Positioning and Measurement
Positioning in New Zealand is built on the geodetic datum NZGD2000 (effectively aligned to ITRF at 2000.0), realised through a network of continuously operating GNSS stations. Positions are projected to NZTM2000: a Transverse Mercator projection with central meridian 173 degrees east and central scale factor k0 = 0.9996. Older surveys may be on meridional circuits (NZGD2000 Mount Eden 2000, Wellington 2000 and so on); know how to recognise them.
Separate the three surfaces: the ellipsoid, the geoid and the ground. Ellipsoidal height h, orthometric height H and geoid separation N satisfy h = H + N. In New Zealand N is of the order of tens of metres, so mixing ellipsoidal and NZVD2016 orthometric heights produces gross errors.
Measurement concepts: static and RTK GNSS, ambiguity fixing (fixed versus float), DOP, multipath, satellite geometry; total station EDM with prism constant and atmospheric correction; resection and traverse; least squares adjustment, redundancy, blunder detection, error ellipse, and the difference between precision and accuracy.
Traps
- Grid distance versus ground distance (scale factor) and grid bearing versus true bearing (grid convergence).
- Quoting accuracy at 1 sigma when the specification is 95% (about 2.45 x 1 sigma in 2D).
- Assuming a fixed RTK solution is error free - it can still be affected by multipath and poor geometry.
- Coordinate order: easting before northing, and degrees/minutes/seconds conversion errors.
Worked example
At 200 km from the central meridian, k = k0 (1 + x squared / 2R squared) = 0.9996 x (1 + (200 000 squared) / (2 x 6 371 000 squared)) = 0.9996 x 1.000493 = 1.000093. A ground distance of 1 000.000 m becomes 1 000.093 m on the grid - trivial for earthworks, significant for control and cadastral checks.