1. Surveying 1 / Dr. Najeh Tamim
CHAPTER 6
ELECTRONIC DISTANCE
MEASUREMENT
(EDM)

2. Need
Accuracy of measurement using chain surveying and tacheometry is very limited, especially when dealing large areas.
EDM was developed because of the need for more accurate means of measuring horizontal distances and elevation differences.

3. ELECTRO-OPTICAL INSTRUMENTS
Composed of transmitter (main instrument) and reflector.

4. MICROWAVE INSTRUMENTS
Composed of two identical units: Master and Remote.

5. BASIC PRINCIPLE OF ELECTROMAGNETIC MEASUREMENTS

7. To go around measuring n in the previous equation (which is difficult to measure with a single frequency), the EDM sends several frequencies successively and measures the phase differences and sums the fractions of the distance to find the total distance (see table below).

8. INDEX OF REFRACTION

9. TYPES OF MOUNTS

11. RETRO-REFLECTORS
Fixed and tilted
Single and multiple

12. SOURCES OF MEASUREMENT ERRORS
Eccentric error.
Inexactness of the instrument in performing phase measurements.
The zero point of the light ray used in phase measurement does not coincide exactly with the theoretical center of the instrument.
The actual center of the reflector does not coincide with the theoretical center.
The actual modulating frequencies differ from the theoretical values of these frequencies.
The refraction index (Na) is not constant through the line to be measured.

13. Standard Error
the manufacturers of EDM instruments provide the standard error for a measured distance in two parts:
1. A constant part that is independent of the length of the distance.
2. A variable part given as parts per million (ppm) of the measured distance.
Example:
If an EDM has a measurement accuracy of ±(10mm + 4ppm), then a distance of m measured with this instrument would have a standard deviation equal to ±( x 10-6 x ) = ± 0.02 m

14. TRIGONOMETRIC LEVELING - SHORT LINE

15. Example