Baseball Field with contour lines and spot grades

Breaklines are essential for creating a precise 3D surface model. It is the data interpolation that establishes the model's shape. Breaklines connect data representing specific features on a surface, such as the centerline of a road, a ridgeline, or a ditch's flowline. This line data manipulates the surface triangles that connect the vertices on the breakline. Whether to use spot elevations or contour lines to determine your breaklines is not always clear. Here, we help shed some light on the differences between contour lines and spot elevations and provide guidance on when to use which one.

CONTOUR LINES AND SPOT ELEVATIONS - 101

Truck turnaround & wall with spot grades

Contour lines are imaginary lines that connect points of equal elevation on the land's surface either above or below a reference surface, for example, mean sea level. A spot elevation is a point on a map that specifically identifies the height above mean sea level. Either of these can be used to determine breaklines, and knowing which one to use in your model depends on the design's intent. Contour lines allow for measuring mountain heights, ocean depths, and slope steepness. Spot elevations tell you the actual elevation of a distinct point (existing and proposed). For example, spot elevation points tell you the elevation of various drainage features and give you the top and bottom of retaining walls, among other things. These points are critical for construction because they provide specific locations above or below their surroundings. Contour lines exist in three formats: index, intermediate, and supplementary. Index lines are the thickest and are usually labeled with a number, making it much easier to know the elevation. Intermediate lines are thinner and don't typically have a number label. These lines are more common and can be found between the index lines. Finally, the dotted lines that indicate the flatter terrain are called supplementary lines. Contour intervals give you the change in elevation between any two contour lines.

EVALUATE THE CONTOURS

Figuring out what type of contour you are dealing with helps you choose a method for elevating a breakline when the contours and spot elevations don't agree. For example, if the contours are curvy or jagged, you may have to disregard a contour if it passes through a breakline at the wrong location. It is not common for spot elevations to be superseded by a contour. They are created far more intentionally by the engineer. Regardless of your choice, it is best to bring up any significant discrepancies with the engineer to check which one is right.

Spot grade layout of a distribution center parking lot

Depending on the engineer's process, contour lines can be molded differently in CAD. Some engineers build the DSM and then generate contours that directly trace that surface. This method can be effective or not, depending on the quality of the surface they've created. It is not common for engineers to take the time to clean up their surface-generated contours or to draw contours that accurately reflect their design intent. You may be able to tell based on how smooth or bumpy the contours look. Ideally, the contours will be made to look nice and clean without hitting any of the spot elevations and without causing any errors in your breaklines. When contours aren't smooth, it is usually an indication of the engineer having used a poor-quality surface to generate them.

GO THE EXTRA MILE WITH ECI

The best approach is always to be diligent about checking for watersheds, ADA standards, and slopes. If these checks are not already a part of your process, then you should add them to your checklist until they become second nature. Having a keen eye for these details sets a good model apart from a poor one. We often find situations where water will puddle and not flow towards the basin in parking lots. We advise our clients of this before they begin construction, saving their project from being delayed or costing more money due to change orders. It is experience, fast turnaround times, and the early identification of issues that sets ECI Technologies apart from our competitors. Contact us to see for yourself what a seamless process it is to include our engineering into your existing workflows.