What is a topographic survey?
A topographic survey determines the location of natural and man-made features (such as buildings, improvements, fences, land contours, trees, streams, etc.) of the land along with its elevations. This type of survey may be required by a government agency but it is mostly used by engineers and/ or architects for the design of improvements or developments on a site.
In other words, a topographic survey or contour survey provides you with the exact figures of the size, height, position of the different changes that have occurred in course of time with regard to their presence of their natural existence. Sometimes, these surveys are also used for the formulation of an optimal plan for drainage, ditches, grading and other features.
A closer look at topographic surveys
Topographic surveys are generally conducted before handing over the ownership of the land or even when the property owner/ industrial owner is intending to make some alterations in the land. In these types of surveys, the main focus is on the distance from the ground rather than the horizontal measurements.
A topographic map is used to depict terrain relief showing ground elevation, usually through either contour lines or spot elevations. The map represents the horizontal and vertical positions of the features represented. The scale of the topographic survey will conform to the needs of the client. A smaller contour interval will result in more field measurements and higher cost.
Topographic mapping is much more efficient and accurate today because of advances in survey techniques, instrumentation, design and printing technologies, and the use of aerial photography and satellite data. Today it is possible to make 3D topographic digital and interactive maps that show the entire landscape roads, mountains, rivers, buildings in amazing detail. This data can be used in AutoCAD programs, which is in turn is worked upon by the engineers and thereby you can easily have the view of how the topography will change after the planned changes.
Methods to conduct topographic surveys
A topographic survey comprises horizontal and vertical plane surveys. It can be carried out using a variety of techniques. Some popular techniques include:
- Geographic information systems (GIS) - They have contributed a lot to the mapping revolution. GIS makes it possible to combine layers of digital data from different sources and to manipulate and analyze how the different layers relate to each other. The process of converting 3D topographic maps to digital form involves raster to vector conversion using CAD-based software such as AutoCAD.
- Theodolite surveys - The theodolite measures the angles, and the distances are measured with either a steel measuring tape or, more commonly, an electronic distance meter (EDM). An EDM can measure great distances (several kilometres) very quickly and accurately. It measures distance with the usage of light and radio waves. Its development was a milestone in survey measurement methods.
- GPS - A constellation of Global Positioning System (GPS) satellites orbiting the earth is used to determine the position(s) of GPS ground receivers as they are moved from point to point. Collected data may either be processed in the office to produce GPS receiver positions (control surveys) or in the field to give the field surveyor immediate receiver positions (real time GPS surveys) for use for example in construction or for subdivision layout surveys.
- LIDAR - Airborne LIDAR (Light Detection And Ranging) systems can produce extremely accurate elevation models for terrain (even measuring ground elevation through trees), while offering a quick and efficient method of surveying terrain that is not easily accessible. LiDAR, like the similar radar technology (which uses radio waves instead of light), determines the range to an object by measuring the time delay between transmission of a pulse and detection of the reflected signal.
- Photogrammetry - In this method, stereographic pairs of photographs are used to indirectly measure objects on the ground and then calculate point coordinates and height differences.
A typical topographic plan includes the following steps:
- Establishing horizontal and vertical control that will serve as the framework of the survey
- Determining enough horizontal location and elevation (usually called side shots) of ground points to provide enough data for plotting when the map is prepared
- Locating natural and man-made features that may be required by the purpose of the survey
- Computing distances, angles, and elevations
- Drawing the topographic map
Topographic surveys are vital for architects, developers and land owners, as they require accurate, three-dimensional representations of the land.