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How Accurate Are The Coordinates From One Gps To Another?


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Is this just me or does everyone run into this while geocaching? I'm using a Garmin eTrex Vista Cx in south Florida. Some coordinates bring me right on top of the cache within a few feet, then others can be way off 15 ft or more away. Some I never find, even after a good search of the area. Was I even in the right area?

 

Things I have thought of that maybe part of the problem:

People not all using True or Magnetic north settings on their GPS's.

One brand to another brand of GPS's.

High end GPS's compared to lower end.

 

While trying to figure this out. I tried a feature on my GPS called Average. I would set the GPS down and let it work for a minute. This seems to improve the accuracy of the coordinate before I save it in my GPS. I am able to go right back to that same spot within a few feet. Do all GPS's work this way when testing there own coordinates?

 

From reading some other post I guess I should be happy getting as close as I am, but I am trying to figure out why this happens and how I maybe to tighten that 15 ft or 20 ft variation, or how to best search for these types of caches. I try walking away and around stopping at times to see were the arrow is pointing, this helps sometimes. Any other ideas?

 

I have thought of placing a cache and ask the finders to also log the coordinates and type of GPS they have at that same spot so I can see how they compare. Has anyone ever tried that?

Edited by SKUNK APE
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I have thought of placing a cache and ask the finders to also log the coordinates and type of GPS they have at that same spot so I can see how they compare. Has anyone ever tried that?

I did a cache in Ohio that asked for that information when you logged the cache.

 

We used to have matching eTrex Legends, and our coords would be fairly consistently 3 feet different from each other. We thought that was interesting.

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I used to cache with a Magellan SporTrak Pro while my caching partner was using a Garmin GPS V. Though we approached the cache differently because of the way our GPS's worked we would eventually end up within 6' of each other.

 

We did find that even though we were very close together that it did not mean we were close to the cache. Sometimes the cache owner had inverted some of his numbers. In one case making us more that 1/2 mile off. We never would have found that cache if a previous hunter had not somehow found the cache and left corrected coordinates.

 

I have also heard of cachers purposely giving incorrect coordinates in order to increase the challenge. You don't want me to get banned so I won't tell you what I think of that.

 

I also think that sometimes a hider will not let his GPS sit long enough to get the best coordinates when he is setting up his new cache.

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From reading some other post I guess I should be happy getting as close as I am, but I am trying to figure out why this happens and how I maybe to tighten that 15 ft or 20 ft variation,

 

You can't. All consumer grade units are only accurate to 15-20 feet in most conditions. You may be able to get a bit better under perfect conditions, but that's not the norm.

 

If you want pinpoint accuracy you'll need a $5,000+ dollar industrial grad unit. But even then you'll still find yourself 15-20 feet off at the cache because of the inaccuracy of the unit used to hide the cache.

 

There is little difference between brands and any difference is measurable in inches.

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Most GPS units, if they are not receiving some sort of differential correction are accurate to the 10 meter range. Today, even with many commercial grade units you can acquie the WAAS differential correction signal which should put you in the 2-3 meter range. There are professional grade units that will achieve sub meter results with the WAAS or BEACON signal. A 2-3 meter professional grade unit with the ability to receive the WAAS signal is $ 1,750.00.

 

You must also be aware that if you are receiving the WAAS signal, it is being broadcast in ITRF coordinates and most probably if you are looking for MONUMENTS OR BENCHMARKS, those values are published in NAD83/WGS84.

 

I have been to only 2 caches and found one to be within the 2-3 meter range but the other position was off +-35 feet. When obtaining a position for a cache, make sure you have clear visibility to the sky as you can loose lock on some SV's (space vehicles) thus fruther degrading your positional accuracy. If you try to get a position under a tree, not only can you loose lock on the SV's but you can also experience high multi path when working next to a wall or building.

 

The most important thing is to have fun!

Edited by rmhurt
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Pay attention!

 

I am only going to do this once and then Fizzy Magic will be along with a test afterwards

 

Your GPSr will be accurate to a given spot by it's stated accuracy. For practical purposes this means 10 meters for a WAAS unit. Accuracy for this statement means the probability that the GPS is at the location it says it is.

 

If you record a position's coordinates and attempt to return to that position by the numbers, with the same unit, this is a digital problem and your accuracy is generalized to 1.414 times (+-) the last digit (generally, .001 degrees) at the equator. This assumes no other errors are present or part of the problem. The amount of error decreases slightly as you move toward the poles because the longitude lines converge.

 

If you try to repeat an approach to a waypoint set by the same unit you are no longer dealing with the digital limit, but rather an analog limit which can indeed be much more fine grained. If you have no multipathing and similar geometry to when the waypoint was set, accuracy can then easily be +- 12 inches. Accuracy defined for this example as being the consistent ability to return to the same marked position (repeatability).

 

Remember that EPE is not accuracy. It does not purport to be an accuracy measurement. It is an estimate of the probability that the numbers match the actual position. This equations used to calculate EPE vary between device manufacturers, and between models. It does not mean that the cache is within 20 feet of the unit, but rather that the unit has a 95% probability that the unit is at the position indicated by the numbers on the screen. That also means that it has a 5% probability it is not, which introduces the possibility it may be very far away.

 

Multipathing is the reflection of signals off of buildings, rock walls, metal and water. Most GPSr antennas are quite directional and must be pointed in the proper direction. Something on the order of 10:1 front to back, or more. If you understand this you can limit the effect of multipathing by tilting the unit appropriately. Multipathing can severely limit accuracy when present, no matter how you are defining accuracy.

