Cartography, Coordinate Systems, and Map Projections
Cartography, Coordinate Systems, and Map Projections
This domain tests map design, scale, projection choice, grid references, and datum transformations, with emphasis on the distortions introduced by flattening the Earth.
Core rules and concepts
- Map scale is the ratio of map distance to ground distance. Large-scale maps show more detail over smaller areas.
- Generalization simplifies, classifies, symbolises, and displaces features to suit scale and purpose.
- Projections distort distance, area, shape, or direction. Mercator preserves angles but exaggerates area at high latitudes; UTM is a transverse Mercator grid with a central-meridian scale factor of 0.9996.
- Grid convergence is the angle between grid north and true north. Datum transformations shift coordinates between reference frames.
- Web Mercator is unsuitable for accurate distance and area measurement.
Worked example
A ground distance of 1,000 m lies at the central meridian of a UTM zone. The grid distance is approximately 1,000 × 0.9996 = 999.6 m. Away from the central meridian, scale factor increases; combined factor also accounts for elevation above the ellipsoid. A survey that mixes ground distances and grid distances without a combined factor will misclose.
Traps
- Measuring area or distance on a Web Mercator map.
- Confusing grid north with true north when reporting bearings.
- Applying a datum transformation in the wrong direction.
- Using a UTM zone outside its designed longitude band without recognising distortion.