Electronic Distance Measurement (EDM)

EDM stands for Electronic Distance Measurement. There are two basic types of EDM equipment, electro-optical (light waves) and electromagnetic (microwaves).

In the past years, the electro-optical instruments have predominated. In the field of civil engineering, the use of EDM in surveying is rapidly increasing.

EDM

EDM is not as complex as it appears to be.  It can be used at several different levels.

  • Dumpy level. EDM can be used to record heights.
  • Theodolite. The EDM will also record vertical and horizontal angles as well as heights. It is Useful for surveying but distance measurements by tachometry are not too accurate.
  • EDM. Does everything a theodolite does but measures distance accurately using a laser beam reflected from a mirror.  This gives great precision when surveying. Many EDM will do the trigonometry and also convert angles and distances.

Setting up EDM

  1. Charge up both sets of batteries in evening and overnight. One fully charged battery will last a day.
  2. Place the tripod at the desired spot, raise the platform to a sensible height. Hold the EDM by its handle and release the horizontal setting lock and screw the tribrach onto the tripod.
  3. Level the EDM.  Adjust the tripod legs so the bubble in the circular level (on the tribrach) is in the center. Now, turn the EDM so long spirit level is in line with 2 of the tribrach adjusting screws. Level the EDM using these 2 screws.  Now turn the EDM through 90 degrees and level using the third screw. Repeat until the EDM is level all round. This is the same levelling procedure used for the theodolite, but the EDM is much more sensitive and knows when it is out of true so the levelling must be done carefully.
  4. Battery.  Align the battery contacts with those on the EDM.  Put the bottom edge into the battery slot and push the top in. The battery will click into place. Switch on using the red on/off button on the control panel.
  5. The display will read Turn Telescope. Gently move the telescope up and down through the horizontal until the display reads horizontal and vertical angles.

Use of EDM for Measuring

  • First, mount the prism on the extendible rod. Extend this to a suitable length i.e. so the prism is level with the EDM telescope.  Lock the locking rings and do not change the length at all if you are taking height readings.
  • Get a friend to hold the prism at the measuring point, use the 3 spirit levels on the rod to make sure it is upright, and rotate the rod so the prism is pointing at the EDM. You may need to angle the prism up or down as well.
  • Sight the prism through the telescope. A suggested way is to release the horizontal lock and with both hands (one still on the lock) rotate the EDM until the rod is in line with the rough setting viewer. Clamp the horizontal lock and make final adjustments through the telescope with the horizontal Vernier. Do the same with the vertical controls. Focus the EDM on the prism. You should see the hexagonal pattern made by the prisms.  If you see a red circle, you have focused on the EDMs reflection!  Get your friend to rotate the prism until it is pointing at you.
  • Take a reading. Press the MEAS button. EDM will bleep and the arrows will appear on the screen.  When the reading is finished, the distance will appear on the third line. If the screen shows a triangle with a line under it, that is the horizontal distance.  If you press the button under the triangle (next to MEAS) the line will move around and the display will read the vertical distance and then the hypotenuse before going back to the horizontal distance.
  • Record the horizontal angle (H.ANG), the horizontal distance and (if you are measuring heights) the vertical distance.

Calculating

There is a spreadsheet where you can enter your readings and all the calculations will be done for you.

All you have to do is plot the results on graph paper or create a graph from within Excel.

Also Read This Article: Remote sensing and its use in surveying

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