SPOT ELEVATIONS: WHEN TO USE THEM INSTEAD OF CONTOUR LINES
Are you having trouble deciding when to use spot elevations versus contour lines to determine your breaklines? Well, don't worry because you're not alone. We are often confronted with that question and are here to help you figure it out.
We can't jump right into answering that question without providing you with some terms related to this topic that are important for you to know. So, we will start with defining DEMs, DSMs, DTMs and TINs. A Digital Elevation Model (DEM) is often used as a generic term for Digital Surface Models (DSMs) and Digital Terrain Models (DTMs). A Triangulated Irregular Networks (TIN) is a representation of a continuous surface consisting entirely of triangular facets. These models provide a 3D representation of terrain elevation data. The difference between the DSM and the DTM is that the DSM includes all objects on the earth's surface, whereas a DTM represents only the bare ground. These models are used for creating topographic maps.
Creating, or even reading, topographic maps can be overwhelming because we live in a 3D world, and it's not necessarily easy to represent it all on a 2D map. For this reason, topographic maps are made up of different line types, colors, and shadings, which give you a visual of the changes in elevation and terrain shape. When it comes to grading plans, the quality of the plan is the key factor. Some reasons for this are: it will help you get your permits easier, and it allows your earthwork contractors to provide you with better estimates. Precise grading plans will provide you with better drainage management, accurate building foundation depths, and they will also account for the various landscaping features.
WHAT IS A SPOT ELEVATION?
A spot elevation is a point on a map that identifies the height above mean sea level. Spot elevations tell you the actual elevation of a particular point, both existing and proposed, which can be used for planning purposes and for creating 3D models. These points are critical for construction because they provide you with specific locations above or below their surroundings. 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.
Road Spot Grade Elevations in CAD
WHAT ARE CONTOUR LINES?
Contour lines are imaginary lines that join points of equal elevation on the land's surface either above or below a reference surface, such as mean sea level. These contour lines allow for measuring things such as mountain heights, ocean depths, and slope steepness. There are three types of contour lines: index, intermediate, and supplementary. Index lines are the thickest and are usually labeled with a number, making it a lot easier to know the elevation. Intermediate lines are thinner and don't typically have a number label on them. 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.
Contour Lines and Spot Grades in CAD
WHAT ARE BREAKLINES?
Breaklines are a way of manipulating the surface triangles. They are lines used to connect data representing a specific surface, such as the centerline of a road, a ridgeline, or a ditch's flowline. This line data forces the surface to create triangles that connect the vertices on the breakline. Breaklines are critical to creating an accurate surface model because it is not enough to have just the data alone; it is the interpolation of that data that gives you the model's shape.
WHEN SHOULD YOU USE SPOT ELEVATIONS vs. CONTOUR LINES TO DETERMINE BREAKLINES?
Contour lines can be molded differently in CAD, depending on the engineer. Some engineers build the DSM and then generate contours that directly trace that surface. This method can be good or bad depending on the quality of the surface they made. If you ever see jagged or zigzagging contours, that means that the engineer probably used a poor-quality surface to generate the contours. Suppose you see nice-looking curving contours that don’t hit any of your spot elevations or that cause flags in your breaklines. In that case, the engineer was probably trying to make aesthetically pleasing contours that don’t reflect the actual CAD model that s/he created. Rarely do engineers take the time to clean up their surface-generated contours or draw contours that accurately reflect their design intent.
Figuring out what type of contour you are dealing with will allow you to formulate a method for elevating a breakline when the contours and spot elevations don’t agree. If you have curvy contours, you may have to ignore a contour if it passes through a breakline at the wrong spot. The same goes for a jagged contour. Rarely are spot elevations superseded by a contour. They are created far more intentionally by the engineer, with thought going into each of them, rather than contours that can be made with a single button click. Regardless of which you choose, it is best to bring up any significant discrepancies with the engineer to check which one is right.
It is necessary to see what the design intent is once the data has been compiled and analyzed. You must check things like a watershed, ADA standards, and slopes to figure out what makes sense. For example, if you’re working on a parking lot that has a water basin but the water is puddling elsewhere and not flowing towards it then something is wrong and needs to be checked. It is through these combined efforts that you can best determine what may work or may not work. If you find yourself in a situation where you cannot figure out what your best course of action should be, contact ECI. Our team of virtual construction engineers is available to answer any questions you may still have on this topic.