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.
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)
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).
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”
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
Reflectors (Prisms) 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