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recommendation for GPS positional and differential accuracy

epsi1on 0 Points
edited July 2014 in GPS Recommendations
Hi Friends.
I need to buy a handheld GPS for surveying the agricultural lands. This is about months i am investigating to find a good device for my purpose. recently i've found a book named 'Landscape Surveying using Handheld GPS Receivers'. That was noted in the book that there are two types of accuracy, 'map accuracy' and 'relative accuracy'. please have a look at the picture below (click on each one to see original size)
image
and this is the practical result:
image

there are also some videos about surveying with handheld GPS : http://www.youtube.com/watch?v=2A1yQv_-DMY

But in the other hand, there are some interesting cases who really indicate very bad differential accuracy, for example have a look at this image, the image is showing the different measurements of the same position within 30 mins (source is here)
image.

I would like to see both of these accuracies (relative accuracy and map accuracy) are how much for the devices like Garmin MAP64S or Monterra or MAP60CX or any other garmin devices. also whould like to know that does GNSS receiver effect on the relative accuracy? the only accuracy matters for me is 'relative accuracy'.

If possible, can you guys measure same set of few locations at different times and give me the raw data (lat and long) of locations with your device? I'll analyses the numbers and extract some information such as maximum difference etc which i think will be suitable for further usages.

Kind Regards

Comments

  • Boyd 1985 Points
    edited July 2014
    I am the author of that image that you posted, it is part of some testing I did for a review of the Garmin GLO here: http://gpstracklog.com/2013/04/garmin-glo-review.html

    Sorry, I can't do the tests that you want. But if you are planning to do "surveying", I think you're looking in the wrong place. The devices you listed are designed for recreational use, not surveying. I think you will need a professional device , such as one of Trimble's units: http://www.trimble.com/mappingGIS/Handheld-Computers-GNSS.aspx

    They are considerably more expensive than consumer devices.
  • epsi1on 0 Points
    Hi,
    Many thanks for your comment. I do not need a very vary accurate device for my surveying, the accuracy about a meter is very good for my few usages.
    I have found 'Trimble Juno 3B Handheld' from the link you've mentioned. I'm not familiar with proffesional properties of a GPS unit but Its about $800 (probably cheapest unit :) ) and in its spec sheet it is noted that it have a accuracy of about 2-5 meters with SBAS. For example if a normal handheld GPS shows same accuracy number on its display, do you think that data from Juno device to be more accurate in average than the normal handheld device?

    Thanks for your time.
  • Boyd 1985 Points
    You will not get anywhere near one meter accuracy with a consumer gps device, 5 meters is typical. Sometimes you can do better, but I wouldn't count on it. The less expensive professional devices, like the one in your link, have accuracy that is similar to consumer models but they have other features that make them better suited to professional use.
  • Tim 1481 Points
    if a normal handheld GPS shows same accuracy number on its display
    Manufacturers can tune/guess that number however their marketing department desires. ;)
  • gatorguy 326 Points
    A little sidenote of interest: Just 25 years ago the very first GPS satellite was launched.
  • privet01 221 Points
    I can tell you from personal experience that the "accuracy" number given by recreational GPS's is not anywhere close to being realistic. I've seen it explained once, maybe even here in this forum, but what the engineers intended this value to mean and what salesman want you to believe this value means are two totally different things.
  • Tim 1481 Points
    Since this resource is no longer online I'm reproducing it here for posterity and preservation, with citation.

    GPS explained: Position accuracy. http://www.kowoma.de/en/gps/accuracy.htm Web. Accessed 22 September 2010.

    Accuracy Values by Garmin Receivers

    imageThe declaration of the accuracy by Garmin GPS receivers often leads to confusion. What does it mean if the receiver states an accuracy of 4 m? This readout refers to the so-called 50 % CEP (Circular Error Probable). This means that 50 % of all measurements are within a radius of 4 m. On the other hand, 50 % of all measured positions are outside of this radius. Furthermore, 95 % of all measured positions are within a circle of twice this radius and 98.9 % of all positions are within a circle of 2.55 the radius. In the given example, nearly all positions are within circle with a radius of 10 m. The determined position is in the worst case accurate to 10 m.
  • Boyd 1985 Points
    This thread might also be of interest: http://forums.gpsreview.net/discussion/14680/redirect/p1
  • epsi1on 0 Points
    There is also a pretty good comparison between handheld and RTK gps devices available from here:
    https://www.crewes.org/ForOurSponsors/ResearchReports/2008/2008-15.pdf
  • privet01 221 Points
    Great reading guys. And I think the info newbie GPS users should take from this is that recreational GPS's should not be used locating your boundaries. Certainly it's okay if all you want is a "there about" idea. But if it might get you into a legal dispute with a neighbor because you cut down his trees or such... you definitely should think twice, maybe three times about it.
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