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Why don't 2 GPS's agree on a waypoint?


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I have been geocaching with my Lowrance Ifinder and my husband has a Magellan 400. It is not unusual for us to put in the same waypoint and have our units send us in two different directions. Sometimes mine is right on and other times his is. This happened several times yesterday. We then went to yet another site where we both walked up to exactly the same tree. There seems to be no rhyme or reason as to which GPS will get us to a cache. Anyone know why this happens?

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I have been geocaching with my Lowrance Ifinder and my husband has a Magellan 400. It is not unusual for us to put in the same waypoint and have our units send us in two different directions. Sometimes mine is right on and other times his is. This happened several times yesterday. We then went to yet another site where we both walked up to exactly the same tree. There seems to be no rhyme or reason as to which GPS will get us to a cache. Anyone know why this happens?
Both GPSs have a limit to their accuracy. It can vary depending on how many satellites are visible, battery level, interference from signal bouncing off rocks/concrete and how long the GPS has been allowed to determine the coords. But basically if you get within 20 feet of the cache put your GPS in your pocket and just look for it! :anicute:
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We are all using civilian standard gps devices with little or no guarantee of accuracy. We can hope for the best and get pretty close, but that's about it. Just as the previous post states, there are many factors at play. Relax, we ain't splittin' the atom here, just lookin' for boxes in the bushes.

 

 

I have been geocaching with my Lowrance Ifinder and my husband has a Magellan 400. It is not unusual for us to put in the same waypoint and have our units send us in two different directions. Sometimes mine is right on and other times his is. This happened several times yesterday. We then went to yet another site where we both walked up to exactly the same tree. There seems to be no rhyme or reason as to which GPS will get us to a cache. Anyone know why this happens?

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What 2 GPS's may not agree upon is exactly how far away a bunch of satellites are that are thousands of miles away. What they may not agree upon is by how many nanoseconds each of the signals are slowed from each of those satellites as they pass through the ionosphere at differing angles. What they may differ on is how relativity has affected their perception of where and when those satellites were. They may also have differing perceptions of which signals have been bounced off of what local objects. Ask 2 witnesses what they saw and you get 2 different answers. GPS's are not people, but what they percieve and process works in simular ways.

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I have been geocaching with my Lowrance Ifinder and my husband has a Magellan 400. It is not unusual for us to put in the same waypoint and have our units send us in two different directions. Sometimes mine is right on and other times his is. This happened several times yesterday. We then went to yet another site where we both walked up to exactly the same tree. There seems to be no rhyme or reason as to which GPS will get us to a cache. Anyone know why this happens?

 

How are you entering the waypoint coordinates? Are you punching them in manually? If so, are you both using the same map datum? Are you both using the same coordinate system?

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It can vary depending on how many satellites are visible, battery level, interference from signal bouncing off rocks/concrete and how long the GPS has been allowed to determine the coords.

 

Can somebody explain to me what "battery level" has to do with the accuracy of a GPSr?

 

The only think I could remotely think of, is that there might be receivers which turn the WAAS mode off if the batteries don't have a lot of power left. But I'm not even sure if there are units which have such kind of power save modes.

 

Otherwise the unit is either on and working to its full potential or not working at all, but there is no degradation depending on how new or charged the batteries are.

 

The other factors like interferance, foilage, ionosphere and so on of course do have an effect.

 

GermanSailor

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The results you and your wife are getting do not seem unusual. Each of you are holding it differently. You may be blocking some sats that she is receiving and vice versa depending on which way you are standing. Your hands are blocking the antennas differently. Reflection of signals bounce differently off of trees and other nearby obstacles changing results from second to second..

 

These seemingly minor differences can cause big differences in what you are seeing at the same time. Notice that even on your own unit how the distance and directions can change so radically from one second to the next.

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Can somebody explain to me what "battery level" has to do with the accuracy of a GPSr?

Exactly nothing. Someone once changed batteries and coincidentally saw a lower EPE or something, and decided the batteries were the problem. As long as the batteries are providing enough voltage to power the unit, then the unit will work as advertised. It's designed to handle different voltages, and will work until the batteries drop below the required threshold voltage.

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I have been geocaching with my Lowrance Ifinder and my husband has a Magellan 400. It is not unusual for us to put in the same waypoint and have our units send us in two different directions. Sometimes mine is right on and other times his is. This happened several times yesterday. We then went to yet another site where we both walked up to exactly the same tree. There seems to be no rhyme or reason as to which GPS will get us to a cache. Anyone know why this happens?

 

How are you entering the waypoint coordinates? Are you punching them in manually? If so, are you both using the same map datum? Are you both using the same coordinate system?

Map datum makes a difference, but coordinate system does not.

 

Why don't both of you get the same GPS and avoid this problem :laughing: .

 

As suggested, I think one GPS will get a lock sooner than the other. I frequently cache with two buddies; they have 60csx's and I have a 60cs. Occasionally, we go in different directions, but never more than 30 feet.

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How are you entering the waypoint coordinates? Are you punching them in manually? If so, are you both using the same map datum? Are you both using the same coordinate system?

