Before jumping straight in, spend a few minutes to decide on which method you will utilise in your surface modelling.

Simple surfaces just need to be slapped together and used as part of a bigger model, or moved into place without so much as another thought given to it. Others, while simple, may need optimisation or re-modelling further down the track. Others are more complex and need an approach that will give the best result while allowing for easy future changes, whether that be you or a colleague who is unfamiliar with the model.

Let us have a look at the different surface modelling options available in Civil 3D:

Modelling method comparison table

Method Setup time Adjustment time Best use Issues
AutoCAD entities and objects 1 (quickest) 3 (slow) Simple models needing few changes – Not dynamic so can be times consuming to iterate designs

– Care needed to ensure grades and distances are correct

Gradings 2 2 Non-linear dynamic models – Does not always work as you hoped

– Can be buggy and break or cause lock-ups

– Care needed to ensure correct use of Sites

Corridors 3 1 Simple to complex linear and non-linear dynamic models – Takes more time to setup

– Care needed with targets, chainages, link coding

A mixture of the above 4 (Slowest) 2 Complex models – Can be very complex so needs strict naming conventions

AutoCAD entities and objects

A useful method for creating quick and dirty surfaces. These models are only dynamic in the sense that whatever entity you adjust will updated in the surface directly, but nothing else will change. You must adjust each entity separately to keep slopes and offsets, for example. Tip: AutoCAD points will not update if you move them, use zero-length polylines instead.


These get little use in my workflow purely down to being unreliable and prone to lock-ups. Use clean the feature lines to have the least number of vertices possible if the model demands them. Separate multiple gradings  into individual Sites if you do not want them to interact with each other. Warning: Upon editing a grading, infills will disappear at will! Sometimes Gradings work brilliantly and other times you cannot get them to do the simplest thing correctly.


Although there is more involved in getting a Corridor up and running, for models needing constant editing and tweaking they are a real time-saver. They will help stave off greying hair more successfully that with the previous two modelling methods. Once you have been doing them for a while it becomes second nature and you will not notice the effort. It goes like this: Create Alignment > Surface Profile > Design Profile > Assembly > Corridor, then edit targets, chainages, assembly frequency, create surfaces. But that is a for another post. If you must adjust the design a dozen times, simply dragging a few alignment vertices or sub-assembly links that update the entire model will save you time.

A mixture of the above

Often the final model will be a composite composed of a corridor surface and multiple entity-based and sometimes grading surfaces. The trick is to be careful and descriptive with naming all the various surfaces so when it comes to ordering them in the Surface Properties Definition dialogue box so it is clear which is which and in what order they go. There is something satisfying about ticking all the boxes with everything in order and the model miraculously looks correct first go!

A note on boundaries

With all of these modelling techniques it is best to use a surface boundary polyline created by intersecting the surface with the topography or other base surface. Grading to a surface, especially when the feature line has a lot of vertices, can be slow and easily cause a lock-up and loss of work. Using corridor assemblies with links that daylight to a surface also slows things down and only accurately intersects the surface at the specified assembly frequency. Also, the start and end of the corridor can get messy. The only time we use a daylight link in an assembly is when using conditional sub-assemblies that need to know where a surface is in order apply the condition.


When it comes to creating your design surfaces, the above is just a quick rundown of the difference methods available to you. As with everything in Civil 3D there is a lot more depth in each of these options and as you explore them you will gain an instinctive understanding of which to use and when.

Feel free to leave any questions or comments below.

The CADD Monkey.


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