A referencing system for locations relative to a one-dimensional object (reference line) as measurement along (and optionally offset from) that object.<br/>A linear positioning system combines a line of reference (here: a collection of PositioningNetElements) with a single number (linear abscissa, i.e. an instance of class LinearCoordinate) allowing positioning within a railway network, at the chosen level (MACRO, MICRO...).<br/>Often called chainage (when yielding the actual "distance along"), or mileage system (when reflecting a conventional kilometric post or mile post system).<br/>The start- and end-measures (optional) provide the valid range of linear coordinates within the chosen linear positioning system. The difference between two coordinates is not necessarily the "true distance along" because of possible deviations (jumps or systematic errors), that are taken care of by anchor points and linear referencing methods. <br/>The units of linear abscissa are the same as the start and end-measure. <br/>For reference, see EN 19148:2021. ISO parameter DistanceAlongDirection is "same", implicitly.<br/><br/>