 

We put them in manually. Why would that matter. I don't know about map datum or coordinate system. Sometimes both units agree and point us both the same way. If they always disagreed I would look at other things. I don't think that would be an issue unless they always disagreed.

Edited by chihuahuak94me
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Can somebody explain to me what "battery level" has to do with the accuracy of a GPSr?

 

Exactly nothing. Someone once changed batteries and coincidentally saw a lower EPE or something, and decided the batteries were the problem. As long as the batteries are providing enough voltage to power the unit, then the unit will work as advertised. It's designed to handle different voltages, and will work until the batteries drop below the required threshold voltage.

 

I guess the dimming of a flashlight bulb as the batteries weaken is ok, because it's operating as advertised? :laughing: What actually does happen when the batteries drop below the required voltage threshold for normal behavior? I have several electronic "toys", and each responds differently when supply voltages drop or dip due to cold weather etc. I have found batteries with intermittant internal shorts that cause operational problems.

 

Unless the unit has a function that shuts it off completely when the voltages reaches a level where operation could begin to suffer then disparities can occur.

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I have been geocaching with my Lowrance Ifinder and my husband has a Magellan 400. It is not unusual for us to put in the same waypoint and have our units send us in two different directions. Sometimes mine is right on and other times his is. This happened several times yesterday. We then went to yet another site where we both walked up to exactly the same tree. There seems to be no rhyme or reason as to which GPS will get us to a cache. Anyone know why this happens?

 

The simple answer is you are using two different units. The software and firmware differ, and how they process things also differ. If you want to compare the 2 units find a survey benchmark with a known coordinate, stand on top of that benchmark with each unit and see how close they are to the published coords. Don't put the units right next to each other as they may affect each other.

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How are you entering the waypoint coordinates? Are you punching them in manually? If so, are you both using the same map datum? Are you both using the same coordinate system?

Map datum makes a difference, but coordinate system does not.

 

 

Sorry, but that is incorrect. If you punch the same digits in on 2 gps's with different coordinate systems, then you are punching in 2 different locations. That's the reason my first question to the poster was were they entering the coordinates manually:

 

47.4747 entered in Degrees, Decimal degrees

is a different position than

47 47.47 entered in Degrees Minutes Decimal Minutes

 

Check these out as latitudes. You will find there is a 20 mile difference in these 2 coordinates. It is NOT an uncommon mistake to make, but a dangerous one nevertheless.

 

Seeing that the original poster went on to later state that they DO enter the coordinates manually, your assumption that "Map datum makes a difference, but coordinate system does not." is not only incorrect, but very dangerous. Anyone who uses a gps should be aware of this. You don't have to be an expert but ANYONE AND EVERYONE that at any time enters coordinates manually into a gps, ABSOLUTELY MUST BE CERTAIN of the coordinate system used when the original coordinates were taken. If you are loading the waypoints from a computer then in most cases the gps will do the conversion for you, so you don't have to worry about the coordinate system when loading waypoints from a computer.

Edited by GreatCanadian
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I have been geocaching with my Lowrance Ifinder and my husband has a Magellan 400. It is not unusual for us to put in the same waypoint and have our units send us in two different directions. Sometimes mine is right on and other times his is. This happened several times yesterday. We then went to yet another site where we both walked up to exactly the same tree. There seems to be no rhyme or reason as to which GPS will get us to a cache. Anyone know why this happens?

The (other) simple answer is that your GPSs are both accurate within their limitations. They have an error of about 20' give or take. Both GPSs can be in 100% agreement about a waypoint and still have the both of you standing 40' apart due to that error. The trick is to remember that the error is specific to the GPS. Your GPS has slightly different sources of error than your husbands thus the differing results at any given time.

 

Edit: tweaked the simple answer verbage since Eraseek used it first.

Edited by Renegade Knight
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If you're putting the coordinates in manually, there is a chance that one or both of you is actually entering a wrong number. All it takes is one number out of the 16 to be wrong, and you can be very far off. If you are absolutely certain that all the numbers are correct on both units, then how you hold them can make a difference. Water, especially impure water, will block GPS signals completely, and you and your husband are both big bags of impure water, even if you're very svelte. You can block the signals from several satellites, and possibly give very bad geometry, which can affect the accuracy.

 

For Rev Jim - yes, your GPS will cut off if the voltage gets too low. Any electronic device will fail to operate without enough voltage. But any properly designed device will operate to spec with enough voltage to operate. Flashbulbs are not electronic devices, and have nothing at all to do with the question.

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My wife has a Magellan Gold and I have a Magellan Platinum and they often show different locations even thought the software is the same (at least the postioning portion). One of the additional things that we've noticed that causes that is what satellites each unit sees. Trees etc. can be blocking 1 satellite for one of us and not the other. Different constellations, different calculation errors.

 

How can you tell which is accurate? You can't, they are both inaccurate to a certain extent. If they are showing a 20ft error then the actual point will be within a 20ft circle of where the GPS is situated.

 

JD

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