1 / 33

# Horizontal Distance Measurement - PowerPoint PPT Presentation

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.

Related searches for Horizontal Distance Measurement

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### Horizontal Distance Measurement

Tape

Odometer

Subtense Bar

EDM

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

• Modern Technology:

• - Electronic Distance Measuring (EDM) devices: 2 +2 ppm

• Accuracy and speed considerations for civil engineers.

• Sources of Errors:

• Incorrect length of the tape

• Temperature difference

• Sag

• Poor alignment

• Tape not horizontal

• Improper Plumbing

• The idea of an odometer.

• Subtense bar: a 2 m rod.

• Distance H= cot(/2) m.

• Subtense Bar

• Distance H = cot(/2) m.

L /2

H

/2

tan ( /2) = (L/2) / H

H = (L/2) / tan( /2)

If L = 2 m, then

H = 1 / tan (/2) = cot (/2)

Example:

what is the horizontal distance between A and B if the angle  was 2?

• Chapter (16)

• Horizontal Distance = 100 rod interceptfor a horizontal line of sight and a vertical rod

• Symbols:

• (I) rod intercept, or stadia interval

• (i) spacing between stadia hair

• (f/i) = k = 100: stadia interval factor

• C = (c + f) approximately 0, Stadia constant

H = 100 I cos2(α) V = 100 I sin(α) cos(α)

• Early types:

• Transmit light, measure up to 25 miles

• Transmit microwaves, measure up to 50 miles

• Classification of EDM:

• Electro-optical: laser or infra red reflected from passive prism or surfaces, the US has installed a prism on the moon.

• Microwave: two positive units, GPS replaced them for most engineering applications such as hydrographic surveys

• We only measure the phase angle shift (change) , different signals of wave length: 10 100 1000 10,000 m are sent. Each fraction provides a digit(s).

• (Phase shift / 360)*wave length = non complete cycle length.

• Example: how a distance 3485.123 is measured.

The Idea:

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).

• Knowing the speed of electromagnetic waves in the air, the EDM computes the distance by measuring the time difference or the shift of the wave phase angle (will be explained in details later).

90

0

180

270

Phase Angle 

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

• Distance = Velocity * Time = ((N *) + ) / 2

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”

B

C

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

EDM Accuracy prisms

• 3mm, and 3ppm is the most common.

• Estimated error in distance =

Ei2 + er2 + ec2 + (ppm X D)2

• Where: Ei and er are the centering errors of the instrument and the reflector, ec is the constant error of the EDM, and PPM is the scalar error of the EDM

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?