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Gps Inaccuracy


Cammac
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I've known about geocaching for a while, but I've only hunted 2 caches. The first one my GPS had me a good 60 feet off from the cache. The second one I haven't found yet, however, having been to the site, and having re-read all of the Logged Visits, I'm certain I know where it is, and its 100+ feet from where my GPS said it was. Both Caches have had no comments about Coords being off.

 

So my question is: Should I expect this much discrepancy when hunting? If not, then what can I do to improve the accuracy of my GPS? It has been suggested that I might be able to calibrate my GPS by finding a National Survey Marker and telling the unit exactly where it is. Is this a good idea? Any advice?

 

--Cam

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100 feet is a bit much, but 30-40 feet is normal, 60 feet off is pretty common and 100 feet isn't totally unheard of. Your GPS is only accurate to 20-30 feet and so was the GPS of the cache owner when he hid it, so even if yours is perfect you still may be 30 feet off.

 

There are also other variables like satellite alignment. It may just have been a bad sat day when you were there. If this problem continues, then maybe there is something wrong with your unit and you should consider sending it back for repair. I'd find a few more caches first though and see what happens.

Edited by briansnat
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I've known about geocaching for a while, but I've only hunted 2 caches.  The first one my GPS had me a good 60 feet off from the cache. The second one I haven't found yet, however, having been to the site, and having re-read all of the Logged Visits, I'm certain I know where it is, and its 100+ feet from where my GPS said it was.  Both Caches have had no comments about Coords being off.

 

So my question is: Should I expect this much discrepancy when hunting? If not, then what can I do to improve the accuracy of my GPS? It has been suggested that I might be able to calibrate my GPS by finding a National Survey Marker and telling the unit exactly where it is. Is this a good idea? Any advice?

 

--Cam

Your unit has a setting called "datum" Be sure it's set to WGS84

 

You cannot calibrate your GPSr. It's results come from digital calculations not measurements as such.

 

100 feet off would be very unusual. Possible, but very unusual. I have never had an error that great.

Edited by Thot
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Your idea of checking your GPSr's accuracy against a known value is a good one. Go to the home page and then to 'find a benchmark'. If there is one located near your home that you can find by 'eyeballing' it then go there. Make about three passes to the benchmark and compare your units readings to the published values.

 

Try and use a benchmark that is clear of overhead obstructions and check that you have at least three sats preferrably four or more.

 

If you consistiently get large errors then the unit is suspect.

 

BTW, your unit is not a Magellan is it?

Edited by Team cotati697
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Your idea of checking your GPSr's accuracy against a known value is a good one. Go to the home page and then to 'find a benchmark'. If there is one located near your home that you can find by 'eyeballing' it then go there. Make about three passes to the benchmark and compare your units readings to the published values.

 

Try and use a benchmark that is clear of overhead obstructions and check that you have at least three sats preferrably four or more.

 

If you consistiently get large errors then the unit is suspect.

 

BTW, your unit is not a Magellan is it?

Many of the published values for the benchmarks are just estimates based on old data. No gaurantee that will do anything for you.

 

Some quick suggestions:

  • fresh batteries
  • upgrade the firmware if possible
  • check the number of sats your unit sees
  • try different times of day

 

just a thought....

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I've known about geocaching for a while, but I've only hunted 2 caches. The first one my GPS had me a good 60 feet off from the cache. The second one I haven't found yet, however, having been to the site, and having re-read all of the Logged Visits, I'm certain I know where it is, and its 100+ feet from where my GPS said it was. Both Caches have had no comments about Coords being off.

 

So my question is: Should I expect this much discrepancy when hunting? If not, then what can I do to improve the accuracy of my GPS? It has been suggested that I might be able to calibrate my GPS by finding a National Survey Marker and telling the unit exactly where it is. Is this a good idea? Any advice?

 

--Cam

There is NO functionality in a GPSr to calibrate it. The unit is self calibrating in it takes measurements for 3 to 4 satelites and makes a calculation based upon the data it recieves it may be more accurate or not. If the unit was consistantly 100+ feet off then it might be the Datum but if it varies it is just poor satallite coverage/calculation. The only thing getting the info for a benchmark will do is tell your sometimes you maybe wrong and sometimes you may be closer but never right.

