Errors can be reduced to a minimum if good measuring practice is used together with a knowledge of potential sources of gross and systematic

**4.2.Sources of error**Errors can be reduced to a minimum if good measuring practice is used together with a knowledge of potential sources of gross and systematic

**errors.**

(a) A good quality steel tape should be used for all setting out work. Fibre glass tapes may be used for some site measurement tasks but not

(a) A good quality steel tape should be used for all setting out work. Fibre glass tapes may be used for some site measurement tasks but not

**where accuracy is required.**

(b) Badly repaired, twisted or kinked tapes will give erroneous distances.

(c) Site steel tapes should be checked against a Department of Trade or similar, standard. If this is not possible a new, good quality tape should be

(b) Badly repaired, twisted or kinked tapes will give erroneous distances.

(c) Site steel tapes should be checked against a Department of Trade or similar, standard. If this is not possible a new, good quality tape should be

**kept solely for the checking of field tapes.**

The procedure for checking is simple.

(i) Using the standard tape, mark out two points on the ground say 29.900 m apart. Apply the standard tension (usually 5 kg) using tension

The procedure for checking is simple.

(i) Using the standard tape, mark out two points on the ground say 29.900 m apart. Apply the standard tension (usually 5 kg) using tension

**handles or by estimation.**

(ii) Lay the site tape beside these marks, holding 0.100 m against one end, and reading the other. Again apply correct tension.

(iii) The reading on the site tape should read 30.000 m. If it does not then the difference indicates how much the tape has stretched or

(ii) Lay the site tape beside these marks, holding 0.100 m against one end, and reading the other. Again apply correct tension.

(iii) The reading on the site tape should read 30.000 m. If it does not then the difference indicates how much the tape has stretched or

**shortened.**

A reading of 29.995, indicates the tape has stretched 5 mm over 30 m. When necessary this correction should be applied to measured distances as

A reading of 29.995, indicates the tape has stretched 5 mm over 30 m. When necessary this correction should be applied to measured distances as

**follows.**

measured distance

Correction = -------------------------- x tape error

30.00

Example: Using the tape checked in (iii), a distance of

15.720 m was measured.

15.720

Correction = -------------- x 0.005

30.00 m

This test indicates that the tape has stretched and

Therefore the correction is added to the measured length to give 15.723 m

(d) Always hold the graduated side of the tape against the measuring mark.

(e) Poor ranging could contribute to inaccurate distances but not to any great extent.

(f) Incorrect reading of tape.

(g) Miscounting tape lengths on long distances.

(h) Failure to take into account the slope of a taped measurement is a major source of error.

Corrections can be made two ways.

Slope distance x cosine of vertical angle

Horizontal distance = ______________________________

/Slope distance – difference in height

(i) Ensure you use the correct “zero” of the tape. On tapes where the “zero” is the hook use the 0.100 m graduation and allow for this in the tape

measured distance

Correction = -------------------------- x tape error

30.00

Example: Using the tape checked in (iii), a distance of

15.720 m was measured.

15.720

Correction = -------------- x 0.005

30.00 m

This test indicates that the tape has stretched and

Therefore the correction is added to the measured length to give 15.723 m

(d) Always hold the graduated side of the tape against the measuring mark.

(e) Poor ranging could contribute to inaccurate distances but not to any great extent.

(f) Incorrect reading of tape.

(g) Miscounting tape lengths on long distances.

(h) Failure to take into account the slope of a taped measurement is a major source of error.

Corrections can be made two ways.

Slope distance x cosine of vertical angle

Horizontal distance = ______________________________

/Slope distance – difference in height

(i) Ensure you use the correct “zero” of the tape. On tapes where the “zero” is the hook use the 0.100 m graduation and allow for this in the tape

**reading.**

(ii) Several other systematic errors can occur with a tape. These can either be calculated and thus eliminated or can be minimized by correct field

(ii) Several other systematic errors can occur with a tape. These can either be calculated and thus eliminated or can be minimized by correct field

**procedure.**

(i) Tension: Tapes read their correct length under standards of tension and temperature. For most site tapes the standard tension is 5 kgf.

(i) Tension: Tapes read their correct length under standards of tension and temperature. For most site tapes the standard tension is 5 kgf.

**Where the accuracy of measurement warrants it, a tension control device should be used. Otherwise attempt to estimate the correct tension.**

(ii) Temperature: Most tapes read their correct length at a standard temperature of 20C (obviously allowing for further corrections for

(ii) Temperature: Most tapes read their correct length at a standard temperature of 20C (obviously allowing for further corrections for

**standard tension, etc.). A correction for temperature, can be calculated from the following:**

Correction = L x a x (t-ts)

Where L = measured length

A = coefficient of linear expansion

(for most steel tapes this is 0.00001125/ C)

t = ambient air temperature

ts = standard temperatue (20C)

(iv) Sag: A sagging tape will always give an overestimate of

The distance. A correction can be computed with the following formulae.

-W2L3

Correction = ----------

24T2

Where W = weight of unit length of tape

L = measured length

T = applied tension

Weights of tape can be found in manufacturers’ handbooks. For example Rabone Chesterman quote the following:

10mm wide tape 0.012 kg/m

13mm wide tape 0.016 kg/m

Sag can be eliminated or reduced if tape lengths of no more than 10 m are used.

Correction = L x a x (t-ts)

Where L = measured length

A = coefficient of linear expansion

(for most steel tapes this is 0.00001125/ C)

t = ambient air temperature

ts = standard temperatue (20C)

(iv) Sag: A sagging tape will always give an overestimate of

The distance. A correction can be computed with the following formulae.

-W2L3

Correction = ----------

24T2

Where W = weight of unit length of tape

L = measured length

T = applied tension

Weights of tape can be found in manufacturers’ handbooks. For example Rabone Chesterman quote the following:

10mm wide tape 0.012 kg/m

13mm wide tape 0.016 kg/m

Sag can be eliminated or reduced if tape lengths of no more than 10 m are used.