What is a Station Equation and How DO They Work?
Both great questions - here is some information that may help you understand how they are used.
A Station Equation on an Alignment is entered in the Station Equation tab of an alignment in the alignment editor. The station equations are always defined using an “Ahead Station” and a “Back Station” value. These equations will normally be defined in the plans for highways projects.
Typically Station Equations are used in Road Improvement schemes where an alignment is being changed to provide better highway operations on a stretch of road - e.g. a widening scheme or a realignment of a road to increase its design driving speed etc. The new alignment ties into the existing alignment at various points (at least at the start and end, but also often at various points in between. At the points where the new alignment rejoins the existing alignment there are effectively 2 station values - one for the original alignment and one for the new alignment which may be shorter or longer than the original alignment at that point.
The following table in TBC Alignment Editor shows a typical alignment with Station Equations applied.
You can see that at all Station Equation Points there is a Back and an Ahead station. At each station equation the road is broken up into “Station Equation Zones”, those zones are then denoted in the alignment station values presented in the software using the nomenclature Station:Zone e.g. 10+00:1 - you can see the :1 or :2 etc in the Station Zone list of the dialog.
In some cases you can get overlapping stations i.e. the Back Station is a greater station value than the Ahead station for the equation - this means that the zone becomes critical when entering station information because a single station value in the alignment can have two possible locations - therefore it is made unique using the zone number as a part of the station entry i.e. if you look at the second station equation in this list you have a back station of 106+14.73 and an ahead station of 106+04.15.
This means that Zone 2 ends at station 106+14.73 and zone 3 starts at station 106+04.15. This also means that a station of 106+10.00 can be in Zone 2 but also in Zone 3 so you would need to enter 106+10:2 or 106+10:3 depending on where on the alignment you want to enter some data etc. if the data location entry happens to fall at that station. If you do not enter a Station Equation when entering data using Station, TBC will assume that you are using the first station equation zone that has that value within it i.e. if I create a point by entering station 106+10.00 it will place it at 106+10.00:2. If I enter it with the station equation zone then I can control which zone the point is created in.
If I use e.g. the Explore Object command to explore the stationing on an alignment, when I approach a station equation point from the down station end of the alignment the read out of the stationing up to the equation point will match up with the Back Station value at the equation point. When I cross the station equation location, the stationing will then read out as the station with reference to the Ahead Station at the equation point.
So for example if my equation is Back 106+14.73 and Ahead 106+04.15 then at 0.01 before the equation point my stationing will read 106+14.72:2 and at 0.01 after the equation point my stationing will read out 106+04.16:3.
Within a Station Equation Zone therefore the Start Station in the Zone is the Ahead Station at the Equation at the start of the Zone and the End Station is the Back Station at the equation at the end of the zone. There will be either a jump (skip a range of stations) or an overlap (station rolls back by an amount) as you cross a station equation.
TBC will label station equations on alignments, on mass haul diagrams, on profile drawings in the following manner
In profiles and mass haul diagrams the station equations are labeled like a Station Label on the X axis of the profile or chart. Typically the profile view shows the Back Station at each station equation, the label being written to the down station side of a vertical grid line denoting the station equation. When you create Profile or Mass Haul drawings both the Back and Ahead labels will be drawn as shown below - in this case the Red Grid Line indicates the Station Equation
If you take an alignment out into the field using e.g. Trimble SCS900 or Siteworks, the station equations are managed in the same way - as you walk up an alignment towards a Station Equation the Stationing will display the Station and Equation Zone in the same way. As you walk up station and cross the Station Equation location, the stationing will jump to the stationing defined by the Ahead station on the up station side of the station equation and will indicate the zone change using the :1 nomenclature. if you walk down station and cross an equation the stationing will jump to the back stationing defined at the equation point.
So between two equations the station at the start of the zone is the Ahead Station at the first equation and at the end of the zone it is the Back Station at the second equation. Within the zone the stationing is consistent and is denoted by the zone number in the station values presented.
Hope that this helps you to understand Station Equations