The thing that will affect a GPSr the most is low batteries as mentioned, I once had 150 ft error from low batteries I was searching while my frienes were still going to the site. Keep fresh batteries on hand.

cheers

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I don't imagine this is just my unit, but I noticed on my Maggie SporTrak that it gets very unreliable once you reach about 25% battery left. I use it mainly for backup for this very reason (or let one of the kids use that one). But, with good batteries, it is great.

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Another Quirk I found with my GPS 315 is if the track log has been on for a while. Once the track gets long, I start getting odd readings. Once I set it down to average on a cache that I was placing, and it showed that it was moving, first 2mph, then slowly speeding up, when it got to 25mph and climbing I jumped on it before it got away. I clearted the track, and all was ok again.

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The GPS Test Station cache sounds like a nice idea.  Howver, it is a virtual cache.  No container.....

No container, but there is a disk in the ground so you know when you are exactly there. And you can place your GPSr right on the disk and see what it has to say for location and accuracy.

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The GPS Test Station cache sounds like a nice idea.

Interesting. All you have to do to put one over on the approvers in SLC and get a virtual approved is to call it a Mystery cache. :P

 

But you don't have to go searching for special caches to find known good geodetic coordintes. There are thousands of accurately located GPS points all over the US. You can look them up in the Benchmark section, or better yet, go to the NGS DATASHEET PAGE. Click on [datasheets][Radial Search]. Choose GPS Sites Only, Stability B or better. Enter you location and a search radius. Download the resulting list of datasheets and sort through them to find one that's accessible. You might have to fiddle with the stability parameter a bit to get a reasonable list for your area. Stability A stations are usually behind chain link fences with gate attendants. Most urban areas have plenty of Stability B stations that are accessible.

 

When you go to one of these sites you won't find out anyting about the so-called "accuracy" of your GPS unit, since it doesn't have any such thing as accuracy. What you can find out is: 1. The GPS system is working. 2. Your Receiver is working. 3. Your unit's datum is set to the right datum (or not).

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Interesting. All you have to do to put one over on the approvers in SLC and get a virtual approved is to call it a Mystery cache.

That was the point I was making. I think it is a good virtual though.

 

Blindleader's makes some good points on the "super Accurate" benchmarks. I recommend doing this, I have found about 8 of these in my local area. You get additional confidence in your GPSr.

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Blindleader's makes some good points on the  "super Accurate" benchmarks.  I recommend doing this, I have found about 8 of these in my local area.  You get additional confidence in your GPSr.

Of course, that doesn't help with the accuracy of the individual that placed the cache. laughing-smiley-007.gif

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Thanks to everyones helpfull replies.

 

BTW, I am using a Lowrance Globalmap100 I bought secondhand last year.

MapDatum has been verified to be WGS84.

 

Update:Most of the benchmarks around here have had thier Lat/Longs updated and verified by a local geocacher. Bad weather here lately, so I haven't been able to vist many of the local benchmarks. However, the one benchmark I was able to get to was 118 feet from where my GPSr said I should be. Earlier today I found the first part of a multi-cache... again 100+ feet from where my GPSr said it should be.

 

So, I'll be trying the suggestion about clearing my track data, however I believe I will be just giving up and buying a second GPS. Perhaps one that is a little easier on the batteries, has WAAS, that I can attach to my bike, and connect to my computer.

 

--Cam

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MapDatum has been verified to be WGS84.

 

Update:Most of the benchmarks around here have had thier Lat/Longs updated and verified by a local geocacher.

Geocaching uses WGS84, but I seem to remember some USGS benchmarks use a different datum. :ph34r:

 

Hoping a benchmark guy will jump in and clarify this if I'm wrong.

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The GPS Test Station cache sounds like a nice idea.  Howver, it is a virtual cache.  No container.....

No container, but there is a disk in the ground so you know when you are exactly there. And you can place your GPSr right on the disk and see what it has to say for location and accuracy.

Not if it's a virtual, you can't. :ph34r:

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Geocaching uses WGS84, but I seem to remember some USGS benchmarks use a different datum.

I don't know about "some" benchmarks, but.

