MEASURING

HORIZONTAL DISTANCES

August 2014 Semester

EGC1164 & EGC2164

OUTLINE

• -1 Rough Distance Measurement – Determining a Unit Pace

– Using a Measuring Wheel

• -2 Taping: Equipment and Methods – Tapes and Accessories

– Taping a Horizontal Distance

– Setting Marks for Line and Distance

• -3 Taping Mistakes, Errors, and Corrections – Taping Mistakes or

Blunders

– Taping Errors

– Correction of Systematic Errors

• -4 Electronic Distance Measurement – Types of EDMIs

– EDMI Operating Procedure

5 Taping Corrections

Common Methods for Measuring

Distances

• One of the fundamentals of surveying is the need to measure

distance. Distances are not necessarily linear, especially if they

occur on the spherical earth. In the present subject we will deal

with distances in Euclidean (geometric) space, which we can

consider a straight line from one point or feature to another.

-Pacing

-Taping

-Tachometry (required theodolite, and graduated rod)

- EDM (Electronic Distance Measurement)

ROUGH DISTANCE

MEASUREMENT

• In certain surveying applications, only a

rough approximation of distance is

necessary; a method called pacing, or

the use of a simple measuring wheel,

may be sufficient in these instances.

• Distances can be measured with an

accuracy of about 1:100 by pacing.

– Distance = unit pace * number of paces

ROUGH DISTANCE

MEASUREMENT

FIGURE -1. Pacing provides a simple yet useful way to make rough distance measurements.

All surveyors and construction technicians should know their own personal unit pace value.

ROUGH DISTANCE

MEASUREMENT

• Determining a Unit

Pace

• Using a Measuring

Wheel

FIGURE -2. A typical measuring

wheel used for making rough

distance measurements. (Courtesy

of Sokkia Corporation)

TAPING: EQUIPMENT AND

METHODS

• Measuring horizontal distances with a tape is

simple in theory, but in actual practice it is not

as easy as it appears at first glance.

– It requires skill and experience for a surveyor to

be able to tape a distance with a relative accuracy

between 1:3000 and 1:5000, which is a generally

accepted range for most preliminary surveys,

ordinary property surveys, and many types of

construction layouts.

TAPING: EQUIPMENT AND

METHODS

• Tapes and Accessories

– Steel Tapes

– Cut Tapes

– Add Tapes

– Invar and Lovar Tapes

– Accessories for Taping

– Precautions to Avoid Damaging the

Tape

9

Methods of Measurement

• Pacing

– Accuracy 1 : 100

• Taping

– Accuracy 1 : 10,000

• Electronic Distance Measurement (EDM)

– Accuracy 1 : 10,000 to 1:100,000

10

Pacing

• Practical measure of distance.

• Don't try to pace out one meter with every step.

Walk casually over 100 m counting the number of

steps. Work out the length of a casual step and

use this instead.

• Varies with uphill, downhill, and your age.

• Low accuracy

• No equipment needed

11

Taping (or chaining) • Chaining is applied to measurement

with a steel tape or synthetic tape (plastic or fiberglass).

All standard in lengths – 100 m, 50m, 30 m, 20 m.

• It is fairly quick, easy and cheap, and hence is the most common form of distance measurement.

• Unfortunately, Chaining is prone to errors and mistakes.

• For high accuracy, steel tape should be used which is graduated in mm and calibrated under standard temp (20 degree) and tension (5kg). Be careful, easily break.

• Synthetic tape is more flexible graduated in 10mm

Equipment Used in Chain

Surveying

14

Measuring wheels

Tapes in lengths up to 100 ft

Some Taping Instruments

15

Taping Procedures

• Ranging rods set up between points A and B

from A to B, set zero of tape at A

• tape unwound towards B

• A third range rod is “ranged” in at C

• Tape straightened, held tight and read at rod C

• C marked with a pin

• for next bay, tape moved from A and zero set at C and so on

TAPING: EQUIPMENT AND

METHODS

FIGURE -3. A steel tape in a convenient reel and typical tape

markings. (See also Figure 1-1d.)

