Working with Surface Instructions - Tips and Tricks

Often when building corridor models for especially Takeoff purposes, I will use Surface Models generated from 3D Strings or 3D Linework (Digitized or imported) to build the Finished Grade surface model. I will also often use Cross Sections (PDF or CAD) to create the surfaces needed for Finished Grade and Subgrade Models.

I will also often use the Apply Site Improvements command to create the “Box Outs” for Subgrade from the Finished Grade Model to get out quick initial volumes to save me having to build complete subgrade models in the early stages of bid work.

When I have the surfaces for Existing, Finished Grade and Subgrade I will then build a Corridor Model for the project to run the volumes or carry out Mass Haul analysis work. These are some tips for you when using Surface Instructions to build your Corridors

When you create the surfaces for Subgrade Adjusted surfaces, you should change their surface properties so that they have the same surface classification of Design, so that they are treated the exact same way as a Finished Grade Surface (that should also be set to classification type Design). In the next release these will then be automatically the same settings in the template editor when used as reference surfaces (See (3) below). If they are both classified as Design, then the need to show them both as reference surfaces below in (2) is not so critical - however it can still be helpful.

When you create the corridor Model, you get to select Reference Surfaces. The natural tendency here is to add just the Existing Ground model so that you can see it in the Template Editor. I am recommending here that you also add the Finished Grade and Subgrade Models as reference surfaces. This is because when we then display the Template Editor, the Finished Grade and Subgrade Models are also added as Cross Sections, and that also gives you the ability to change the way the cross sections are used in the model (see (3) below)

The surfaces when used in the corridor model as Surface Instructions are then sliced by the template editor at the defined interval locations or wherever you pause the template editor at a specified station value. When we slice them there are two settings that you can access that affect the way the sections are sliced. You want these settings to be the same for both the Finished Grade and Subgrade Surfaces. When they have the same classification they will both be given the same settings in the model (when not used as reference surfaces), however we just found that when you add them as reference surfaces, they are not treated the same way - one gets the following settings set to Yes and the other gets the settings set to No and this is what causes the failure to tie out correctly problem shown in the video below.

Setting 1 - Remove Diagonal
When a cross section slices through a surface created using breaklines, it will create nodes in the sections where the slice cuts the breaklines and also where the slice cuts through the diagonal sides of a triangle. Take the example below

  • The two Red lines are breaklines that represent top and bottom of bank
  • The Black lines show the additional triangle sides created by a TIN Model
  • The Magenta line shows where we cut the cross section
  • The two Red Spots are points we always want in the model
  • The Black dot is an extra node in the section computed at the Section / TIN intersection that we often do not want in the model.

If the line elevations create a “twist” in the surface in this section, that will create a pair of triangles that can form in one of two ways, the two ways can generate a different result in the surface (causing a hump or a sag) in the resulting TIN Model. Removing the diagonal in this case will allow the section to be formed between the elevations of the two strings only, i.e. generating the correct slope of surface wherever it is sliced. Setting the Remove diagonal setting to Yes will help in this scenario.

Setting 2 - Adjust for arc/chord offset
When a cross section is cut through a curved section of a road, the curvature of the alignment causes a chording of the surface model, based on the Interval settings or other corridor / surface model settings (Alignment based, Density, Table locations etc.) to form the TIN Model. When we cut sections we can apply an Arc/Chord Offset to the sections to adjust for the curvature of the alignment at each offset point in the section. This has the effect of moving the nodes of a section away from the center line on the outside of the curve and towards the centerline on the inside of a curve. These settings can be applied to each surface used in the corridor model - however the Finished Grade and Subgrade surfaces need to be treated the same way - either both On or both Off - you should not have One Off and One On because that will create mismatches between the surfaces - The classification of the surfaces (Design, Existing etc.) should make them the same, but for example you may have a Finished Grade set to Design and a subgrade set to unclassified and that will create the mismatch.

If you have added the surfaces as reference surfaces, then in the template editor you will see them as surface cross sections - you can click on the sections and check those properties of the surface sections. If you do not add them as reference surfaces, you have no way to see those settings - I am confirming with development that the same classification does drive the exact same behavior settings.


In the Corridor Template editor, you will now add the surfaces for Finished Grade and Subgrade as Surface Instructions. These surface instructions are affected by the settings described above.

Typically the subgrade will tie up vertically automatically to the finished grade surface where the subgrade is completely beneath the finished grade

However along the edges of the model, depending on the data, this may or may not happen automatically. It is good practice to either add ties between subgrade ends up to Finished Grade or to add connectors between the ends of subgrade to the finished grade ends to close out the areas of the model - this should then give you closed areas and accurate volumes.

Lastly one more trick that I use is to sometimes create a surface edge breakline around the finished grade model. Then offset that line by 0.01’ inside to make it slightly smaller and then add that smaller offset line as a boundary to your subgrade adjusted model - that way you will find the two surfaces tie more reliably along the edges eg where you have applied topsoil removals to finished grade on all the embankments.

The video shows how all these things work