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CO Land Surveyor study materials
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CO Land Surveyor: Geodetic and Control Surveying

CO Land Surveyor · Work through your exam, one topic at a time.

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Geodetic and Control Surveying

Geodetic and Control Surveying

This domain tests datum literacy, state plane coordinate operations, and control-network practice - the material that separates a survey which closes on paper from one that closes on the ground.

Datums and heights

  • Horizontal datum NAD83(2011); vertical datum NAVD88; ellipsoid GRS80; geoid model GEOID18. NGS is modernizing to NATRF2022 and NAPGD2022.
  • h = H + N links ellipsoid height (GNSS) to orthometric height (leveling). In Colorado the geoid lies on the order of 20 m below the ellipsoid, so skipping N produces errors of tens of meters.
  • Colorado State Plane has three zones - North, Central and South - all Lambert Conformal Conic on NAD83. Scale factor is slightly below 1 near each zone central meridian and grows toward the zone edges.

Grid versus ground

  • Combined factor = scale factor × elevation factor, with elevation factor ≈ R / (R + H) and R ≈ 20,906,000 ft.
  • Grid distance = ground distance × combined factor. Divide to return to ground.
  • Convergence rotates geodetic azimuth to grid azimuth; a GNSS-derived bearing must be rotated before it is used with state plane coordinates.

Control practice

  • Static GNSS sessions tied to CORS, processed through OPUS, checked with repeat baselines and loop closures.
  • Differential leveling for vertical control, closed on published benchmarks and adjusted before use.
  • Publish coordinates with datum, epoch, adjustment, and method stated.

Worked example

A distance of 2,500.00 ft is measured at elevation 6,500 ft where the state plane scale factor is 0.99994.

Elevation factor = 20,906,000 / (20,906,000 + 6,500) = 0.999689. Combined factor = 0.99994 × 0.999689 = 0.999629. Grid distance = 2,500.00 × 0.999629 = 2,499.07 ft.

The 0.93 ft difference is about 370 ppm - enough to fail a closure check if it is ignored.

Traps

  • Applying only the scale factor and forgetting elevation in the mountains.
  • Mixing ground distances into state plane COGO.
  • Combining NAD27 coordinates with NAD83 observations.
  • Treating a rapid-static OPUS solution as equal in quality to a long static session.