Spatial / GIS
A GIS combines geometry, attributes and coordinate reference information. Know the data models - vector (point, line, polygon) and raster (cells, resolution) - and the operations: selection by attribute or location, buffer, clip, intersect, union, spatial join, dissolve and network analysis. Know that topology rules enforce logical consistency (no gaps, no overlaps, polygons closed), and that geoprocessing inherits the accuracy of its worst input.
Coordinate reference systems are the most examined topic. Distinguish the datum (NZGD2000) from the projection (NZTM2000) and from the EPSG code (2193 for NZTM2000, 4167 for NZGD2000 geographic, 4326 for WGS84). Reprojection does not improve accuracy: it changes how existing accuracy is expressed and may introduce transformation error if a datum shift is applied. Data quality is assessed on positional accuracy, attribute accuracy, logical consistency, completeness, temporal accuracy and lineage - all recorded in metadata.
Traps
- Confusing a datum with a projection, or assuming an on-the-fly reprojection in a GIS has changed the stored data.
- Mixing DEM (bare earth) with DSM (includes buildings and vegetation) when calculating flood levels or solar exposure.
- Buffering in a projected CRS using grid metres when the specification is ground metres - the scale factor applies, though in NZTM the effect is small.
- Treating a raster cell value as a point measurement rather than an average over the cell.
- Coordinate transposition (x/y swapped) when importing CSV or shapefiles.
Worked example
Area is scaled by the square of the scale factor. At k = 1.0001, k squared = 1.0002, so a 1 hectare grid-computed area corresponds to about 10 002 m2 on the ground - only 2 m2 per hectare, negligible for most purposes but worth stating. At k = 1.0009 the discrepancy rises to about 18 m2 per hectare.