 

Lastly, you cannot compare the approach to a set waypoint between two units until you have set the same waypoint in the same manner in both units. In practice, this is not so simple as it seems. Even using averaging between two identical units is not sufficient to guarantee identical measurement.

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I flew into Texas a couple of months ago to tackle Quantum Leap and several other Houston-area caches, all in SW Houston, bringing with me our SporTrak Pro GPSr, which is our primary GPSr (our backup and foreign travel GPSr is a Magellan Explorist 100; very good reception under dense forest cover, but only $65 new.) Not only was Quantum Leap a Snoogans cache, but several of the other caches which I sought were also Snoogans caches. Before I proceed, I must point out that SW Houston/Missouri City is a pretty good area to be hunting caches with a GPSr, due to the very flat terrain, lack of mountains or ridges to block satellites, and thus the large number of satellites visible to the GPSr. When I first started hunting caches that weekend, I noticed instantly -- to my amazement -- that our SporTrak Pro had a lock on 9 or 10 satellites -- this would be unheard of on the East Coast where we live. For the first stage of Quantum Leap, and for all other Snoogans caches which I found that day -- including one cache which was missing in action, but I found the hide spot anyway, which Snoogans confirmed (no, I did not claim a find!) -- I noticed that when I was standing at the hide spot, my GPSr read from zero to 4 feet to target. I later reported this rather amazing fact to Snoogans, and also mentioned it in several of my log entries, and Snoogans did confirm that he had used a very "tight" Magellan GPSr to record waypoints when placing these caches. I must admit that I was quite impressed with these tight renderings of the waypoint!

 

I have also had similar results in searching for a few other caches, even on the East Coast, all again using the SporTrak PRo, and in the cases where I later asked the hider, it turned out that they too had used a Magellan (most often a SporTrak Pro) to record the waypoint for the hide spot.

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I make a game of this at events.

 

Pick a spot - an unubtrusive rock on the ground, anything that you will remember but that isn't noticeable.

 

Set a waypoint using your GPS, getting the coords the same way as when you place a cache.

 

Give those coords and a small flag on a stick to all the event attendees and have them place their flag at the coordinates.

 

Give a stocked ammo can cache to the person whose flag is the closest to the item you chose as your waypoint for the coords.

 

You will be amazed at the spread!

 

Most of the time 50% will be within 50 feet, but the other half may be as far as 120' off!

 

The last time we did this at an event in Arkansas we had maybe 35 participants and marked a spot using the city's big-bucks Trimble pro unit, operated by a surveyer, who buried a nail at the marked spot in an open field - the nearest geocacher's GPS was 19', the farthest 60+' (and that farthest happened to be my new Garmin 60CSx, bummer!) !

 

It really shows the neccessity to put the GPS away once you are close and actually look!

 

Ed

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There are a lot of variables that affect the accuracy. I have an old Magellan 300 and a GPS Companion that I use with a palm M125. The 300 is the older unit I started with. It is less sensitive, slow to get a lock and unusable in overcast weather. The 300 accepted waypoints with 2 decimal places in the degrees. This translates roughly to +-50 feet error if you round off to the second decimal place when entering coordinates.

 

The GPS Companion is more sensitive, and and will usually aquire an initial location fix within a minute. I use the Cetus GPS software on an M125 with this. Cetus accepts waypoints with 3 decimal place precision, which theoretically specifies the location within +-5 feet. The catch is that atmospheric effect mess with the signal and to acurately calculate the location, the software has to average the data over a period of time. What makes it complicated is that the software must assume that you are moving, so it uses a more complex averaging algorithm than the arithmatic mean ( "add em all up and divide by how many you have") that most people think of as the "average".

 

WAAS enabled GPSr units use ground based signals used for aircraft navigation to rapidly correct for the error caused by atmospheric disturbances. This gives a quicker and usually more accurate response that averaging the readings over time.

 

The result is that most consumer gps receivers, even the same brand will not lead yo to the exact same location. Sometimes the same unit at different times will not take you to the exact same location.

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I have a story to tell about this topic. I set a cache with a Garmin Legend C, I did not know about averageing at the time. This cache has been archived, but this is what happend. I loged the cache with the cords. that the Garmin told me. I recieved a log for this cache that it was missing. I went back to the cache and found it was still there. I took another cord. reading and noticed that it was not even close to the orginal cords. I also use Trimbal Outdoors on my cell phone, so I marked the spot with my cell phone also. When I got back home I decided to put all three cords. into the Trimbal Outdoors Adventure Planner and see where the cords. would lead me. Well one set of cords. at first told me that I was in the middle of the Bearing Sea and the Arabian Sea. I put the other cords. into the Adventure planner the Trimble Outdoors software was right on the money. Showing me exactly were the cache was suppossed to be. I put the 2 different sets of cords. from my Garmin into the Adventure Planner 2 or 3 more times and was shown spots a lot closer to home but still on the other side of the state. I have also used the Garmin and my cell phone to find a cache and most of the time the Garmin is way off from my cell phone with my cell phone being more accurate because we found the cache with it and not the Garmin. At other times the cell phone software and the Garmin are very close to exactly the same readings. So go figure, I am going to keep using the Garmin for now anyway, I might decide to sell it some day. My wife uses the garmin to geocache but I might put T.O on her phone.

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