Topographic and Cadastral Mapping
This domain tests whether you understand a map as a generalised, dated, specified product rather than a picture of reality. Topographic mapping questions focus on specification and quality control; cadastral questions focus on what a boundary plan does and does not prove.
Topographic mapping
Hong Kong's large-scale mapping is produced by the Survey and Mapping Office of the Lands Department, with digital 1:1000 topographic data (the iB1000 series) and smaller-scale derivatives, plus orthophotography. A specification fixes: the datum and projection, the scale, feature classification and coding, positional accuracy in plan and height, contour interval, generalisation rules, and the currency of the survey. Accuracy is normally stated as a root-mean-square error checked against independently surveyed points, not against the same data used to build the map.
Cadastral mapping
Cadastral products include lot index plans and land boundary plans showing lot numbers, boundary dimensions and areas. A boundary plan is a record of the lease geometry; it is not a guarantee that a peg, wall or fence in the field sits on the line, and it does not resolve occupation or title. Where the plan and the ground disagree, the surveyor must establish and report the discrepancy with its uncertainty rather than adjusting the plan silently.
Production methods
- Ground survey by total station and level, with control established from the geodetic network.
- Photogrammetry and LiDAR, which need ground control points and independent check points.
- UAV survey, which needs ground control, sufficient overlap and clear statements on vegetation and shadow areas.
- Digitising from existing mapping, which carries forward the errors of the source and must be labelled as such.
Worked example
A topographic survey is required for a slope site with a 50 mm vertical accuracy requirement. Control is transferred by closed level run to two temporary benchmarks; ground detail is fixed by total station from two control stations. Twenty check points are independently observed and compared with the interpolated model. The mean difference is 12 mm and the RMSE 28 mm, so the requirement is met. The survey report must record the datum, the control hierarchy, the instrument, the check point results and the date, because a later user will otherwise not know what the model represents.
Traps that cost marks
Treating a cadastral plan as a title document or as proof of occupation. Mixing data from different scales and epochs. Quoting a manufacturer's accuracy as the map accuracy. Omitting the datum from a drawing so nobody knows whether the levels are mPD.