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Waypoint Accuracy - A brief tutorial (Read 3922 times)
RedMan
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Waypoint Accuracy - A brief tutorial
07/20/06 at 12:31:56
 
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:  Multimedia file viewing and clickable links are available for registered members only!!  You need to Login or Register!!


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."
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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 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.
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    RedMan
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #1 - 07/20/06 at 13:22:06
     
    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 from 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.

    Rule of thumb: If you are in a valley/ravine/canyon etc. spend more time averaging.


    This picture shows what happens if you use the wrong DATUM for a given waypoint.

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    monty
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #2 - 07/20/06 at 18:19:54
     
    great information. my Vista CX works just like yours in this respect. this is a feature i knew nothing about, but will now use.

    thank you.

    monty
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    mcgeed
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #3 - 07/20/06 at 18:53:22
     
    RedMan wrote on 07/20/06 at 13:22:06:
    This picture shows what happens if you use the wrong DATUM for a given waypoint.


    Some of us have a GPS and would love to know how to use it better......

    I don't know what you're even talking about when you mention using the wrong DATUM.

    Help me out here.  Is there an online tutorial that would help me learn how to use my GPS and help me understand this stuff?

    I have a Garmin e-Trex Vista.  Boutght it several years ago.  I got it primarily for use with the compass feature.  I was installing some wireless network equipment and bought a pair of them, one for each team at each end of the link.  They worked great.

    I know enough to figure out how to use it to find a geocache when I want to.  I can tell you the elevation and coordinates where I am at.\

    But, I'd like to learn how to import a map into my GPS and track my rides on that map.  How to use it to find my way to a place on my map, other than following a pointer this way or that way.

    So, any suggestions from you who have gathered that knowledge?

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    RedMan
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #4 - 07/20/06 at 19:04:27
     
    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.  Was it in degrees.min.sec or what?

    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.
    The other Datum that you can expect to see regularly is NAD83/WGS84.

    Click the link above for "GPS 101".

    Better than that buy the "GPS for Dummies" book.  It really is good and you can take it out in the field with you as a reference.  Gary sells it here on ATVUtah.

    The only way to learn this is by doing it, go walk around and play with the GPS.  You can mumble to yourself and people will leave you alone. Smiley

    Yuo can buy Garmin Mapsource software that will download a map into your GPS.  BUT it is not a high quality map so guranteed you will be saying "hey, lost of my favorite trails do not appear on this map".  Which is why you need to buy Nat Geos Topo map software too.  It has the quality you want but will not download the actual map to yuor GPS. Catch 22.

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    mcgeed
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #5 - 07/20/06 at 19:53:50
     
    RedMan wrote on 07/20/06 at 19:04:27:
    Yuo can buy Garmin Mapsource software that will download a map into your GPS.  BUT it is not a high quality map so guranteed you will be saying "hey, lost of my favorite trails do not appear on this map".  Which is why you need to buy Nat Geos Topo map software too.  It has the quality you want but will not download the actual map to yuor GPS. Catch 22.



    I have the Garmin Topo set and I also have Delorme Topo USA 5.0.  

    Have you used the Delorme?  How does it compare?  What do you know about it?

    I'll have to try the link you provided and get started learning.  I'll also check out the GPS for Dummies.

    Thanks.
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    an attractive and well preserved body, but
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    up, totally worn out, and screaming
    "Woo Hoo -- What A Ride!"
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    RedMan
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #6 - 07/20/06 at 22:00:32
     
    They have so many names for these things I can't keep them straight.

    I believe Delorme Topo Quads is the one that has 1:24000 maps.

    Look at the box for Topo USA snd it should say what resolution it is.
    If it says 1:100,000 it is about the same quality as the Garmin Mapsource software.

    Delorme makes good products, you just need the one with the 1:24000 resolution.
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #7 - 07/21/06 at 07:41:23
     
    RedMan wrote on 07/20/06 at 22:00:32:
    Delorme makes good products, you just need the one with the 1:24000 resolution.


