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What is EDM in survey?

Electronic distance measurement in surveying

Electronic Distance Measurement

Electronic distance measurement (EDM) is a method of determining the length between two points using electromagnetic waves. EDM is commonly carried out with digital instruments called theodolites. EDM instruments are highly reliable and convenient pieces of surveying equipment and can be used to measure distances of up to 100 kilometers. Each piece of EDM equipment available at Engineer Supply provides dependably accurate distance measurements displayed on an easy-to-read digital screen. Devices known as total stations share similarities with theodolites and can be used to measure distances as well as angles.


A Theodolite is a instrument for measuring both horizontal and vertical angles, as used in triangulation networks. It is a tool used in the surveying and engineering industry, but theodolites have been adapted for other specialized purposes as well. A theodolite consists of a telescope mounted movably within two perpendicular axes, the horizontal or trunnion axis, and the vertical axis. When the telescope is pointed at a desired object, the angle of each of these axes can be measured with great precision, typically on the scale of arcseconds. The measurements are typically recorded by hand as they are not recorded by a computer or data collector.

Total Stations

A total station is an optical instrument used in modern surveying. It is a combination of an electronic theodolite (transit), an electronic distance measuring device (EDM) and software running on an external computer, such as a laptop or data collector.

Learn more about Transits, Theodolites, Total Stations, & levels

Many professionals depend on accurate measurements. Whether you are an engineer, surveyor, designer, architect, realtor, contractor, construction worker, estimator, or in another profession you know that you could lose business due to incorrect measurements. There are so many methods and tools available for measuring how do you know which tool to purchase for your measuring needs?

Read this brief guide on selecting the best measuring tool: How to measure almost anything

Frequently Asked Questions

How does electronic distance measurement work?

Electronic distance measurement (EDM) is a way of determining the length between two points by looking a phase changes that occur as electronic energy waves, which travel from one end of a straight line to the other. But when large variations occur in the terrain or when there's a great deal of obstruction, this method isn’t as effective. So, this method of measurement is avoided in difficult terrain.

What are the types of EDM instruments?

Here are the three types of instruments that are used for electronic distance measurement, which are based on the methods being used:
  • Microwave Instruments — Also called tellurometers, these instruments use microwaves. And they have been around since the 1950’s.

  • Infrared Wave Instruments — Uses prism reflectors that pick up amplitude modulated infrared waves at the end of a line.

  • Visible Light Wave Instruments — Uses modulated light waves to measure up to a specific range.

If you’re looking for a place to buy a broad range of theodolites, total stations, or other tools used for electronic distance measurement, be sure to look at what we have at Engineering Supply.

How does a total station measure distance?

Total stations use the principle of a theodolite and combine it with electronic distance measurement. They also come with a microprocessor and electronic data collector, which is combined with a storage system. Like theodolites, they can be used to measure the sloping distance of certain objects from the instrument, as well as horizontal and vertical angles. Once the data has been collected, it can be uploaded to a computer or laptop for further processing.

Does a transit measure distance?

A transit is a telescope that moves around horizontal and vertical axes, and it’s used to measure angles. The readings will allow you to map a specific site by measuring the position of all of its features, which you can do with a great deal of precision.

How do I measure dimensions?

Three-dimensional measurement is similar to measuring in two dimensions. But in 3-D, the boundaries of a solid object are called the surface area and not the perimeter (which is measured in square inches or feet). The volume refers to the space that it occupies, which is measured in cubic inches or feet. To measure a specific dimension (whether it’s length, width, or height), simply measure any side of a specific surface. If you need to measure two or three dimensions, you’ll need to measure the distance of each side that relates to that specific property.
If you’re looking for a place that has a broad selection of total stations, theodolites, or other electronic distance measurement tools, be sure to look at what we have at Engineering Supply.
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