What is a Landranger?

What is a ratio of two numbers that have different units?

If the measurements are of like things (eg distances) to find the ratio convert one of them into the units of the other, then the units an be dropped and the ratio simplified; eg: What is the ratio of 2 cm = 1 km? 1 m = 100 cm 1 km = 1000 m = 1000 × 100 cm = 100,000 cm → 2 cm : 1 km = 2 cm : 100,000 cm = 2 : 100,000 = 1 : 50,000 (This is the scale of the OS Landranger maps). If they are different things, then the units cannot be dropped, and the ratio requires them, eg making a brine solution may require 1 g of salt per 100 ml of distilled water the ratio of salt : water = 1 g : 100 ml.