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View Full Version : GPS Waypoint Accuracy - Or why can't I fnd the Geocache



RedMan
07-20-2006, 12:10 PM
One of the things I hear all the time is "That waypoint you gave me is wrong!" Ouch. I hate giving out bad info and feeling stupid.

Well, I know better and so I thought I might explain why a lot of waypoints seem to be wrong.

As we all know the accuracy of your GPS is limited to something like 15 meters at best. We see that in all of the manufacturer material. Well guess what, thats not quite accurate. :rolf:

There are a bunch of factors that contribute to accuracy and I will not dive into them all.

The quickest way to summarize all of this is to consider this picture which is from a more complete discussion at: GPS 101 (http://www.aximsite.com/articles/link.php?id=18#_Toc78550411)

The author says "Consider this picture. It represents about 9000 consecutive fixes at my house, taken once every two seconds. For scale, the cul-de-sac I live on is almost exactly 100 yards long. For the bulk of the time, the GPS was somewhere close to accurate. However, you can see some excursions, particularly one large one to the south that at the worst was close to 800 yards in error! I did some statistics and the overall average of the 9000 points is very accurate, and the 95% accuracy was just as advertised, about 15 meters, even with that one large deviation included."
http://files.aximsite.com/reviews/gpstutorial/gpswandersmall.jpg

So the manufactureres are not lying to you, because if you take a lot of waypoints at a given location and average them together, then 95% of the time you will be within 15 meters of your real location.

But thats not typically what most people do when they collect a waypoint. In fact most of us arrive at some destination, whip out the GPS, and Mark the waypoint. Were done. Then we give that to our friends and they try to find the same spot. They do the same thing.

So what if the waypoint you collected was one of the fringe points in the above picture? Maybe its the one that was off by 800 yards?? Ouch.

So how do we get more accurate waypoints? We collect many of them at the same location and average them together? Do not assume that your GPS is doing this automagically. If you have a GPS that does waypoint averaging you will likely have to manually tell it to start averaging and stop averaging.

On my new whizbang Gamin GP60csx it works like this.

Stop moving completely
MARK a waypoint and save it
FIND that waypoint from the list of saved waypoints
MENU select "Average location"
Let it run, its collecting waypoints and averaging them together. Perhap 30 seconds.
SAVE the waypoint
Go back to FIND and do it AGAIN. 4-5 times for best accuracy.


If you spread the 4-5 samples over a longer period you will be getting signals from satellites that have come into view since the very first sample was taken and you are essentially getting a second , third or fourth opinion.

Is that all? Well no. If you and a friend do this with two GPSs sitting side by side there will still be a discrepency. But the longer you sample, the closer the two GPSs will get to having the same last couple of digits.

But let say that you have gotten tired of sampling and decided your waypoint if good enough. Its potentially still incorrect by 15 meters in just about any direction despite what your GPS is saying about its accuracy level.

You give it to your friend who then proceeds to go to the waypoint. Now he/she has a similar problem in that he/she can probably only get within 15 meters of truly finding your waypoint. (Maybe)

Now consider that you left a geocache or small item at the waypoint you created. It sits at the center of a 15 meter (radius) circle of inaccuracy. Your friend stands at the center of a similar circle. These two circles may intersect at the very edge.

Stop Jabbering and get to the point.

It is possible that even if you and your friend have made every effort to be as accurate as possible by taking lots of samples and averaging the waypoints over a period of time, that he/she is still standing 30 meters way from the actual item you left for them to find. Ouch.

They might search in vain and decide you gave them a bad waypoint. In fact that may have happened if all you did was whip out the gps and mark the waypoint.

So learn to waypoint average. Then make sure your friends understand this concept.

Parting shot: I'm the guy who sold the computers to the government to make the whole GPS thing work and I'm surprised the thing works at all so I think we are pretty lucky.

RedMan
07-20-2006, 12:21 PM
BTW, do you notice a sort of star pattern in the picture? That correlates with the path of the satellites.

If you are in an area that is blocking signals from one direction (say form the south) then the inaccuracy will be scewed toward the north since you are not getting any offset waypoints from the south to correct your averaging.

jumar
07-20-2006, 01:51 PM
I'll have to see if my eTrex has that averaging option.

Thanks!

RedMan
07-20-2006, 04:18 PM
This picture shows what happens if you use the wrong DATUM for a given waypoint.

http://www.nb.net/%7Eresteele/newsites/images/gps_datum.gif

Alex
07-20-2006, 04:38 PM
What is DATUM and how do I set it or how do I use it? I have Garmin VistaC

RedMan
07-20-2006, 05:58 PM
Kazak,

Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth. Hundreds of different datums have been used to frame position descriptions since the first estimates of the earth's size were made by Aristotle. Datums have evolved from those describing a spherical earth to ellipsoidal models derived from years of satellite measurements.

So just about anywhere you go there are different Datum models that can be used or are in regular use in that area.

When sharing coordinates just make sure you understand which DATUM was in use when the waypoint or track or route was recorded.

If someone gives you waypoints or routes the first question should be "What format is this?" Get the Datum and the coordinate specs.

Go into your "Setup" menu and select "Units Setup". This is where you choose the coordinate system and the Datum. If you want your Datum to match a USGS map which is typically DATUM NAD27 (Its written on the map usually) then select it from the pull down menu.

The list in your Garmin will show several NAD 27 Datums. Yuo want to use NAD 27 CONUS.

Udink
07-20-2006, 06:17 PM
Also, I believe most GPS units come out of the box using WGS84. The coordinates on geocaching.com also use the WGS84 datum, and it's something that most people never need to change on their GPS units.

RedMan
07-20-2006, 06:37 PM
Unfortunatley DATUMS are something we all need to understand and consider in order to use the GPS properly.

Most maps are still NAD27 and nothing is more frustrating than not being able to understand why the map in your left hand does not jive with the GPS in your right.

Alex
07-20-2006, 08:11 PM
I must be dumb or something, I don't see the setting to change DATUM :ne_nau:

Udink
07-20-2006, 08:58 PM
I must be dumb or something, I don't see the setting to change DATUM :ne_nau:
From the "Main Menu," go to "Setup," then from that menu go to "Units." "Map Datum" should be the second field down on the "Units Setup" screen. I used an eTrex Legend for a few years, then switched to a GPS 60, and the map datum is one option I never had to mess with.

Alex
07-20-2006, 09:48 PM
Thank you, my setting is set to WGS84, that is correct, right? I have used GPS for a very long time, but this was the first time I found out about this setting....