Corridor Template Model and Corridor Surface Model - What is the difference?

When you create a Corridor Model using the TBC Corridor Model functions, you will use the Template Editor to create the Corridor Model. This model is 100% accurate everywhere - i.e. when you say show me the section at Station 123.24’ it computes that for you exactly based on the template Instructions that exist.

The Corridor Model however is asked to then generate a Surface Model output that can be visualized in Plan, 3D and Cross Section Editors / Surface Slicers etc. and that can be sent out to the field for use by Site Positioning, Surveying and Machine Control Systems. This Corridor Surface Model is computed from the Template Model at specified locations along the road, so that the Corridor Surface Model closely resembles the Corridor Template Model. It will however have minor differences depending on where the cross section drops are placed, how far apart the section drops are and depending on whether the section drops are actually created as you thought / hoped.

When you build a corridor, if you are going to transition things over very short distances, I would not push this right up against the tolerance of .016’ (.02’), you might want to loosen that up to .05’ or something so that the section drops are a little further apart in the corridor surface model - however 0.02 should be the smallest gap that you use and if you use that then you need to have that tolerance set to Best and BL across in order for it to be effective.

The tolerance is there so that people do not create sections every 0.01 down the alignment and it does cause you to have a clean approach to how you define your tables. You do have to watch what happens with the HAL and VAL points as well.

The Corridor Surface will be formed with Section Drops at

  1. HAL Points (segment ends)

  2. VAL Points (segment Ends)

  3. Superelevation Points (transitions in cross slopes defined by Stations computed on HAL)

  4. Shareable Slope Table Points

  5. All Table Instructions where you use a Table defining Stations (for any instruction and Offsets, Elevations, Slopes, Sideslopes etc.)

  6. All Instructions that use 2D lines either stand alone or in tables - the node points of the 2D lines will create section drops

  7. Interval Locations defined in Project Settings, Computations - Corridors e.g. 25’

7a) Interval locations defined in a Template properties (which will override the default intervals for the range of that template)

  1. Template Locations - all Template Start Stations

  2. Densification locations - if the Corridor Surface Models properties are set to Alignment Based and Densification is turned on then the model is densified using the HAL and VAL curvature as well as where Slopes and width changes are happening

  3. Additional Stations defined in Define Extra stations command

The above creates a list of Stations, and starting from the low station the software checks to see if a station of one section drop is closer to the previous station drop than the corridor tolerance setting, if it is it can be thrown out and then the surface model derived from the remaining sections may be wrong (depending on where the next drop location happens to be). You need to become aware of this when building a model and try to avoid getting 2 sections too close to each other by using appropriate separations in your tables etc.

Hope that this shines a light on a few Corridor Modeling tips that you need to use to be successful

  1. Use a gap of 0.02’ minimum between two stations in a Table
  2. Set the Road Tolerance to Best and BL Across
  3. Watch your section drop locations to make sure that you are getting all the ones that you want and that none are getting skipped
  4. Be aware of all the things that can generate section drops and how those aggregate together to cause a drop for tolerance reasons, and if you see that happen, find the instructions or causes for those locations and move them slightly so that they do not get dropped
  5. Be consistent in where you create table or 2D line locations - it helps to create a master list somewhere in Excel, to run checks on this as you build out complex models.
  6. Be consistent in the use of step distances - if you are using 0.02 or 0.05, always use the same values unless there is good reason to change.
  7. Be consistent on all instructions as to where transitions will start and end - i.e. where possible always start and end them at the same stations.

With these in mind you will be more successful when building corridors.