Lab EDM Calibration
Purpose:
This lab will teach the students to understand the basic principles of
Electronic Distance Measuring using an electro-optical EDM on a total
station, and EDM calibration techniques.
Overview:
The basic principle of an EDM is the indirect determination of time
required for a beam of light to travel between two points. This principle
was put to use in the early 1950’s in Sweden with the introduction of the
Geodimeter.
How an EDM Works:
The instrument at one station emits a modulated beam of light, usually
infrared, to a reflector at another station. The reflector sends the light
back to the instrument, where a phase comparison is made between the
projected and reflected pulses. The velocity of light is the basis for the
computation of the distance. The infrared emitted light leaving the
instrument is modulated into wavelengths that are proportional to the
modulating frequency, where the wavelength is given by:
= V/f
(Eq. 1-1)
 = wave length
V = velocity of light through the atmosphere, m/sec
f = modulating frequency, Hz (hertz = cycles per second)
The velocity of light in the atmosphere will vary with temperature,
humidity, and pressure. In order to obtain an accurate velocity,
atmospheric interference must be accounted for.
Measurement Principle of EDM:
D

Light
Instrument
Reflector
Light
d
A
C
D = ½ (m + d)
D = distance
d = the amount the received beam is out of phase with the
transmitted signal.
m = integer number of wavelengths in the double path of light.
Lab #1 (Continued)
Objective:
Using the Topcon Total Station GTS 4, do the following:
Fig. 1-1
(Eq. 1-2)
1. Determine the proper ppm correction of the EDM.
2. Determine the EDM total offset constant using a baseline of unknown
length. (TOC)
3. Determine the proper Glass Constant and Instrument Offset Constant
(IOC, GC)
4. Take proper field notes.
Field Work Procedure for TOC, GC, and IOC:
STA A
STA B
STA C
1. Find a level area and mark three points A, B, C on a straight line.
2. Set the instrument’s TOC (instrument constant) to zero.
3. Measure AC and AB using three tripods with tribrach’s erected over the three points
with the instrument at A.
4. Leave tripods and tribrach’s in place and place the instrument at B and prism at A.
Measure BA and BC.
5. Place the instrument at C with prisms at A and B. Measure CB and CA.
6. Calculate the average distances AB, AC and BC.
7. Calculate the Instrument Offset Constant (IOC).
8. Measure the prism and calculate the Glass Constant (GC).
9. Calculate the IOC.
Calculation Procedure for ppm Correction:
Given:
Wavelength modulation frequency, f0 = 14985.4 kHz
Wavelength of infrared carrier beam, 1 = 0.900m
Wavelength, 0 = 20 m
Velocity of light in a vacuum, V0 = 299,792.458 km/sec
Can be found
in
Manufacturer
Manual
Observe the Following at Time of Observations:
p = Atmospheric pressure (Use average of 762.0 mmHg)
t = Air temperature, C
1. Calculate the refractive index of air under standard conditions, ng:

 4.8864   0.068  6
   4 10
nq  1  287.604  
2
 

Lab
#1
(Continued)

 1   1 

(Eq. 1-3)
2. Calculate the refractive index of air under the condition at time of observation, na:
 0.359474 nq  1p 

na  1  
273
.
2

t


(Eq. 1-4)
3. Calculate the reference index of refraction for the instrument, n0:
n0 = V0 / (0 f0)
(Eq. 1-5)
4. Calculate the manufacture’s EDM reference index of refraction, N0, and the refractive
index for the conditions at the time of observation, Na:
N0 = (n0 – 1)106
(Eq. 1-6)
Na = (na – 1)106
(Eq. 1-7)
5. Finally compute the Correction (ppm):
Correction (ppm) = (N0 - Na)
Calculation for Glass Constant (GC)
t
a
GC
Glass Constant (GC) = 1.517 * t - a
Calculation for IOC and TOC:
(Eq. 1-8)
A
B
Segment
Horizontal
distance (m)
C
Average
distance(m)
AB
BA
BC
CB
AC
CA
ABmeasure + TOC + BCmeasure = ACmeasure
TOC = ACmeasure – (ABmeasure + BCmeasure)
TOC = IOC - GC
IOC = TOC + GC
Submit:
Lab write-up containing the following:
a. Procedures and record of measurements taken.
b. Calculations, substitutions, with formulas used. (repetitive calcs in
spread sheet OK)
c. Analysis and conclusions
Items for your analysis:
1. Does your prism constant match the value advertised by the manufacturer?
2. Does your ppm correction match that from the manufacturer chart or slide rule?
3. Does your IOC match that of the manufacturer?
4. In your opinion what are the TRUE values for AC, AB, and BC?
5. Other observations and conclusions?