ELECTRONIC DISTANCE MEASURING INSTRUMENT:(EDMI)

Electronic distance measuring instrument is a surveying instrument used to measure distance electronically between two points through electromagnetic waves.

Components of EDMI

Figure 2.1 and the list given below indicates major components constitututing a typical EDMI (Wolf and Ghilani, 2001):

carrier signal modulation signals and modulator signal transmitter and signal receiver beam splitter reflector filter Operations with EDMI Measurement with EDMI involves four basic steps:  Setting up: The instrument is centered over a station by means of tribrach or by mounting over a compatible theodolite. Reflector prisms are set over the remote station either on tribrach or on a prism pole. Observations related to height or instrument and prism are recorded. These are usually kept the same to avoid any additional corrections. Aiming: The instrument is aimed at prisms by using sighting devices or theodolite telescope. Slow motion screws are used to intersect the prism centre. Some kind of electronic sound or beeping signal helps the user to indicate the status of centering. Measurement: The operator presses the measure button to record the slope distance which is displayed on LCD panel. Recording: The information on LCD panel can be recored manually or automatically. All meteorological parameters are also recorded ERRORS SOURCES: Measurement with EDMI has the following error sources which have to be accounted for while reporting the distance (Kennie and Petrie, 1990):  (i) Instrument operation errors One has to be careful for precise centering at the master and slave station pointing/sighting of reflector entry of correct values of prevailing atmospheric conditions (ii) Atmospheric errors Meteorological conditions (temperature, pressure, humidity, etc.) have to be taken into account to correct for the systematic error arising due to this. These errors can be removed by applying an appropriate atmospheric correction model that takes care of different meteorological parameters from the standard (nominal) one. (iii) Instrument error: Consists of three components - scale error, zero error and cyclic error. These are systematic in nature. Market Value: Its price in US $600 to $800 in PKR 85000 to 111000.