1. WGS84 is the standard for GPS.

2. For our purposes, NAD83 is identical to WGS84. I.e. they use the GRS80 spheroid, though their "datum points" have drifted apart by about 2 meters in the past twenty years. I put "datum point" in quotes because neither system really has a datum point as such. They are both based on the earth's center of mass, the measurement of which has improved over time, causing WGS84 to move as it is adjusted, while NAD83 is tied to a usable network of survey monuments.

3. GPS points in North America are defined in NAD83.

example:

SY4595                        *CURRENT SURVEY CONTROL

SY4595  ___________________________________________________________________

SY4595* NAD 83(1998)-  47 35 42.53929(N)    122 23 14.84424(W)    ADJUSTED 

SY4595* NAVD 88    -        5.2    (meters)      17.    (feet)  GPS OBS 

SY4595  ___________________________________________________________________

 

edit: stupid typo

Edited by blindleader
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Thanks to everyones helpfull replies.

 

BTW, I am using a Lowrance Globalmap100 I bought secondhand last year.

MapDatum has been verified to be WGS84.

 

Update:Most of the benchmarks around here have had thier Lat/Longs updated and verified by a local geocacher.  Bad weather here lately, so I haven't been able to vist many of the local benchmarks. However, the one benchmark I was able to get to was 118 feet from where my GPSr said I should be. Earlier today I found the first part of a multi-cache... again 100+ feet from where my GPSr said it should be.

 

So, I'll be trying the suggestion about clearing my track data, however I believe I will be just giving up and buying a second GPS.  Perhaps one that is a little easier on the batteries, has WAAS, that I can attach to my bike, and connect to my computer.

 

--Cam

The Lowrance Globalmap 100 was introduced in about 1999. Here's what Joe Mehaffey and Jack Yeazel had to say about the accuracy:

 

Lowrance tells us that the GM's specification for position  accuracy  is 100 meters with SA (95% confidence).  No  accuracy  is specified  exclusive of SA  or for GM-100 accuracy with DGPS signals.

 

You can read the entire review here: Lowrance GlobalMap 100-GPS Receiver

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Checking bench marks for accuracy, ROFLMROLOLOLOLOL! I've had them 1/3 of a mile off. As someone earlier said, benchmark coords are merely guesstimates. When the mark was placed, there was no such thing as coords, hence the detailed directions. Decades and possibly centuries later, someone sitting in their cubicle looked at a map and said, "ok, that's close enough" between online chats.

 

As others have said the weather, nearby structures (natural and manmade), your gps vs. the cache owner's gps, and whether the last person placed it back in the correct place can all be variables. The biggie one is *recheck* that you typed the coords in correctly! Look at the logs of a few nearby and check it with a couple that state the coords were right on - of course depending on the above variables, your mileage may vary. My Garmin Etrex is usually within 20 feet.

 

Also, until you get more experience, stick with the regular sized caches. Those micros and multis can be buggers. Cache on!

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Checking bench marks for accuracy, ROFLMROLOLOLOLOL! caches. Those micros and multis can be buggers. Cache on!

NOT rolling on floor

 

The horizontal position of a benchmark comes in kinds "Scaled" and "adjusted"

 

For scaled - your right - someone looked at a map, and said - looks to be about there

 

For ADJUSTED - they are right on the money - a heck of a lot closer than your GPS can measure (unless you are running a Multi 10s of K$ total station) - we are talking within milimeters - How do you think they defined the grid system

 

So, if you want to check, go find an adjusted point, and knock yourself out

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Checking bench marks for accuracy, ROFLMROLOLOLOLOL!  I've had them 1/3 of a mile off.  As someone earlier said, benchmark coords are merely guesstimates.  When the mark was placed, there was no such thing as coords, hence the detailed directions.  Decades and possibly centuries later, someone sitting in their cubicle looked at a map and said, "ok, that's close enough" between online chats.

There isn't a single GPS point in the database that fits your remarks above. I can assure you that where ever you happen to live in the lower 48, there is a nearby GPS "bench mark" with coordinates published with centimeter accuracy.

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blindleader is correct.

 

NGS Data Sheets

 

Highlight State, click choose Counties.

Highlight County and GPS Sites only, click Get Marks.

 

Look for disks, not CORS.

 

When you round the coords and enter them in your GPSr, the rounded coords will be within 3 feet from the disk. If you want to be more precise, you can calculate this spot and stand over it and look at your goto distance.

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