Understanding a Survey

  • Understanding a Survey

    Posted by Rafagus on January 31, 2022 at 12:09 am

    The survey reads: S 38 degrees 15′ 00″ E

    Does this mean: From South move 38 degrees 15 min in a easterly direction? If so then I’d subtract 38 degrees 15 min from 180 degrees and get: 141 degrees 45 min. Then I would subtract 1 degree because declination in my area is +1 degree and get a compass heading of 140 degrees 45 min ?

    Thanks!

     

    bill93 replied 2 years ago 7 Members · 8 Replies
  • 8 Replies
  • Mark Mayer

    Mark Mayer

    Member
    January 31, 2022 at 1:13 am

    You’ve got it. Keep in mind that bearings determined by the typical compass will be accurate to +/- 2?ø or so, at best. 

  • dave-karoly

    dave-karoly

    Member
    January 31, 2022 at 3:35 am

    The other thing is survey bearings are not necessarily related directly to astronomic north. Typical practice is to use the record bearing of a line and build the survey off of that. Since the survey is on a plane the convergence of meridians is ignored. Therefore as the surveyor moves east or west the bearing diverge from north, about 1 or 2 minutes per mile depending on latitude.

  • mike-marks

    mike-marks

    Member
    January 31, 2022 at 4:22 am

    Done a lot of preliminary surveys using a compass (Jacob’s Staff) and tape and that’s the way to interpret the record compass bearings *except*:

    • Declination changes over the years in some areas so 50 year old record bearings have to be adjusted by as much as 5+?ø over time in northern climes.
    • Local magnetic attraction from iron rich rocks (basalt) will severely skew magnetic bearings so a transit line using optics will be different than that run using a compass.
    • Metal on your body affects the compass, for example using a Silva eye level compass is highly degraded if you wear eyeglasses with metal frames.

    Mark Mayer nails it; do not expect to run a compass line better than 2?ø and if you’re running with handheld tools expect 5-10?ø errors in difficult conditions.  Worrying over 15′ corrections is not warranted.  A compass is a useful tool for recovery of monuments by establishing  a search locus but certainly not usable for a reestablishment of a lost corner.

  • nate-the-surveyor

    nate-the-surveyor

    Member
    January 31, 2022 at 9:47 am
    Posted by: @mark-mayer

    typical compass will be accurate to +/- 2?ø or so, at best. 

    If you can find 2 consecutive monuments, remove any eyewear, and carefully observe the direction, NOW use this number like you did above, as the declination number, you can improve your mechanism. 

    @Rafagus you didnƒ??t tell us what kind of compass you have, but we are curious ??? 

    And, if you use this compass at both ends, of the same line, it may help you find local attraction issues. 
    Compasses tend to act like some people, and will act differently, around certain other people. But for the compass, itƒ??s metal, or magnets. ESPECIALLY magnets. Click glasses, and all magnets, or cell phones will addle a compass. 
    enjoy

    Nate

  • Rafagus

    Rafagus

    Member
    January 31, 2022 at 2:47 pm

    @nate-the-surveyor  I have a Harbin Geological Compass DQL-8 it’s a $65 one from the jungle site 🙂 

  • BStrand

    BStrand

    Member
    February 1, 2022 at 2:00 am

    Technically, I think you’re supposed to look up what the declination was at the time the survey was done, research what it is now, find the difference between those and then apply that to the measurement that you’re doing today.  If the survey you’re following was done relatively recently then it might not matter but if you’re referring to a survey that’s decades old…

  • Rafagus

    Rafagus

    Member
    February 1, 2022 at 3:44 am

    Today I rented a metal detector and quickly found two known points along one line of my survey. I have a theodolite and I want to try and follow the survey. I think I should setup over one point, aim at the second one and set to zero. Then do angle math following the survey. I want to see how close I am at the end when I come back to the POB. My property is a 2.48 acre triangle with a dog leg addition in one spot.

    One of the surveyors that did some of our property showed me some of his tools. Got me real interested in this. He told me surveys in my area are “messy”, he didn’t elaborate. 

    We’re selling off one parcel later this year. I want to map out my version of the property lines and see how close I am to the Professional and figure out where I messed up if we don’t agree. I’m hoping when he sees the effort I’ve put into learning he’ll spend some time with me.

  • bill93

    bill93

    Member
    February 1, 2022 at 4:05 am

    What Dave Karoly said above.


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