Surveying, Mapping and Field Measurement for Conservation Works
Surveying, Mapping and Field Measurement tests the instruments and arithmetic used to lay out conservation works: chain and tape, prismatic compass, dumpy level, contour surveys, planimeter, total station and GPS, plus scale and map reading.
Core ideas
- Chain/tape survey measures horizontal distances; compass measures bearings; levelling measures vertical differences with a dumpy or auto level and staff.
- Contouring joins points of equal elevation; the contour interval is the vertical gap between successive contours, chosen from scale, relief and purpose.
- Horizontal equivalent = contour interval / ground slope.
- Representative fraction (RF) = map distance / ground distance, e.g. 1:5000; it is a pure ratio with no units.
- A planimeter measures area on a map; GPS gives position, and remote sensing/GIS give land use and slope layers.
Worked example
Map scale 1:5000; a bund measures 4 cm on the map. Ground distance = 4 cm × 5000 = 20,000 cm = 200 m. Reasoning: multiply the map distance by the denominator of the RF, then convert centimetres to metres by dividing by 100.
Second check: with a 2 m contour interval on a uniform 4% slope, horizontal equivalent = 2/0.04 = 50 m between contours.
Traps
Writing a unit after the RF; contour lines crossing each other (impossible on a normal map); reading a V in a valley as pointing downhill (it points upstream); confusing contour interval with horizontal equivalent; using sloping distance instead of horizontal distance in chain survey.