How accurate is your existing ground surface: Creating Surfaces from various data types

Kate Brown
Jan 1
2 min read

Updated: Jan 4

There are a many ways to create surfaces in Civil 3D. I knew some of the "best practices" that Autodesk recommends for creating surfaces, but I wanted to do more of a deep dive into raw data types and how Civil 3D handled that data.

Raw Data Types

I collaborated with our GIS team to have public LiDAR data exported in multiple formats. Each dataset uses the same source data and matching extents, except for the xyz/csv files. For those files, the GIS team resampled the data in half to keep the points close to 1 million points.

XYZ/CSV point files: Point file containing x, y, and z data.
Geotiff DEM file: Gridded looking tin data that a surface can be created from.
ASCII Grid DEM file: Gridded looking tin data that a surface can be created from.
ASCII text file: Basically a specialty xyz file.
CAD file: CAD file with polyline contours with elevations assigned to the polylines.

Surfaces Created

Import XYZ/CSV files as Cogo Points: Import points as cogo points, then create the surface using those cogo points
Surface using XYZ/CSV point files: Create the surface, then point the the point file as a definition
Geotiff DEM file: DEM file surface**
ASCII Grid DEM file: DEM file surface**
ASCII text file: DEM file surface**
CAD file Polylines to surface as breaklines: Add polylines as breaklines to create the surface
CAD file Polylines to surface as contours: Add polylines as contours to create the surface

**At our company, based on testing, we do not create surfaces directly from DEM files. Instead, we’ve found the most reliable workflow is to first create an empty surface in Toolspace and then add the DEM as a surface definition.

Surface Information and raw data Results

Each surface was created in a separate dwg file in order to see file size once the surface is created.

Although surfaces built from polylines initially seem to produce the smallest file size, the next section shows why this method is not ideal. The note about shifted data refers to our observation that ASCII DEM files often convert to International Feet instead of staying in U.S. Survey Feet. See this blog post for an explanation.

Profiles of all surfaces

All surfaces were data-shortcutted into one file and then ran an alignment and profile through the data set.

The accuracy differences between the surfaces are evident, with the polyline-based surface showing a substantial loss in precision.

Final Thoughts

At our company, our best practice is to use GeoTIFF DEM files when creating surfaces. We add the DEM only as a surface definition to avoid potential coordinate shifting and to ensure we are working with the most accurate existing-ground data for design.

Thanks for stopping by the Den! Civil 3D: It’s not a bug, it’s a feature. Allegedly.