S+SNZ Geodetic Survey common mistakes
S+SNZ Geodetic Survey · Understand the traps and how to avoid them.
Applying a single scale factor to convert between grid and ground distances without checking whether the combined factor (point scale × elevation factor) is appropriate for the specific line, especially near zone boundaries or over long distances.
Candidates memorise the projection scale factor formula but forget that the elevation factor varies with height and that the line's mean scale factor differs from the point scale factor at each end. They also confuse the projection scale factor with the combined factor.
What to do instead
For every grid-to-ground conversion, compute the combined factor at each end of the line (or at the midpoint for short lines), then use the mean combined factor. In NZGD2000, confirm the projection parameters (central meridian, false origin, scale factor at CM) and test with a known baseline. Always state the method and assumptions in the answer.
Treating NZGD2000 coordinates as if they were in terms of the local vertical datum (e.g., NZVD2016) without applying a geoid model, or mixing ellipsoidal heights with orthometric heights in a level network adjustment.
The candidate sees 'height' in a dataset and assumes it is usable for engineering design or cadastral purposes without checking the datum metadata. The difference between ellipsoidal and orthometric heights can be tens of metres in New Zealand.
What to do instead
Always identify the height datum from the coordinate metadata. Convert ellipsoidal heights to orthometric heights using the official geoid model (e.g., NZGeoid2016) and document the transformation. In level networks, reduce all observations to a consistent datum before adjustment.
In a cadastral survey, adopting a bearing and distance from an old plan without checking its reliability, or failing to search for and reconcile all available evidence (e.g., physical marks, abuttal measurements, adopted boundaries).
Candidates focus on the mathematical closure of their own survey and overlook the hierarchy of evidence required by cadastral rules. They may assume the old plan is correct because it is recorded.
What to do instead
Follow the evidence hierarchy: physical marks first, then measurements from the original survey, then abuttals, then areas. Recompute the old plan's geometry and compare with your observations. Document any discrepancies and your reasoning for adopting or rejecting each value.
Failing to correctly identify and apply the appropriate accuracy standard (e.g., cadastral accuracy, engineering setting-out tolerance) to the survey observations, or using a generic 'good enough' approach without justifying the chosen tolerance.
Candidates memorise accuracy classes but do not connect them to the specific task. They may also confuse relative accuracy with absolute accuracy, or ignore the effect of instrument precision and environmental conditions.
What to do instead
For each task, state the required accuracy from the relevant standard (e.g., Cadastral Survey Rules, engineering specification). Then design the survey to meet it: choose instruments, observation procedures, and redundancy accordingly. In the answer, show the error budget and how the chosen method satisfies the standard.
Ignoring the effect of refraction and tide on hydrographic soundings, or failing to reduce soundings to the correct vertical datum (e.g., Chart Datum, LAT) using the appropriate method.
Candidates treat hydrographic surveying like land surveying, forgetting that water level, sound velocity, and tide introduce systematic errors. They may also confuse the various vertical datums used in charts.
What to do instead
Apply sound velocity corrections from a measured profile or a reliable model. Reduce soundings to Chart Datum using observed tide or a tide model, and document the method. For precision work, use a real-time kinematic tide reduction. Always check the datum definition and the separation between ellipsoid, Chart Datum, and any local datum.
In civil engineering setting-out, using coordinates directly from a design model without verifying the transformation between the design datum and the site control, or without checking for a scale factor difference.
Candidates assume the design coordinates are in the same system as the survey control. Often the design is on a local grid or a different projection, and a scale factor or translation is required.
What to do instead
Obtain the design coordinate system definition and compare it with the site control. Compute the transformation using at least three common points and check residuals. If a scale factor is involved, apply it consistently to all design coordinates. Verify by setting out a known point and comparing with the design.
In land development and urban design, proposing a subdivision or road layout that ignores the existing cadastral boundaries, easements, or the requirements of the district plan (e.g., minimum lot size, setbacks, height-to-boundary).
Candidates focus on the geometric design and forget the legal and planning constraints. They may not have checked the district plan rules or the underlying survey network.
What to do instead
Before designing, gather all relevant information: cadastral survey data, easements, district plan rules, and infrastructure constraints. Use a checklist to verify compliance. In the answer, explicitly state the constraints and show how the design meets them. Where a non-compliance is proposed, justify it with a resource consent pathway.
In spatial/GIS tasks, failing to document the coordinate reference system (CRS) and datum transformation, leading to misalignment of layers or incorrect area/length calculations.
Candidates treat GIS as a mapping tool and overlook that different layers may be in different CRSs. They may also assume that on-the-fly projection in GIS software preserves geometric accuracy.
What to do instead
Always define the CRS for each dataset and the project. When combining layers, explicitly transform them to a common CRS using the correct datum transformation (e.g., NZGD2000 to NZVD2016). For area and length calculations, use an appropriate projected CRS or compute on the ellipsoid. Document the CRS and transformation in metadata.
Misinterpreting or misapplying the New Zealand Geodetic Datum 2000 (NZGD2000) and its relationship to the International Terrestrial Reference Frame (ITRF), including the effect of plate motion and the need for a deformation model.
Candidates learn that NZGD2000 is 'fixed' to the Australian plate but forget that it is a semi-dynamic datum. They may not account for the accumulation of tectonic deformation since 2000, leading to coordinate discrepancies.
What to do instead
Understand that NZGD2000 coordinates are propagated from ITRF96 using a deformation model. When working with precise GNSS, either use the official NZGD2000 coordinates (which include the deformation model) or transform ITRF coordinates to NZGD2000 using the model. Always state the epoch and frame of any coordinates.
In cadastral surveying, failing to prepare a proper survey report or plan that clearly documents the survey methodology, evidence found, and any decisions made, leading to a plan that cannot be understood or relied upon by others.
Candidates focus on the fieldwork and computation and treat reporting as an afterthought. They may not be familiar with the requirements of the Cadastral Survey Rules for plan documentation.
What to do instead
Use a standard checklist for plan and report content: datum, control, observations, reductions, evidence, adopted boundaries, and any discrepancies. Write clearly and concisely, including diagrams where helpful. Before submission, review the plan against the rules and ask a colleague to check it.