    I think that IS the one I have.  I'll double check it this afternoon.

    Thanks again.


    Hey, you sure you can't find some extra time off for that Big Ride???  Sure is gonna be fun!

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    "Woo Hoo -- What A Ride!"
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #8 - 07/21/06 at 07:56:30
     
    Working for a living bites.

    I guess if people can be ski bumbs and survive I could be an ATV bumb.
    Anyone need a guide?
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    mcgeed
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #9 - 07/21/06 at 09:08:07
     
    RedMan wrote on 07/21/06 at 07:56:30:
    Working for a living bites.

    I guess if people can be ski bumbs and survive I could be an ATV bumb.
    Anyone need a guide?



    Can't we find a way to do this for a living?  We could guide ATV tours and fishing trips and the like.

    Think we could make enough to live on?

    Boy, I'd sure like to try.  But I'd have to clear it with my wife first and I'm not so sure it'd go over to well with her.....   Unless we could make enough money!

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    an attractive and well preserved body, but
    rather to skid in sideways, thoroughly used
    up, totally worn out, and screaming
    "Woo Hoo -- What A Ride!"
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #10 - 07/21/06 at 10:46:08
     
    I have used GPS units for years and love them. I won't hit the trail without one. I have my likes and dislikes. I use a Garmin 60CS and love it. I just read the GPS for Dummies and learned some things, even though I thought I was pretty knowledgeable to begin with. Mgeed, I am not a huge fan  of your type of GPS, it is mostly on the navigation buttons and menus. To each his own. Perhaps we can spend some time going over them on the big ride.

    I use the Delorme maps and their GPS with my laptop for driving trips. I do have the topo map but don't use it for ATVing.
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    "Life should not be a journey to the grave
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    an attractive and well preserved body, but
    rather to skid in sideways, thoroughly used
    up, totally worn out, and screaming
    "Woo Hoo -- What A Ride!" "

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    Re: Waypoint Accuracy - A brief tutorial
    Reply #11 - 07/21/06 at 11:55:50
     
    RedMan wrote on 07/20/06 at 12:31:56:
    ......As we all know the accuracy of your GPS is limited to something like 15 meters at best.
    That's probably true for most GPS's out there but a WAAS-capable receiver can give you a position accuracy of better than 3 meters 95 percent of the time. For the other 5% of the time you better have a bread crumb trail to follow.
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #12 - 07/21/06 at 12:27:27
     
    mcgeed wrote on 07/21/06 at 09:08:07:
    Can't we find a way to do this for a living?


    I'm trying really hard at it Cheesy

    mcgeed wrote on 07/21/06 at 09:08:07:
    But I'd have to clear it with my wife first and I'm not so sure it'd go over to well with her.....  


    Hmmm maybe that's why mine left the kids with me and ran off to New York.   Oh well her loss Cheesy Me and the kids are having a blast with the ATVs



    Effer wrote on 07/21/06 at 11:55:50:
    That's probably true for most GPS's out there but a WAAS-capable receiver can give you a position accuracy of better than 3 meters 95 percent of the time. For the other 5% of the time you better have a bread crumb trail to follow.


    Actually I was reading last night that WAAS doesn't work too well in much of the remote areas of OHV trails. Deep canyons, heavy forestation and foliage can interfere with the signals reception.

    this is from Garmins information
    Quote:
    WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications.


    It's awesome for Aviation and that was it's intended purpose, to improve aviation GPS accuracy.

    I'm not saying it's not a cool improvement on the system. It just has some limitations at the moment for land use in rugged terrain. It is not available anywhere but in the US right now but other countries are working on similar systems. Wasn't it nice of us to provide the world with GPS satellites?
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    Re: Waypoint Accuracy - A brief tutorial
    Reply #13 - 07/27/06 at 21:34:06
     
    McGeed , I bought the same unit you have a year ago and bought the instructional video that goes with it .  I have since traded it for the  60csx . If the video will do you some good you're more than welcome to it . You can get the video and the bumper at the same time . Just run up the street and pick them up .
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