Using maps to measure

Geographers, planners and resource scientists use maps to measure distance, direction and area.

Distance:

• Straight features (roads, railway lines, canals): measure directly with a pair of dividers or a scale.
• Winding features (coastlines, rivers, streams): lay a thread along the line from start to end, then stretch it and measure it; or roll the wheel of a rotameter along the route.
• Then convert the map distance to ground distance using the map's scale.

Direction:

• Direction is an imaginary line showing the angle from a common base direction. North is the zero or base direction, and every map shows it.
• The four cardinal points are North, South, East and West; between them are intermediate directions such as north-east and south-west.

Area:

• Squares method (simple but not very accurate): put graph paper under the map on a lit tracing table, or trace the area onto squared paper. Count the whole squares and the partial squares; partial squares are counted as halves. Area = (whole squares + partial squares ÷ 2) × the ground area of one square.
• Polar planimeter: one end of its arm is fixed; the tracing point is moved clockwise around the boundary, back to the starting point. The dial readings before and after, multiplied by the instrument's constant, give the area.

From the video: the teacher notes that these hand methods are now mostly taught rather than used: today remote sensing, GIS and GPS work together, and a phone map measures distance and direction in seconds. Knowing how the old tools work still explains what the digital ones do.

Example (new): on a map, a region covers 36 whole squares and 14 partial squares, and each square stands for 4 sq km on the ground. The partial squares count as 7, so 43 squares × 4 = 172 sq km.