Cartography and Map Projections
This domain tests projection properties, scale conversions, north references, and map accuracy standards. Expect questions that require you to choose a projection for a mapping task, convert between map and ground distances, and interpret accuracy statements correctly.
Projection properties
- Conformal projections preserve local angles and shape; scale at a point is the same in all directions but varies across the map. Lambert Conformal Conic and Transverse Mercator are conformal.
- Equal-area (equivalent) projections preserve area at the cost of shape, such as Albers for national small-scale maps. Equidistant projections preserve distance from a point or along a standard line only.
- Every projection distorts something; the choice follows the measurement the map must support.
Scale and accuracy
- Scale is a ratio: 1:1,200 is large scale (much detail, small area); 1:1,000,000 is small scale. A graphic scale bar survives reproduction and scanning better than a representative fraction.
- At 1:24,000, one inch equals 2,000 ft and one centimeter equals 240 m. Contour intervals are set so adjacent contours remain legible, typically about 0.5 mm apart on the printed map.
- Horizontal accuracy under national map accuracy practice is reported at 95 percent confidence as the RMSE multiplied by 1.7308; vertical accuracy uses the RMSE multiplied by 1.9600.
- True north, grid north, and magnetic north differ. Convergence separates grid from true north; magnetic declination separates magnetic from true north and changes over time. Magnetic declination is not the same quantity as convergence.
Worked example
A plan is at 1:24,000 and a line measures 3.5 in on the sheet. Ground distance = 3.5 times 24,000 = 84,000 in = 7,000 ft, which is 7,000 / 5,280 = 1.33 miles. Check the shortcut: at 1:24,000 one inch represents 24,000/12 = 2,000 ft, so 3.5 in is 7,000 ft. For the inverse, a 1-mile ground line on a 1:62,500 map measures 5,280 ft times 12 = 63,360 in, divided by 62,500 = 1.014 in.
Traps that cost marks
Reversing large and small scale; trusting the printed ratio on a scanned or photocopied map instead of the scale bar; treating grid north as true north; using 1.9600 as the horizontal 95 percent factor; assuming a conformal projection preserves distance; reporting the RMSE itself as the map accuracy.