TAPING: EQUIPMENT AND

METHODS

FIGURE -4. Some surveyors’ tapes have the zero mark at the endpoint of the tape, while

others have zero offset from the end. (Courtesy of The Lietz Company)

TAPING: EQUIPMENT AND

METHODS

FIGURE -5. A cut tape.

TAPING: EQUIPMENT AND

METHODS

FIGURE -6. A 100-ft tape with graduations outside the 100-ft length. In use, the graduated end and the add mark are

kept forward, with the 100-ft mark at the rear. In laying out 100-ft intervals, the 100 mark and the zero mark are used.

In measuring distances less than 100 ft, for example, 64.32 ft, the head chainman stops when point B is reached and

holds the zero mark at that point. The rear chainman finds that the previous point marked on the ground comes

between the 64- and the 65-ft mark. Choosing the smaller, he or she calls “holding 64” and holds the 64-ft mark over

the ground mark. The head chainman reads the value of the backward graduation, that is, 0.32 ft, at point B.

TAPING: EQUIPMENT AND

METHODS

FIGURE -7. A plumb bob is one of the simplest yet most

important accessories for accurate surveying. The vertical

cord transfers a position from the steel tape to the wooden

stake in the ground. (Courtesy of The Lietz Company)

FIGURE -8. A tape clamp handle.

(Courtesy of The Lietz Company)

TAPING: EQUIPMENT AND

METHODS

FIGURE -9. A nonmetallic 15-m fiberglass

tape. (Courtesy of The Lietz Company) FIGURE -10. How a loop breaks a tape.

TAPING: EQUIPMENT AND

METHODS

• Taping a Horizontal Distance

– Setting Out and Aligning the Tape

– Marking an Intermediate Station on Line

– Completing the Measurement

– Breaking Tape

TAPING: EQUIPMENT AND

METHODS

FIGURE -11. Methods of supporting a

tape.

TAPING: EQUIPMENT AND

METHODS

FIGURE -12. Holding the plumb-bob cord

on the tape.

FIGURE -13. Steps in marking a point on a

pavement.

TAPING: EQUIPMENT AND

METHODS

FIGURE -14. Breaking tape over steeply sloping ground.

TAPING: EQUIPMENT AND

METHODS

• Setting Marks for Line and Distance

– Identifying Stations

– Field Procedure

• Driving a Stake

• Setting a Tack

• Making Marks on Other Surfaces

TAPING: EQUIPMENT AND

METHODS

FIGURE -15. The positions along a measured line are called stations.

TAPING: EQUIPMENT AND

METHODS

FIGURE -16. Holding the plump-bob

cord taut against the tack. FIGURE -17. Plumb-bob cord bent

over tape.

TAPING: EQUIPMENT AND

METHODS

FIGURE -18. Driving a wooden stake (or hub) at a station.

TAPING: EQUIPMENT AND

METHODS

FIGURE -19. Setting a tack on a wooden hub.

TAPING: EQUIPMENT AND

METHODS

FIGURE -20. Handling a plumb bob to set a tack.

TAPING: EQUIPMENT AND

METHODS

FIGURE -21. Measuring for a tack

with a short hold.

TAPING MISTAKES, ERRORS,

AND CORRECTIONS

• Taping Mistakes or Blunders

– Misreading the tape

– Misrecording the reading

– Mistaking the endpoint of the tape

– Miscounting full tape lengths

– Mistaking station markers

TAPING MISTAKES, ERRORS,

AND CORRECTIONS

• Taping Errors

• Correction of Systematic Errors

– Common Tape Corrections

– Correction for Tape Length

– Use of CL for Correct Distance

– The Sign of the Tape Length Correction

– Correction for Temperature

TAPING MISTAKES, ERRORS,

AND CORRECTIONS

FIGURE -22. Accidental errors occur when the tape is (a) misaligned or (b) off-

level. For good accuracy, the tape should be on line and horizontal within 1

ft/100 ft (0.3m/100m).

