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Accuracy test among devices

Ramaprem 112 Points
edited November -1 in GPS Discussions
I played today with getting my location ("Where Am I", or whatever) while sitting in front of my home. I used 3 devices: Garmin eTrex20; Garmin nuvi 3590; TomTom 930.
I then went to my computer and, on Google Maps, marked that exact location.

Here are my results:
The nuvi and TT were close to one another - but 20-25 meters "off" from where the eTrex and Google showed me to be. The eTrex and Google were within 1-2 meters of each other.

Questions:
Is Google Maps "dead-on" accurate - beyond what the devices are capable of?
And, if so, is my eTrex most accurate because I have the Russian and US satellites in use? Is the eTrex always going to be the most accurate of my present devices?

Thanks.

Comments

  • sussamb 813 Points
    Bear in mind auto GPS 'snap to road' so will indicate the closest point on a road to where you are provided it's not too far away.

    Having said that I find my Etrex 20 running GPS and GLONASS more accurate than any other handheld I've owned/tried :wink:
  • Boyd 1985 Points
    You might find this thread interesting: http://forums.gpsreview.net/discussion/27533/x/p1/

    I don't know if you can generalize about the accuracy of Google Maps/Google Earth. It is data-dependent. A more ideal situation would be to find aerial imagery that includes metadata regarding accuracy. In the link above I explained that the imagery I was using from the State of New Jersey is certified to be accurate within +/- 4 feet (1.25 meters) with a 95% confidence level.

    Now New Jersey is a small state and they shot and processed their own aerial imagery, so it's a closely controlled situation. Googe is using a variety of imagery from all over the world and I don't think they could make any blanket statements regarding accuracy.

    Nevertheless I'd say it's very likely that Google will be more accurate than any consumer GPS most of the time.
  • Ramaprem 112 Points
    It must be a new day, 'cause I'm learning something.

    "Bear in mind auto GPS 'snap to road' so will indicate the closest point on a road to where you are provided it's not too far away."
    So, sussamb, this "snap-to-road" is a feature that is always in operation - given certain parameters of the software? Never knew of it; always thought that it gives the actual location - as best it can.
    Since I was, in fact, sitting about 20-25 meters from the street, that is what the auto devices were doing, it seems.

    "Having said that I find my Etrex 20 running GPS and GLONASS more accurate than any other handheld I've owned/tried"
    And you are basing this on...the assumed accuracy of Google Earth, yes? Or something else?

    I did think that the double-satellite-system in the eTrex was providing more accuracy.

    Thanks, Boyd.
  • sussamb 813 Points
    Well I geocache, and my etrex 20 places me far closer to caches than my etrex legend H ever did, so I'm 'assuming' that has something to do with GLONASS. Either way the Etrex 20 appears more accurate :)
  • Boyd 1985 Points
    On the Nuvi, road-snap is only supported with City Navigator. If you load a non-routable map - like a topo from GPSFileDepot - it won't snap to the road.

    But you can also use the raw track logging feature available in the developer menu on most models. Tracks recorded this way are just the raw data without being snapped to the roads, regardless of what map you use.
  • sussamb 813 Points
    And that's true also if you run a CN map on the Etrex and other handhelds, although you can select whether you 'lock on road' or not :wink:

    However I don't think it's 'only if you use a CN map' on a nuvi, I have a routeable topo map and my GPS seems to 'snap' to that also, so I suspect it's whether or not you have a routeable map loaded, CN or not :?
  • Ramaprem 112 Points
    Thanks for that.

    I was experimenting because, tomorrow, I will go to some location in the forest. I will need to mark the location, and communicate its co-ordinates to a group of people.
    So my best (meaning, most likely to be accurate) device to use for this purpose turns out to be the cheapest: the eTrex20.

    But that is so, in this case, because of its dual-system. Because of the location being in the forest, the auto devices would not be snapping anywhere. Right? I mean, they're not tree-huggers, are they? :wink:
  • sussamb 813 Points
    Certainly if it was me I'd rely on my Etrex 20 :D
  • Boyd 1985 Points
    however I don't think it's 'only if you use a CN map' on a nuvi
    Yes, I'm sure you're right. I just mentioned CN because that's what's already installed on the Nuvi. :)

    If you're concerned about road-snap, you can just disable the City Navigator map and only the the basemap. The basemap is also routable, but it doesn't have very many roads on it.

    Really, it would be best to use raw logging for tracks on the Nuvi. Do a google search, it's been discussed here and elsewhere.
  • wbport 92 Points
    Bear in mind auto GPS 'snap to road' so will indicate the closest point on a road to where you are provided it's not too far away.
    I run into that every time I come home. My driveway is 75' from a T intersection and whenever I pull in, my Nuvi says I am driving on the road from which I just turned.
  • My experience with my Garmin Vista HCx using the older Garmin USA topo is that anywhere near my home it always showed me to be about 30 feet east of the road or tail or lake's edge. That suggested to me that the map itself was shifted 30 feet to the west.

    I use that unit bike riding and it would always show me riding through my neighbors shrubs or, riding 20 feet into the lake. That is if a zoomed in that close. Normally I'd be zoomed out such that 30 feet wouldn't make a different. My bike was basically on the road when zoomed out.

    Away from about a 50 mile vicinity of my home the error was less or different.
  • roadster 96 Points
    what about in motion.

    glad companies do their testing LOL
  • Boyd 1985 Points
    My experience with my Garmin Vista HCx using the older Garmin USA topo is that anywhere near my home it always showed me to be about 30 feet east of the road or tail or lake's edge..
    That map (Topo US) is notoriously inaccurate. It was based on the Census Bureau TIGER road data and the errors there are well known. 30 foot errors are not that bad (10 meters), and are close to the accuracy limit for consumer devices (+/- 5 meters). But that map has errors in the hundreds of feet.
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