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GPS for Boundary Surveys

Discussion in 'General Land Surveying' started by surveysc, Jan 10, 2013.

  1. surveysc

    surveysc 5-Year Member

    Joined:
    Jul 2, 2010
    Messages:
    194
    Location:
    Conway, SC
    This is a follow up from last year. Have any of you changed your opinion in the last year?

    I would like for some of you GPS Gurus to explain to me how to do property line surveys in the woods under canopy with any degree of acceptable accuracy.
    1. Is it accurate?
    2. Are the readings repeatable?
    3. How consistent is the data?
    4. Can you replace property corners with any accuracy?
    5. Is this a magic box?
    These are just a few of my concerns.

    We have a Trimble Pro XR that is over 10 years old that we use for wetland surveys and I have seen it get to within 1.0 feet with shots and they are repeatable if you use the correct methodology (shoot the same control points before each session and post process the readings just to name a couple). Of course that is only for horizontal accuracy. The elevations are too rough for topographic mapping.

    Happy New Year!!:-P
     
  2. RFB

    RFB 5-Year Member

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    Location:
    Old Florida
    Licensed in:
    FL
    I would like for some of you GPS Gurus to explain to me how to do property line surveys in the woods under canopy with any degree of acceptable accuracy.


    I'd use conventional methods.

    :stakeout: :totalstation:
     
  3. cptdent

    cptdent 4-Year Member

    Joined:
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    Location:
    Clinton, MS
    Use the GPS system to set up your 2 base control points, then run conventional in the wooded areas.
    GPS is good for locating stuff in open areas. Setting corners and locating stuff in the woods is better done with conventional methods.
    Worst case scenario is that you can only set your control points and then have to run conventional, but that still has its benefits.
     
  4. Nate The Surveyor

    Nate The Surveyor 5-Year Member

    Joined:
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    Location:
    SW Arkansas
    Licensed in:
    AR
    There are 150 answers for you.

    Gimme a call.

    N
     
  5. dmyhill

    dmyhill 4-Year Member

    Joined:
    Jan 26, 2011
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    829
    Location:
    Bellevue, Washington
    > 1. Is it accurate?
    > 2. Are the readings repeatable?
    > 3. How consistent is the data?
    > 4. Can you replace property corners with any accuracy?
    > 5. Is this a magic box?


    I find that using industry best practices for GNSS, it is faster for me to use a total station. For me, using GNSS's in that situation is frustrating.

    We are a generation back in our GNSS tech (Topcon GR3).
     
  6. jcoutsrls

    jcoutsrls 3-Year Member

    Joined:
    Jul 6, 2011
    Messages:
    32
    Location:
    Lafayette, IN
    This is an issue we've had to drill into our field crew's head(s). Just because you get a "fixed" position from GPS in the woods (or for that matter in an urban environment) does NOT mean it's an accurate position. For example, I had the crew set control points with GPS for a subdivision, then I followed behind with conventional for actual lot corner staking. All of the points that were out in the open checked within 0.015' between GPS and conventional. The one point that they set in the woods (of course it was in a small "clearing" and the crew felt like they had sufficient clearance from nearby trees) was off by 0.09', which was not within acceptable RPA tolerances for this particular subdivision. If you're dealing with larger RPA tolerances, then by all means give it a shot. But I would always recommend double-checking with conventional if there is any possible source for multipath.
     
  7. Dave Ingram

    Dave Ingram 5-Year Member

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    Location:
    Shenandoah Valley, Virginia
    OK - sarcasm is on

    So what? According to some posters here you can set the corner anywhere you want and once it's set it is good!
     
  8. surveysc

    surveysc 5-Year Member

    Joined:
    Jul 2, 2010
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    Location:
    Conway, SC
    OK - sarcasm is on

    That didn't take long. I just wish some of the local guys here would look on this site and see what the experienced GPS guys say. But as they say in church, here I am preaching to the choir. Thanks guys.B-)
     
  9. David Livingstone

    David Livingstone 5-Year Member

    Joined:
    Jul 22, 2010
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    531
    Location:
    Macomb Illinois
    OK - sarcasm is on

    Like others have said, you can't use GPS in the woods and shouldn't try. Use GPS to set control points and break out the total station.
     
