ELECTRONIC DISTANCE MEASUREMENT
Total station instruments
Modern surveying system typically consists of an electronic total station, electronic field book and software used in the office for processing data.
The total stations function is to measure horizontal and vertical angles and slope distances in a single integrated unit. It is usually connected to an electronic field book. The field system (total station and field book) is usually controlled via the field book. The principal reason is that the field book keyboard does not transfer the force used to press the keys to the total station.
In operation the total station is set up over the required point and its height over the survey station measured. Then the operator points at a prism/target and initiates a reading. Usually this is done by pressing a key on the field book. In some system it may be a key on the electronic total station.
While the basic data sent by the electronic total station consist of slope distance, horizontal and vertical angle, other data may be included in the data stream. This may include units setting, parts per million (ppm) value, prism constant being used, etc. Additionally, calculated values such as coordinates, azimuths, and horizontal distances may be transmitted Electronic total stations can also have a variety of functions to improve efficiency and accuracy.
Some of these may be corrections for collimations, curvature and refraction and horizontal and vertical angles to compensate for the title of the vertical axis. The electronic field book's basic function is to store the raw data gathered in the field, including horizontal and vertical angles, slope distances, heights and instruments and targets, temperature and pressure, point numbers and descriptive codes.
One of the most crucial aspects of the electronic data collection concept is data flow. Traditional surveying techniques force one to view surveying as a data gathering activity. This view does not recognize the fact that once a design is completed based on the survey data there usually is a need to transfer this design on to the topography. In modern surveying, setting out is a given equal importance as data gathering.
Effect of atmospheric connections on wave velocity
The velocity of electromagnetic radiation is constant in vacuum (at the velocity of light) but when affected by the atmosphere it retarded in direct proportion to the density of air. Because of refraction the direction and speed change iſ the refractive index usually symbolized as n is related to the dielectric constant u of the air in the following way:
n =√u
The instantaneous velocity C of the radiation at any point within the atmosphere is a function of the speed of light C, and the refractive index n and is given as C = Co/n
Where Co is a constant and is taken as 299,792.5 Km/s. Refractive index is a function of a temperature, pressure and humidity. Humidity is given as the partial pressure of the water vapour in air. In field observation it is almost always obtained by the simultaneous observations of wet and dry bulb readings of a psychrometer.
International Association of Geodesy General Assembly (1963) has recommended that the Barrel and Sears (1939) formula may be used to calculate ng or group index of refraction when λ is the equivalent or effective wavelength of radiation in micrometers.
ng( -1)10⁷=2876.04+{3(16.288)}/λ² +{5(0.136)}/λ⁴
For the kinds of light used in EDM's the values of λ are
(a) Mercury vapour = 0.5500
(b) Incandescent = 0.5650
(c) Red laser = 0.6328
(d) Infrared = 0.900 – 0.930
Refractive index for light waves as per Barrel and Sears formula
Na = [ng-1/1+do . P/760 - 55(10)-⁸/1+dt . e ] 10⁶
Where Na = (na –1)10⁶
P = Total pressure
e= Partial pressure of water vapor in millimeters of mercury.
d = Heat expansión coefficient of air 0.00367
t = Temperature in degree centigrade
Effect of water vapour is small on propagation of light waves but is of great significance when microwaves are used. For microwaves Essen-Froome formula can be used which is as follows:
Nm = 103.49/T(p–e)+86.26/T(1+5748/T)e
T is in kelvin units , p and e are in mm of Hg .
Question 1 :-
If electromagnetic energy travels 299792.5 km/sec under conditions what unit of distance corresponds to each millimicro second of time ? (ii) The speed of electromagnetic energy through the atmosphere at a standard barometric pressure of 760 mm of mercury is accepted as 299792.5 km/sec for measurements with an EDM instrument. What time lag in the equipment will produce an error of 15 m in the distance to a target 80 km away? (üi) (a) If an EDM has a purported accuracy capability of (5 mm + 5 ppm) what error can be expected in a measured distance of 800 m? (b) If a certain EDM instrument has an accuracy capability of (7 mm + 7 ppm). What is the precision of measurements in terms of 1/x for line length of 3000 m?
Answer
Question 2 :-
What is the refractive index of red laser light at a temperature of 20°C and barometric pressure of 710 torr? Neglect the effect of vapour pressure? What is the velocity through this air? What is the modulated wavelength if the modulating frequency is 24 MHz?
Answer
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