Land Surveyor Forum

Replies

• Survey Legend

  Justin Farrow March 4, 2025 at 1:01pm

  Great question, Andy! Welcome. It sounds like you have a strong technical grasp of the concepts, and you're asking exactly the right questions about how PLSS surveys work in practice. Let’s break it down step by step.

  1. PLSS Bearings and Monument Placement

  You're absolutely right to question whether the section corners are "mathematically accurate" or if they are defined by where the original surveyor placed the monuments. The answer is: the monuments control. In the PLSS (Public Land Survey System), monuments placed by the original government surveyors (or their officially recognized replacements) are the definitive legal boundaries—not the mathematically derived locations.

  Even if later surveys use GPS or state plane coordinates, the official survey bearings and distances are based on where those original monuments were placed, even if they don’t perfectly match modern coordinate calculations. In fact, the original PLSS surveys, especially in rough terrain, often had significant errors, but their placements became legally binding.

  2. Understanding Your Bearing Differences

  • The bearing from the survey is given as N 88°55'57" E.
  • Your mathematically calculated bearing (NW to NE) is 89.87566° (which is closer to 89°52'32").
  • That’s about 0.94° difference—which is not insignificant.

  This difference could be caused by a number of factors:

  • The survey may not be using True North, but a grid north from the Washington State Plane Coordinate System (NAD83(91)).
  • The convergence angle (difference between True North and Grid North at your location) could be affecting the reported bearing.
  • Small local variations in how surveyors adjusted for magnetic declination (if referenced) or local distortions.

  How to Correct for It?

  • Check the convergence angle for WA North Zone (NAD83) at your location. You can compute this using GIS tools or a NAD83 grid-to-ground calculator.
  • The bearing might also have been referenced to Geodetic North vs. Grid North, which causes differences.
  • If using State Plane Coordinates, apply the grid-to-ground correction if necessary.

  3. Distance Discrepancies

  • Your survey distance (after ground correction) is 5324.00'.
  • Your mathematically computed distance is 5274.92'—about a 49.08' difference.

  Why the Distance Error?

  1. PLSS Sections are Not Perfectly 5280' per Side PLSS sections were originally meant to be 1 mile per side (5280’), but in reality, sections often deviate due to:

     • Earth curvature adjustments.
     • Surveyor measurement errors (especially in older surveys).
     • Fractional sections along the northern tier due to convergence of meridians.

  2. Ground vs. Grid Distance The survey applies a scale factor of 1.00007431, meaning it converts grid distances to ground distances. This correction factor accounts for how state plane coordinates are mapped on a 2D grid while the actual ground is curved.

  3. What Should You Do?

     • Verify if you’re using grid or ground distances in your calculations.
     • If using State Plane Coordinates, make sure to apply the same scale factor when converting distances.
     • If your source data is from an online tool, check if it assumes an ideal 5280' section or uses actual surveyed data.

  4. What About Closure?

  You mentioned “closure” in four-sided lots. If you’re getting 0.01' closure errors, that’s excellent accuracy. However, PLSS sections aren’t perfect rectangles, and section lines often have jogs due to:

  • Fractional lots (especially in northern/western tiers).
  • Adjustments for convergence.
  • Original surveyor errors that became legal definitions.

  Final Thoughts

  • Monuments control—not computed coordinates.
  • Bearings can be affected by grid vs. geodetic north, convergence angles, and datum reference.
  • Distances can be affected by PLSS inaccuracies, ground vs. grid scale factors, and historical surveying errors.

  Next Steps for You

  • Try computing the convergence angle for your section in WA North Zone.
  • Double-check if your tools are using Grid North, True North, or Geodetic North.
  • Use State Plane coordinate transformations (e.g., from NOAA’s NGS tools) to convert your survey bearings and distances to KML properly.

  Let me know if you need a deeper dive into any of these areas!
  • 

    Andy > Justin Farrow August 24, 2025 at 7:10am

    

    Hi Justin

    Sorry - only just seen your comprehensive reply 

    I solved some of the issues, but your details above will likely further help with my understanding... so when I get back to the project and look at your info - I may well ask more!

    Many thanks
    Andy
    • 

      Andy > Andy August 24, 2025 at 9:26am

      

      <<The survey may not be using True North, but a grid north from the Washington State Plane Coordinate System (NAD83(91)).>>

      The survey sheet in the Notes says:
      "5. Basis of Bearing is Washington State Plane Coordinate System NAD 83 (91) 4601 Washington North Zone" which concurs with your comment above.  However, I have failed to find where/how to find out how that Grid North relates to True North?

      <<not the mathematically derived locations>>
      I assume the online tool I used above to get the Section corners provides "mathematically derived locations"?  And that nobody has visted the monuments and documented their WGS84 Lat/Longs 

      <<convergence angle (difference between True North and Grid North at your location)>>

      I think this area is based on the 'Willamette Principal Meridian'?  So I'm guessing I can calculate/derive the Convergance angle given the Lat/Long of the area, and the Long difference from Willamette [122°44′34″W]?

      <<Use State Plane coordinate transformations (e.g., from NOAA’s NGS tools)>>
      Thanks very much... another area to explore right now