Yabancı Dil III (YDL285)

Distance measurements

Yrd. Doç. Dr. Saygın ABDİKAN

2016-2017 Öğretim Yılı Güz Dönemi

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Distance measurements

Why do we need?

In science, measurement is the process of

estimating or determining the magnitude of a

quantity, such as length or mass, relative to a

unit of measurement, such as a meter or a

kilogram.

The term measurement can also be used to

refer to a specific result obtained from the

measurement process

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The word geometry is derived from the Greek

meaning geo- "earth", -metron "measurement“

Types of surveys:

Land surveys

Topographic surveys - location of objects and

measuring the relief, roughness, or three–

dimensional variations

Route surveys - location of natural and artificial

objects along a proposed route for a highway,

railroad, canal, pipeline, power line, or other

utility3

City or municipal surveys - use to lay out streets, plan sewer

(kanalizasyon) systems, and prepare maps

Construction (yapi) surveys - locating structures and providing

required elevation points during their construction

Hydrographic surveys - pertain to lakes, streams (akarsu), and other

bodies of water

Marine surveys - related to hydrographic surveys, but they are

thought to cover a broader area

Mine surveys - relative positions and elevations of underground

shafts, geological formations, etc.

Forestry and geological surveys

Photogrammetric surveys - photographs (generally aerial) are used

in conjunction with limited ground surveys

Control surveys - provides vertical and horizontal reference points

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TYPES OF MEASUREMENT

• Horizontal distances

• Vertical distances

• Slope distances

• Horizontal angles

• Vertical angles

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UNITS OF MEASUREMENT

Length: Feet, Meters, Chains

Angles: Degrees Minutess Seconds (DMS), Grads (400gr = 360°)

Radians (2 PI = 360°)

Area: Ft2, acres, square meters, hectares, square kms

(1 ha = 2.471 acres) acres-dönüm

(1 ha = 10,000 m2)

Measurement techniques

Distance is one of the most basic engineering measurements

Early measurements were made in terms of the dimensions of the

body

Cubits: the distance between the tip of your middle finger to the

elbow (~1.83m) (arşın)

Fathom: distance between the tips of your middle finger when

your arms are outstretched (~1.83m) (kulaç)

Foot/Feet: distance from the tip of a man’s big toe to the heel

(1 feet ~30.48cm)

A pace is defined as one step and a stride is two steps

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http://surveying.structural-analyser.com/chapter04/

Chaining (Gunter’s chain/ Gunter’s measurement)

A Gunter’s chain (named after its inventor Edmund

Gunter 1581-1626) is 66 feet long and is divided into 100

links, each 7.92 inches long. (1 inch=0,08333 feet)

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Taping/chaining

clear line

lay out tape on ground making sure there are no kinks

(bukulme/kirilma)

apply tension (~5 kg) (gerilim)

use plumb-bob on uneven ground

plumb-bob or a plummet is a weight, usually with a pointed tip on the

bottom, that is suspended from a string and used as a vertical

reference line, or plumb-line.

read the tape

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Taping on steep slope

The term breaking tape is used to describe the procedure for

measuring directly horizontal distance on sloping ground

Breaking tape (basamaklı ölçüm) – range pole

The sum of horizontal lengths (L1 & L2 & L3) equals the horizontal

distance of the slope from A to D.

The horizontal distance is always shorter than the measurement

on the slope.

To reduce the sag break tape measurement

into shorter lengths.

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L1

L2

L3

A

B

C

D

Slope

angle

COMMON TAPING

ERRORS

1. Incorrect length of tape

2. Temperature variation

3. Non-standard tension

4. Sag (catenary) (sarkma)

5. Poor alignment (kötü

doğrultma)

6. Slope

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TAPE STANDARDIZATION

- Done by comparison with standard

tape or known baseline.

- Standardization usually defined in

terms of:

- tension (e.g. ~5 kg)

- temperature (e.g. 20°C)

Short measurements may minimize sag effect

EDM (electronic distance measurement)

Electronic distance measurement (EDM) instruments

determine lengths using phase changes that occur as

electromagnetic energy of known wavelength travels

from one end of a line to the other end and returns.

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Gediometer (geodetic

distance meter) – which

is developed in 1948

based on a modulated

light beam (measuring

the distance between

two points by means of

a laser)

Tellurometer employs

modulated microwaves

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EDMs are now being incorporated with

theodolites having automatic angle readout

capabilities, so called total stations

(electronic tacheometers).

They can simultaneously and automatically

measure both distances and angles.

There are two kinds of EDM

Electro-optical System

Microwave instruments

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Electro-optical System

An EDM uses electromagnetic (EM) energy to determine the

length of a line. The energy originates at an instrument at one

end of a line and is transmitted to a "reflector" at the other

end from where it is returned to the originating instrument.

The nature of the "reflector" is dependent on the type of EM.

They transmit light having wavelengths in the range of 0.7 to 1.2

micrometers within or slightly beyond the visible region of the

spectrum.

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Microwave instruments

They transmit microwaves with frequencies in the

range of 3 to 35 GHz corresponding to wavelengths of

about 1.0 to 8.6 millimeters.

In either case the measurement is the total distance

from the instrument to the reflector and back to the

instrument.

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Comparing the two EM types:

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EM Type Advantages Disadvantages

Electro-optical Less susceptible to atmospheric

conditions.

Less expensive: only a single

transmitter needed.

Shorter range.

Microwave Can penetrate fog and rain.

Longer range.

Transmitter at both ends allows

voice communication.

Atmospheric affects are

greater.

Susceptible to ground

reflected signals.

More expensive: requires

two transmitters.

An EDM does not determine distance by measuring the travel

time of the EM signal. Instead, an EDM uses the signal structure

and determines the phase shift. The EM signal has

a sinusoidal wave form. This wave form repeats every 360°.

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Different wavelengths are generated at different

modulation frequencies, f. Wavelength,

frequency, and the speed of light are related by:

An EDM measures the line of sight distance between

the instrument and reflector. This is a slope

distance and not horizontal unless the EDM and

reflector are at the same elevation.

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In order to determine a horizontal or vertical distance

additional information is needed. Combining an EDM with

a digital theodolite results in a Total Station

Instrument (TSI). When distance measurement is made,

the TSI measures the slope distance and a zenith angle.

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Total station

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Single reflector Multiple reflector

FACTS ABOUT MEASUREMENTS

• No measurement is exact

• Every measurement contains error

• The true value of a measurement is never known

• Exact error in measurement is always unknown

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SOURCES OF ERROR

1. NATURAL ERRORS:

Errors caused by natural phenomena, such as wind, temperature,

humidity, multiple refraction of the signal, etc

2. INSTRUMENT ERRORS:

Errors resulting from imperfections in measuring instruments,

such as graduations on theodolite circle or tape

3. PERSONAL ERRORS:

Errors caused by human limitations of the observer, such as

vertical crosshair not aligned perfectly on target

Inaccuracy in initial setups of EDMs and the reflectors over the

preferred stations

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TYPES OF ERRORS

1. SYSTEMATIC ERRORS

These errors conform to mathematical and physical laws (e.g.

temperature) and can generally be computed and correction

applied

2. RANDOM ERRORS

Errors that obey the laws of probability (beyond control of

observer) - no absolute method to compute and eliminate these

errors, but they can be estimated using statistical methods

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Yrd.Doç.Dr. Himmet Karaman, Lecture notes on Geodesy, GPS

and GIS

Prof.Dr. Uğur Şanlı, Lecture notes on Vocational English

http://jerrymahun.com/library/Distance/a.htm

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