One of the instruments used to survey the track is the rail shoe – an L shaped device placed horizontally on the rail head, that touches laterally the running edge at 14mm below its horizontal arm – see figure 1. The horizontal placement of the shoe is checked using a spirit level. There might be variations of the device from what figure 1 shows, but the principle is the same.
The device measures a point X, located at the intersection of the level plane tangent to the rail head and of the vertical plane passing through the running edge at 14mm below the level plane. That point is the one we find in the rail running edge strings of the survey CAD files (see Figure 2).
When the track is canted, a difference in level exists between the level planes of the two rail shoes used for measurement – see figure 2.
This level difference is equal in value to the track cant.
So, the answer to the question in the title is: “Yes, the survey cant is the actual cant!”
( If you are not careful, I will now confuse you! )
… However, the position of the points that define the cant, understood as the transversal inclination of the track, have changed. On track, the level difference that defines the cant is measured to the top of the rail heads. That is what defines the plane of rail and the actual inclination of the track (the angle α in figure 3).
The survey output, due to the nature of the measurement, translates these points from the cant base to a Gauge base. Although the cant value in mm is practically the same, the inclination angle in the survey output αRE , is different, steeper than the actual inclination angle of the track, α. The difference is not as dramatic as Figure 3 might suggest. But it is sometimes significant.
Is it anything wrong with this?
There is no mistake here. That’s the way the track is and has always been measured. The point X allows us to get two main cross-sectional properties of the track: the cant and the track gauge.
We just need to be aware of what these points really are and what the survey data gives us!