Length (km and Tape Measure)

Reading the scale on a tape measure comes first, and 1 km is grasped as a block of 1000 m. Conversions from km to m and from m to km and m, and down as far as m to mm, are covered, along with adding and subtracting lengths and reading straight-line distance and route distance off a map.

Misreading 1 km = 1000 m as 100 m turns 2 km 300 m into 230 m rather than 2300 m, and counting the zeros settles it. On the distance-versus-route question, children take the straight-line length on a U-Shaped Map and add that up as though it were the route.

The sense of expressing length in cm and mm stays the same here. What changes is the addition of a new unit, km. The idea of rewriting one unit as another, built up in Length (cm, mm, m) in grade 2, is later replaced by a form that uses decimals, once Decimal Units (Capacity and Length) is studied.