A milepost equation, milepoint equation, or postmile equation is a place where mileposts on a linear feature, such as a highway or rail line, fail to increase normally, usually due to realignment or changes in planned alignment.[1][2] In order to make mileposts consistent with the real mileage, every milepost beyond the equation would need to be moved.[3]
For example, an equation of 7.6 back = 9.2 ahead means that the feature does not have any section between mile 7.6 and mile 9.2, and the distance between mileposts 7 and 10 is only 1.4 miles. This would usually be caused by a relocation that shortened the distance by 1.6 miles. It is also possible for an equation to add mileage to what it would otherwise be; the duplicated mileposts receive a special prefix, such as Z.[1]
