Using Surveyors to Map Contours

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What is a Contour?
A contour line-known simply as a “contour” in the mapping business-is a line drawn on a topographic map connecting a series of points that all have the same elevation. A topographic map shows both man-made features-roads, towns, and political boundaries-as well as natural features, such as lakes, rivers, and all sorts of terrain. The squiggly lines one sees on the map are the contours.

Although various methods have been used historically from which to measure a base line, today’s standard involves using mean sea level as the “zero” or starting point. Primary contours will have their elevation marked somewhere along the line. Depending upon the map scale, there may be a contour line for every 10-meter rise in elevation, or every 20 meters, or every 50 meters, and so on.

Secondary contours will remain unmarked but nonetheless are an equal representative height from each other. For example, with two primary contours marked 1000m and 1100m, if there are five secondary contours in between, we know that each shows a 20-meter change in elevation. If contours are widely spaced, we know that the area is relatively flat. If they are bunched close together, it is clear this location is steep.

Why Are Contours Important?
Topographic maps are employed by a wide variety of individuals for various purposes. Hikers and campers will use them to determine how steep or level an area might be. This is important because it’s good to know if your planned campsite area is flat enough to pitch a tent. One can also make informed decisions on the best hiking trail to traverse based upon its relative steepness to alternative trails. In the construction business, contours are immensely valuable for road and building design.

How Are Contours Identified?
During the mapping process, contours can be determined by several different methods, both on the ground and from the air. Surveyors use sophisticated equipment such as total stations (electronic versions of the old surveyor’s transit) to map out contours onsite, while it is possible to cover much larger areas from the air by using a LiDAR (Light Detection and Ranging) sensor in conjunction with other sophisticated airborne equipment, such as GPS navigation.

A primitive way to map contours on the ground, but one that easily illustrates the same process undertaken by electronic instruments, is through the use of a line level. A string of a specific length (oftentimes eight meters) is attached between two poles of equal height. A builder’s level-the kind with a bubble suspended in some sort of liquid-hangs midway between the two poles.

Person A stands at the starting point in a field, holding his pole perfectly upright. Person B walks to the next target area and plants his pole so that the string is taut. A third person observes the level and indicates verbally or with hand motions whether Person B should move forward or back until the bubble is in the exact center of the level. Once this state is achieved, the supervisor marks both pole locations with a rock or a wooden stake. Person B remains stationary while Person A proceeds across the field and plants his pole accordingly. By leapfrogging each other in this manner, a line can be drawn from stake to stake that shows a contour line of equal elevation.

Measuring a Slope
The same primitive method can be used to measure how high one point is in relation to another. This requires the same three-person team, but this time holding poles that are cut with notches every centimeter. Person A holds his pole while Person B proceeds to the target spot. The string on his pole is moved up or down in order that the bubble in the builder’s level is centered. In noting which notch the string rests in on Person B’s pole, it is easy to determine exactly how many centimeters difference there is in elevation from one pole position to another.

Land Surveys Pty Ltd are licensed surveyors and provide a complete array of surveying services in Perth & Karratha, Western Australia.

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