1. BASIC DISTANCE MEASUREMENT

2. contents Types of distance measurements Measurement methods
Taping methods
Errors and corrections in distance measurements

3. Linear measurements
Linear distance measurements in plane surveying are made in such a way that they are horizontal.
Where a distance is measured along a slope it will have to be projected.

4. Types of Distance Measurement

5. Types of Distance Measurement

6. Types of Distance Measurement

7. Methods of Measurement
Pacing
Accuracy 1 : 100
Taping
Accuracy 1 : 10,000
Electronic Distance Measurement (EDM)
Accuracy 1 : 10,000 to 1:100,000

8. Taping Direct distance measurement Equipment for measurement of lines
Material
Chain
Tempered steel
Steel band
steel
tapes
Varies (synthetic material, glass fibre, coated steel

9. Other equipment Optical square Ranging rods Abney level Pegs
Used to measure right angles
Either made of prism or mirror
Ranging rods
Used for marking any points that needs to be seen
Abney level
Measures ground slope
Pegs
Used to mark points that need to be permanently placed on the ground

10. Tape measurements techniques
Surface taping
Tape is laid on the ground and fully supported on the ground. Usually done where the ground is even.
Catenary taping
Tape is suspended clear of the ground. This eliminates the effects of surface irregularities.
Reduces uncertainty in the determination of tape temperature

11. Step taping
Used in ground with variable slope. The distance is measured in steps with the tape held horizontally.
Measurements are done in steps of 5 – 10 m
A plumb bob is used to transfer the distance to the ground.
Easier to work downhill than up hill

12. Errors and corrections
Systematic errors
Standardisation
Slope
Tension
Temperature
Sag
alignment

13. Correction Standardisation
Occurs when the length of the tape is not equal to the standard one
Due to wear and tear, tape tends to stretch
The error per tape length can be measured by checking the tape against a standard.

14. ls = standard length of tape (m) l = nominal length of tape (m)
L = measured length (m)
ls = standard length of tape (m)
l = nominal length of tape (m)
The sign of correction is given by magnitude of l and ls

15. Slope Measurement should be on the horizontal plane
Slope lines must be reduced to horizontal
Correction is always negative
Step taping
No need to measure slope angle or vertical distance
No corrections needed
Correction always -ve

16. Projecting slope distance to horizontal plane
Applicable where the ground runs in long regular slopes
Slope measured by
Leveling
Abney level or
Procedure which gives surface heights at points along slope
Using slope angle
Slope angle
Measured length

17. Using difference in level between the two points
h = height difference between the two points
Acceptable if h/L is less than 1 in 25

18. Tension Caters for elasticity of tape material
Occurs when the tape is tensioned to a value at which it was not standardized at
For Steel tapes
E is 200,000 N/ mm2
P, Ps = field and standard tension respectively (N)
A = cross sectional area of band (mm2)
E = Young’s modulus of elasticity (N mm-2)
L = length measured (m)
The sign of correction is given by magnitude of P and Ps

19. Temperature Due to thermal characteristics
Required if the temperature is not the same as when the tape was standardized
For Steel tapes
 is per °C
coefficient of linear expansion
temperature
Standard temperature
Measured length
The sign of correction is given by magnitude of t and ts

20. Sag
If a tape was standardized on flat level, using catenary will introduce a sag error due to its weight.
w = weight per unit length (N/m)
W = total weight of tape (N)
l = measured length of span (m)
= angle of slope between supports
P = tension applied
Correction always -ve

21. Alignment
Applied when there is an obstruction that causes the tape to be misaligned
Correction is usually small
e = amount of deviation from tape
L = measured length

22. Combined Formula Steel Taping: Examples Actual length is:
For vertical length is:

23. A steel tape of nominal length 30 m was used to measure a line AB by suspending it between supports. The following measurements were recorded
Line Length Measured Slope Angle Mean Temp. Tension
AB m 3o 40’ 5oC N
The standardisation length of the tape against a reference tape was known to be m at 20oC and 50 N.
If the tape weighs 0.17 N/m and has a cross sectional area of 2 mm2, calculate the horizontal length of AB.

24. Mistakes/ Gross errors
Inexperience or carelessness of surveyor
Mis – reading
Erroneous booking
Omitting entire band/tape length
Displacement of station errors
Mis alignment of tape