  10. your-other-right!

    your-other-right! 2-Year Member

    Joined:
    Jun 29, 2012
    Messages:
    69
    Location:
    Athens, TN
    Line Staking

    I think it depends on what you are trying to accomplish. Are you staking the line through the woods for a fence? Or for the owner to put up no trespassing signs? or for logging? it is a judgement call. I have had pretty good success staking/flagging lines with my RTK unit in the winter time. I did not trust it at first, so I took it out to a job where we had staked a 2000' line with the total station the summer before. all of the stakes were .2' or less from where the GPS said they should be. That's about the tolerance we staked them with in the first place. Sometimes .20' just doesn't matter.
     
  11. Kris Morgan

    Kris Morgan 5-Year Member

    Joined:
    Jul 1, 2010
    Messages:
    3,266
    Location:
    Rusk, Texas
    GPS for Boundary Surveys *Sarcasm WAY ON*

    > This is a follow up from last year. Have any of you changed your opinion in the last year?
    >
    > I would like for some of you GPS Gurus to explain to me how to do property line surveys in the woods under canopy with any degree of acceptable accuracy.
    > 1. Is it accurate?
    > 2. Are the readings repeatable?
    > 3. How consistent is the data?
    > 4. Can you replace property corners with any accuracy?
    > 5. Is this a magic box?
    > These are just a few of my concerns.
    >
    > We have a Trimble Pro XR that is over 10 years old that we use for wetland surveys and I have seen it get to within 1.0 feet with shots and they are repeatable if you use the correct methodology (shoot the same control points before each session and post process the readings just to name a couple). Of course that is only for horizontal accuracy. The elevations are too rough for topographic mapping.
    >
    > Happy New Year!!:-P

    1. Maybe
    2. Maybe
    3. As consistent as the wind, constantly changing
    4. Really? No.
    5. Evidently it's a box for people who shouldn't use GPS

    The Pro XR is an aweseome piece of equipment and should be used for what it was designed for. If you try to go into the woods with survey grade GPS, you MAY get the right answer, or you may not, but unless you check many times, you won't know, thereby making the profit you should have realized from the gear, spent in checking for trying something that shouldn't have been done.

    Set pairs of control, check the control, get the damn gun out, tie or set the corner, pack it up, move on. Then, near the end of the day, check the control again. Multiple shots will generate the positional tolerance you need for any feel good test.
     
  12. gschrock

    gschrock 4-Year Member

    Joined:
    Aug 23, 2010
    Messages:
    1,864
    Location:
    The Nice Washington
    GNSS, trees, assumptions

    Given the scenario you painted, and the gear you described it almost sounded like a loaded question. Of course you'd have to use conventional.

    There are situations where GNSS can be utilized, and situations where it should never be utilized. But when it can, it can be a godsend. Gauging the difference can take a lot of hands on experience.

    It seems counterproductive to lump the value of any tool into two narrow camps of "good" and "evil". There is a lot of real estate in between.

    And a side note: assumptions about GNSS, even from a decade ago are questionable, even on such things as trees. There are new features that help in canopy situations; we've tested quite a few new ones, and each time we seem to be able to work effectively just a wee bit deeper into the woods. And when L5 gets fully deployed, watch out trees...
     
  13. Bruce Small

    Bruce Small 4-Year Member

    Joined:
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    Location:
    Tucson, Arizona
    (1) In the woods, under canopy, you cannot get accurate results, and they won't be consistent. On the other hand, if a couple of feet is good enough for what you are doing, then have at it.

    (2) You will waste so much time waiting for lock that you will give it up as a bad idea (which is good, because it is indeed a bad idea).
     
  14. James Fleming

    James Fleming 5-Year Member

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    2,901
    Location:
    Washington DC
    Licensed in:
    MD
    GNSS, trees, assumptions

    > There are situations where GNSS can be utilized, and situations where it should never be utilized.

    I use it for first floor elevations on topo surveys; but you have to run in the building real fast so you can get the shot before you lose lock.
     
