A clock provides a linear reference to measure duration of motion. Similarly, there is a linear reference to measure length of movement. What is this linear reference?

In mapping and geographic information systems (GIS) a *linear referencing system* (LRS) “is a method of spatial referencing, in which the locations of features are described in terms of measurements along a linear element, from a defined starting point, for example a milestone along a road.”

This may be extended to 3D space by a *reference frame*, “a space-time coordinate system and set of reference points in space-time that assigns unique space positions and reference durations.” From such a reference frame, one can derive linear references from path lengths.

Alternately, one may attach an odometer (cyclometer, pedometer) to each vehicle or subject in motion, and measure their transit length directly.

Thus there is a strict parallel between the reference provided by a clock and a linear reference such as an odometer. As the former is said to constitute time, an ordering by duration, so the latter constitutes an ordering of space by length.

*Related*