TAPING MISTAKES, ERRORS,

AND CORRECTIONS

FIGURE -23. A steel tape always tends to sag between supports, no matter how hard it is

pulled.

TAPING MISTAKES, ERRORS,

AND CORRECTIONS

FIGURE -24. When measuring a distance with a tape whose actual length is shorter than its

last marking, a correction must be subtracted from the recorded distance.

ELECTRONIC DISTANCE

MEASUREMENT

• Electronic distance measurement (EDM) is the measurement method of choice not only for large-scale geodetic surveys but also for ordinary plane surveys.

– Compared with taping, EDM offers the advantages of increased speed, accuracy, and dollar economy when routinely determining or setting relatively long horizontal distances.

ELECTRONIC DISTANCE

MEASUREMENT

• Types of EDMIs

– Reflecting Prisms

– Accuracy of EDM

• EDMI Operating Procedure

– Setting a Mark with EDM

ELECTRONIC DISTANCE

MEASUREMENT

FIGURE -25. EDM depends on the constant speed of electromagnetic waves (e.g.,

infrared light). The measured slope distances must be converted to corresponding

horizontal distances.

ELECTRONIC DISTANCE

MEASUREMENT

FIGURE -26. A total station.

Distances (and angles) are

displayed digitally. (Courtesy of

Leica Geosystems, Inc.)

ELECTRONIC DISTANCE

MEASUREMENT

FIGURE -27. (a–c) Electronic total

stations. (Courtesy of Leica

Geosystems, Inc.)

ELECTRONIC DISTANCE

MEASUREMENT

FIGURE -28. (a) A triple-prism assembly with sighting pole. (Courtesy of Sokkia Corporation) (b)

The internal reflecting surfaces of a corner prism, with the path of a single beam of light coming

from any direction and being reflected in a direction parallel to its original direction. (Philip

Kissam, Surveying for Civil Engineers, 2nd edn,New York:McGraw-Hill Company, 1981.

Reprinted by permission.)

ELECTRONIC DISTANCE

MEASUREMENT

FIGURE-29. Instrument person pointing the line of sight at a prism pole

held plumb on a point.

Taping Corrections

47

Taping: Corrections

• Erroneous Tape Length

• Slope

• Tension

• Temperature

• Sag

Once a line is being measured, it is necessary to convert the measured

length into a horizontal length. Series corrections have to be applied.

Five possible corrections have to be considered. These are

48

Taping: Corrections

For synthetic tapes, only Erroneous Tape Length

and slope corrections will be applied

The best accuracy that can be achieved is the

order of 1:1000

When using steel tapes, if only Erroneous Tape

Length and slope corrections are considered, the

best possible accuracy that can be obtained in the

range 1:5000 If tension and temperature are

added into consideration, accuracy can be

increased to better than 1:10000 ~ 1: 20000

Sag only applies if tape is supported only at ends

49

Erroneous Tape Length

• Tape has a nominal length under certain

conditions, a tape stretches with time.

• standardisation needs to be carried out frequently

by using reference tape or baseline.

n

nsmst

L

LLLe

standardisation

length

(actual tape length)

nominal length

(assumed tape length)

For a 30m Nominal Length Tape

Tape used

When comparing to a standard tape, the tape used indeed has a length

For every 30m measurement, the small elongated amount should be added for correction.

30 m + ∆l

60

Tension Correction

• A tape is a given length when pulled with a

certain tension. If the tension changes then so

does the tape length.

AE

LTTe ms

tension

Standardisation tension

Modulus of Elasticity

of tape material

For steel, E = 200,000 N/mm2

Cross section

Area

Tension applied

63

Sag Correction

• If the tape cannot be supported for its length then it will hang freely under the influence of gravity. The shape of the tape will take is known as (sag) and can be determined mathematically.

2

232

24

cos

T

Lwe m

catenary

Tension applied to the ends

Angle of slope

Weight of tape

per unit length

66

Combined Errors

tensionslopesagsttempma eeeeeLL

Actual length is:

Steel Taping: Examples