I came across this (above) thread and feel it's my duty to correct some misunderstandings throughout it.
Chainage is a linear distance measurement used for linear transportation infrastructure. Chainage can be either slope distance or horizontal distance. Generally, this is dependent on what stage of the project the chainage is referring to.
At construction planning phase, chainage will be horizontal as it corresponds to legal drawings and ROWs which are defined in 2D.
The surveyor will then relate the plan to real world, which will allow topographic (3D) information for better planning of material QTO, safety related design work, etc.
As-built chainage is the slope distance. For a road, the as-built chainage does actually represent the length of the road.