Just think of it as a little extra challenge - Geocaching wouldn't be nearly as much fun if these little gizmos were 100% accurate all of the time...

[2oldfarts (the rockhounders)]

I believe that it is more important to have as many sats locked in as possible than to wait on the unit to "settle-in".

 

I just went ouside and turned on my GPSr and as soon as 5 sats were locked in I set a waypoint. I then walked around my house and did a goto for that waypoint. It put me within 5 ft of where I was standing when I set that waypoint. I still only had 5 sats locked in at this point. I proceeded to walk around the block and the GPSr brought me back to within 13 ft of the waypoint, but now I had 9 sats locked in.

 

I know, this isn't very professional, but then I'm only in this for the sport of it anyways.

 

More sats mean a better position definition.

 

I firmly believe that you do NOT have to wait for your GPSr to "catch-up" with you. If it can keep my position when I'm doing 75mph going down the Highway it sure can keep up at a 3.5mph walking pace!

 

Your display screen should show which sats are overhead and can be tracked. The more you have locked in the better.

 

Just the musings of an oldfart.

 

John

[+robert]

one problem i see many newbies encounter is with technique. i've seen cachers walk around with the gps in their pocket, pull it out to check it, then put it back in their pocket. i've seen them find a cache, set the gps back in their pocket, then pull it out and wonder why it shows them as so far off.

 

when finding or placing a cache, always set the gps down face-up so it can continue to receive signals. this is especially true with the etrex series and its patch style antenna.

 

good for you for practicing your hiding and marking in your yard before setting one out in the wild!

[+bons]

Turn WAAS off and see if predictability improves.

Also, you may want to load the latest changes to the software from the Garmin Website.

Edited June 7, 2004 by bons

[+tirediron]

Did you double-check that your GPSr is set to WGS84 Datum and not something else?

Note to self: Read the WHOLE ****ing post next time!

[+briansnat]

I firmly believe that you do NOT have to wait for your GPSr to "catch-up" with you. If it can keep my position when I'm doing 75mph going down the Highway it sure can keep up at a 3.5mph walking pace!

 

Actually, you do with some of the Magellans. They have what users call the "slingsht effect", which sends them past the cache. It has to do with the averaging featue I think. Not an issue with Garmins though.

[dsandbro]

Backyards can be poor areas. I have the same problem, but a short walk down the street improves significantly.

 

My yard has powerlines along the alley and lots of metal roofs and chain link fences in the neighborhood, so I blame it on those.

[+Rogue_monkey]

i have a etrex legend, i found it to be way off the first time i used it but then it settled down and was fine.

 

here are my suggestions:

 

1) make sure the unit is set to normal and not battery saver

2) try turning waas on and off like some one else mentioned.

3) hold the unit horizontal when in use

4) when marking a way point lay the gps down and walk away from it for a few min then walk back up and quickly mark the way point. your body can affect the view of the sky your gpsr gets.

5) listen to everyone elses idea's

6) they are not 100% accurate many things affect there use.

 

enjoy and i hope this helps

[+dmanc53]

Interesting posts. I agree that the GPS should read faster- I was wondering why it was so slow walking but on the highway it seemed to keep up. I'm going to blame it so far on the location. to give you an idea...my backdoor and west neighbor is 30 ft up. so if only a few satilites can be read as I turn?? I'm holding the GPS up at chest level- which should be good.

 

Up here in New Hampshire, a GPS can be a good thing to have- lots of trails to hike on, but need to be respected. If I'm going to exert my 50 yer old body to a mountain hike, I really don't want ot waste steps going the wrong way- but who would. I'll have some free time to play with this in a few days.

[+Destitute]

...Seamus said a bunch of good stuff, no sense repeating

Recently, I had this same thing happen to me. Now, I see why. Possibly reflections or maybe because I only got a few "birds" at any given time. at any rate, I was off by 300+ ft.

 

We found the cache anyway by using the hint.

[2oldfarts (the rockhounders)]

I firmly believe that you do NOT have to wait for your GPSr to "catch-up" with you. If it can keep my position when I'm doing 75mph going down the Highway it sure can keep up at a 3.5mph walking pace!

 

Actually, you do with some of the Magellans. They have what users call the "slingsht effect", which sends them past the cache. It has to do with the averaging featue I think. Not an issue with Garmins though.

 

Our Magellan does averaging and we have never had such an effect. I don't know why some people insist that it does happen. We leave it in averaging mode all the time and have yet to have the unit take us past the waypoint we were going to.

 

If your unit will give your speed while going down the highway and show where all the crossroads are and they match the map obviously it is tracking properly. If your unit shows your walking speed in 10th of miles per hour and it changes as your speed changes there is no problems with the unit. If it can figure out your speed and position at high speeds why can't it do the same at slow walking speeds? All of ours have always been able to do this.

