6 Flashcards
Pacing
Accurate for preliminary measurements and good to detect blunders. Sometimes use passometer. Accuracy 1/50 to 1/100
Odometer
Determining distances along curved lines by converts number of revolutions of a wheel. Accuracy 1/20p
Optical rangefinders
Same operation with single-lens cameras. Accuracy 1/50 up to 50 m
Tacheometry/stadia
By sighting from telescope with 2 perpendicular cross hairs. Accuracy 1/500
Subtense bar
Using another angle-measuring instrument to read the subtended two targets spaced on a subtense bar. Accuracy 1/3000 for 150m
Taping
Can be done by 2 scenarios:
- observing distance between two points
- laying out a known distance with the starting mark
One example way to taping is RANGE POLES: mark the line being measured so that the alignment can be maintained
Principle of EDM
Electromagnetic energy spreads through the atmosphere, which is affected by temperature, atmospheric pressure,humidity. It employs near infrared light. It travels double path distance.
Distance is determined by the number of full and partial waves of energy.
Different of with and without reflectors EDM
With reflectors can measure until greater than 3 km. Without reflectors only up to 300m, but can be made to inaccessible objects.
Major errors in EDM
Instrument and reflectors miscentering
Instrument has constant error or scalar error
Total error of EDM Formula
Ed = root(Ea^2 + Eb^2 + (distance *Eppm)^2
Sources of errors in EDM
Personal error, inaccurate setups, mis-determining atmospheric pressure and temperature
Personal mistake, reading/recording incorrectly, failing to set the temperature and pressure
Instrumental error, reflector constant because the light travels at lower velocity in glass than in air so the effective center is behind the prism which is systematic error
Natural error, because temperature and perssure
