Determining how far an object is from you often requires comparison against a second object of known distance.

Then someone else a known distance away simultaneously takes the same measurement.

You must mark on the shore two points with known distance between them (base line).

The time delay can be varied up to the maximum corresponding to the flow time over that known distance.

Before the availability of such technology the police would time drivers over a known distance to calculate their speed.

Grain diameter is calculated from a known distance and time of the fall of the particle.

Now you need some sighting device of known size held at a known distance.

The computer's reading is then compared with the known distance and any necessary corrections made.

The time that it subsequently takes for the particle to reach a detector at a known distance is measured.

These cameras time vehicles over a known fixed distance, then calculate the vehicle's average speed for the journey.