  15. FLS

    FLS 4-Year Member

    Joined:
    Sep 15, 2010
    Messages:
    485
    Location:
    UPSTATE NY
    Do not set just a "pair of control points" set 3 so you have a solid check. I have caught bad shots by doing this.

    I have used GNSS in the works to locate ledges and creeks, where 1' is accurate enough and the feature changes everyday.

    I have located long fence line in the woods using GNSS, between total station located end points of the fence line.

    I locate hedgerow and fence lines on an offset. This really saves a ton of time, especially if or on a wheeler or snowmobile.

    Remember your going to have error in your TS traverse, so if it equals your GPS location, then you've saved a lot of work and time.

    And always it all depends on your situation (sats, distance to base, terrain, accuracies needed).

    I just completed 90 percent of a 50 acre farm survey including buildings using GNSS. With hedgerows and woods.
     
  16. MightyMoe

    MightyMoe 4-Year Member

    Joined:
    Sep 22, 2010
    Messages:
    2,851
    GNSS, trees, assumptions

    I use it for first floor elevations on topo surveys; but you have to run in the building real fast so you can get the shot before you lose lock.

    That's good advice! I should have thought of that myself. ;-)
     
  17. gschrock

    gschrock 4-Year Member

    Joined:
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    1,864
    Location:
    The Nice Washington
    GNSS, trees, javelins, bungees...

    Of course, one would need to be capable of still running that fast. Bazinga.

    You could launch the rover on monopole like a javelin in to the woods or first floor area then run after it to gain those few critical fractions of a second. takes a bit of practice.
    The addition of a bungee chord can, if carefully done, eliminate the running part, but you have to be careful with the returning rod. I suggest body armor, faceguard, breatplate, cod-piece or other protection.
     
  18. Marc Anderson

    Marc Anderson 4-Year Member

    Joined:
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    Location:
    Springfield, IL
    We have used the Trimble RTN receivers and have had extremely good results, even in foilage free trees (winter) when re-tracing previous work done by others. We stayed within Bill Henning's guidelines documents for RTN and repeated about half the observations on a different day and a different time. Everything repeated within .03' and a check to a nearby NGS monument checked .03' N-S, and .01' E-W. This was for rural landfill boundary work however.

    Conventional RTK, I'm not quite so sure about, but if I had the time to observe at a differnt day and time, that would improve my confidence.

    Most of the time it's just as fast to set up a TS and go conventional though.
     
  19. adamsurveyor

    adamsurveyor 4-Year Member

    Joined:
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    Messages:
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    Location:
    Colorado
    A thought:
    Go set your corner locations using GPS. In the treed areas (or perhaps all areas) set a hub. You can use the hubs location to help you figure out how to traverse into it using intervisible points, and traverse in using conventional methods. Set your corner, and see how close you hit to the hub. This will be a check for a gross error blunder, and can be used to see what kind of precision and/or consistency you get. Regardless of the responses you get on this "poll" you would want to build up your confidence level and/or know you limitations better anyway (I would think).
     
  20. jprice

    jprice 2-Year Member

    Joined:
    Jun 27, 2012
    Messages:
    24
    I have ran a lot of Large Boundary Projects using GPS as a tool 5000 t0 30,000 acres and NO you can not get reliable results in woods, No it is not a magic box but for someone who has been there here is my sugestion on how to incorporate GPS on large boundaries where corners are covered by canopy.
    First spend a few days reconing and recovering property corners. then with google map or aerial photo pick an open area near the corner or a location that you suspect might be a corner that is not covered by canopy . Set two intervisible GPS control points in these open areas.
    Then take your trusty total station an run a traverse tie to your corners.
    Once this is complete you start at one of your tied corners and inverse to the next corner on line you have traversed to. Then with the total station,transit or compass start your traverse and marking toward your ending point and carry your coordinates with you and inverse at each foresight until you reach your next tied corner on line.
    Once you have completed your traverse then take your information to the office and start plotting your deeds and accept or set the property corners.
    You will save time since I have experienced 30 minute delays in trying to get GPS positions in canopy and a lot of positions are suspect.
    GPS is a great tool but to run a large boundary in woods I suspect that to meet your state mapping standards you will need to use your convential surveying skills.
    Jerry K. Price
     

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