 

Maybe what you're seeing is a false reading due to overhead cover or cliffs or buildings, but not a slingshot effect. With all the different units I have had the opportunity to use I've never seen one do what you're claiming they can do.

 

I have over 14 years experience owning & using different brands of GPSrs and have yet to see this type of slingshot effect.

 

We walked right up to a 1/2" diameter drill hole benchmark and when the GPSr showed we were 2 feet away we were actually 3 feet away. It was an easy find out in the middle of nowhere.

 

It all boils down to the number of sats you are tracking and the type of cover you are in.

 

John

[+G-Squad]

I have over 14 years experience owning & using different brands of GPSrs and have yet to see this type of slingshot effect.

 

We walked right up to a 1/2" diameter drill hole benchmark and when the GPSr showed we were 2 feet away we were actually 3 feet away. It was an easy find out in the middle of nowhere.

Sorry, John, but I have to agree with Brian here. For my first year of geocaching, I used a Magellan 315. For the next two, I have been using a Magellan 330. The sling-shot effect happens to me quite often. Even including hunts out in the wide open where I have several satellites locked. Because of this, when I hear the 100 foot alarm, I stop and stand still for a minute letting the unit settle down. I then get my bearing and distance from there and start moving the appropriate direction and scan along my path line looking for the cache. If I haven't found it after about 5 or 10 minutes, I stop and wait again. More often than not, by then I am usually within 20-30 feet so I just hook my 330 to the outside of my pants pocket and hunt harder.

 

As for going 75 mph and being accurate, what scale do you use on the highway? Typically on the highway, I am scaled out to 7 miles, 12 miles, or even 25. At that high of scale, yeah, the map looks spot on. But if you take the scale down to 100 feet or 250 feet going 75 mph, I think you will find you physically will overshoot your mark by the time the arrow hits the mark.

 

AJ

[2oldfarts (the rockhounders)]

I haven't used the 300 series magellans, but I have used the Magellan 2000, 4 different Meridians, and Lowrance GPSrs. If the 300's do this then it must be specific to those units. Even with the old Maggie 2000 unit we never had a slingshot.

 

As for the highway driving, when I am getting close to where I think I want to turn I usually go into the 250 or 500 foot range. The only problem with that is when the Mapsend shows roads that no longer exist!

 

Perhaps I don't get the slingshot because of how I use the GPSr when out looking for a waypoint. I tend to switch back and forth between the map screen and the distance screen quite frequently.

 

Perhaps some unusual atmospheric phenomena could be the cause?

 

John

[+robert]

I haven't used the 300 series magellans, but I have used the Magellan 2000, 4 different Meridians, and Lowrance GPSrs. If the 300's do this then it must be specific to those units. Even with the old Maggie 2000 unit we never had a slingshot.

I witnessed it while using my SporTrak color. On one particular instance it took me 80' past before the needle flipped 180 degrees. That's just one example. Might have to do with the speed at which I cache... none of the "oldfarts" I cache with can keep up... I know moving slower will help it keep up, maybe that's the case here?

[+boulist44]

My faithfull etrex is never wrong,

maybe you are in the wrong place!!!!

[+Team Aspenoro]

The GPS satellites are in 24-hour orbits, so they will be in approximately the same locations at the same times day after day.

I realized this when I first got my unit and did a lot of reading about GPS as a whole. Perhaps some IT or astrophysist person could explain why the government set up 12-hour orbits. It would seem to me that a big ol' Cray could figure out a different velocity for each satelite that would eliminate the 12-hour periodicity of the system without also lumping the satelites all on one side of the earth on occasion, i.e. keep a reasonable spread.

[+Seamus]

The GPS satellites are in 24-hour orbits, so they will be in approximately the same locations at the same times day after day.

I realized this when I first got my unit and did a lot of reading about GPS as a whole. Perhaps some IT or astrophysist person could explain why the government set up 12-hour orbits. It would seem to me that a big ol' Cray could figure out a different velocity for each satelite that would eliminate the 12-hour periodicity of the system without also lumping the satelites all on one side of the earth on occasion, i.e. keep a reasonable spread.

To the best of my knowledge, there are a number of orbits, each with different declinations (inclinations?), and each with a number of satellites (4?) in them, spaced regularly around the "track". They're arranged so that they provide "pretty good" coverage most of the time, but there are times when the orbital mechanics work out "just so" and you go through a period of questionable geometry on the satellites you can see. I'm sure that if it weren't for hills, trees, buildings and such, these periods of "clusters and lines" would be far fewer because you'd have better coverage down near the horizon.

 

As it is, probably the only way to achieve a more consistent spread throughout the day would be to add more satellites to the orbits, which would probably make the existing systems useless, since they wouldn't be able to compensate for the new geometry.

 

It should be noted that I am by no means an expert on the GPS system, and should probably do more reading on the subject before making such broad statements as these, but for what it's worth, these are my two cents...