Horizontal Distance Measurement. Tape Odometer Subtense Bar Stadia EDM. Distance Measurement. Devices and accuracy:. Older technologies “ quick look” Pacing : 1:50 Optical rangefinders: 1: 50 Odometers: 1:200 Tacheometry / stadia 1: 500 Subtense bar 1: 3000 Tapes: 1: 10,000.
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Horizontal Distance Measurement
Devices and accuracy:
tan ( /2) = (L/2) / H
H = (L/2) / tan( /2)
If L = 2 m, then
H = 1 / tan (/2) = cot (/2)
what is the horizontal distance between A and B if the angle was 2?
H = 100 I cos2(α) V = 100 I sin(α) cos(α)
Electronic Distance MeasurementEDM
To measure the distance between two points (A) and (B) the EDM on point (A) sends electromagnetic waves. The waves received at (B) are reflected back or resent to (A) by a device on (B).
Concept of the fraction
Assume that = 2 m
If 1 = 80, it corresponds to a distance = (80/360) * = 0.44 m
If 2 = 135, it corresponds to a distance = (135/360) * = 0.75 m
If 3 = 240, it corresponds to a distance = (80/360) * = 1.33 m
Where is a fraction of wave length = (/360) *
N is the number of full cycles, ambiguity?
Since is divided by 2, so is , we call /2 “effective wave length”
EDM Mounted on a theodolite
The angle between the rays A and C is double the angle between the two mirrors = 2 *90 = 180
Notice that the objects will look upside down, notice the box at the tail of the arrow
Aiming at a prism through the telescope of a total station in a zoo!
Fully rotating prism
Prism and sighting target
Pole and bipod
Ei2 + er2 + ec2 + (ppm X D)2
Example 6-5 page 156: if the estimates errors of centering the instrument and target were ± 3mm and ± 5mm respectively, the EDM had a specified error of ± (2mm +2ppm) what is the estimated error in measuring 827.329 m?
Answer: ± 6.4 mm
Handheld laser measuring devices