Using Survey Offsets in AutoCAD Civil 3D

Collecting linework in the field is standard practice for today’s surveyor. In AutoCAD Civil 3D, we use linework code sets with our standard field descriptions to define edges for any linear object observed on site. The challenge comes when picking up consistent cross-section features such as curb and gutter. The long way to do this is to collect the edge of pavement, the flow line, the top, and, if necessary, the back of curb at set intervals while making our way up the road.

This really isn’t necessary when the curb and gutter has been cast-in-place, unless it has been badly damaged or worn down over time. Concrete holds up very well over time, so the sections will stay consistent. Asphalt or stone may settle so those materials may require manual shots to be taken to get accurate survey data.

A lot of the time, we have curb and gutter in perfect shape.

In the linework code sets of AutoCAD Civil 3D, we have tools to collect horizontal and vertical offsets:

Using the horizontal and vertical offset codes can be tedious. To issue the linework codes for the offsets, we must collect the point at the edge of pavement and enter the code for a standard curb and gutter section in the data collector in this manner:

EP2 H2 V-.167 H2.5 V.33

This translates to:

1. Create a stepped offset at 2’ to right of the EP and 2” lower than the EP.
2. Create another stepped offset at 2.5’ to the right of the EP and 4” higher than the EP.

Thereafter, every EP2 collected would continue applying the offsets until a stop offset code is entered:

EP2 SO

There is a way to save us from monotonous code entry.

What if instead of a long string of offset codes we had a code that described the standard DOT, city, or county curb and gutter? In the field, one short code representing this detail could be entered: for example, CG1, or CG2, etc.

When the field data is imported into AutoCAD Civil 3D, any time this code is used, a cross-section with linework needs to be generated. We can produce the cross-section and linework quickly using any of the following methods:

1. AutoCAD Offset: Survey figures are related to feature lines. When a survey figure is offset, it produces a feature line and retains the elevation of the survey figure. This feature line could then be raised or lowered as required by the cross section. The advantages are that linework is generated and this linework can be used to accurately depict the curb and gutter in the surface. The disadvantage is that to generate one breakline, this process requires the use of two commands and the manual entry of distance and elevation offsets.
   
2. Civil 3D Stepped Offset: This one command would do the work of the AutoCAD offset and raise/lower command. The advantages are the same as the AutoCAD offset process and that only one command is necessary. The disadvantage is that manual entry of distance and elevation offsets are still required to generate one breakline.
   
3. Civil 3D Assemblies: Since a figure is so closely related to a feature line, an assembly can be used to create the cross-sectional shape as well as the linework. In the image below, the offsets on the left were created with field codes as the label suggests. The offsets on the right were created using an assembly in combination with a plan-friendly code set. The advantages are the same, however the linework produced is segmented and not truly curved. But there is no manual entry of numeric data since assemblies representing these standard curb and gutter shapes can be defined in advance in the standard survey template. The Create Corridor dialog where we select the assembly is the only interaction we have with the software. If the wrong assembly is selected, there is no need to delete and start over; we just need to swap out the wrong assembly with the right assembly.
   

Creating consistent offset linework in the field has always been tedious, with either having to collect multiple shots to define a cross section at every collection point or entering in a combination of horizontal and vertical offset codes manually. A happy medium would be to identify the cross-section with a standard code and apply the cross-section in the office by someone who isn’t trying to out-race time and weather to complete the survey of the entire site. There is more than one way to do this, but the result gives us intelligent information and an accurate surface based on what was observed in the field.

– Cyndy Davenport