CHAPTER 2CHAIN SURVEYINGTape Measurements

Mapping Details using chain surveying In chain surveying all ground features (natural or industrial) are located and

mapping by measuring lengths using tapes to a selected lines called chain lines .This done by one of two ways:

1- Ties method : 2- Offsets method:

BuildingPole

Ties

BuildingPole

offsetsChain lines

Ground Details may be located by the method of ties or offsets or a combination of both. The two methods depend on measuring horizontal distances between points and setting Right angles using some of surveying equipments.

Chain Surveying Equipments

1- Equipments used for the measurements of lines :

A- The chain

B- Tapes: synthetic, glass fiber, coated steel or plain steel of lengths: 10m , 20m, 30m, 50m . , tapes are more accurate than chain.

C- Invar Tapes: accurate tape made of steel (65%) & Nickel (35%) but they are very expensive.

2 -Equipments used for making right angles: A- The cross staff: B- Site square: consists of two

telescopes, their lines of sight at 90º.

C- The optical square: There are two types:

mirrors & prism

3 -Other equipments: a- Ranging Rods: 2m, 2.5m, 3m long & painted red and white each 0.5m

with a pointed steel shoe . b- Arrows: 40cm long & 3- 4 mm in diameter.

c- Pegs: wooden (square or circular cross-section) & steel

d- Plumb Bob: a metallic cone object used to erect vertical lines.e- Clinometer & Abney level: used to measure the inclination angle of slope lines .

Processes in Chain Surveying

1 -Ranging and measurement of lines: a- Level ground.

b- Uniformly Sloping Ground.

c- Uneven Ground (non-uniformly sloping ground): using stepping process.

2 -Setting out right angles:

A- Dropping a perpendicular from a point to a line:

1- Using the optical square.

2- The equilateral triangle method.

3- Pythagoras theorem method.

B- Setting out a perpendicular to a line from a specific point:

Plan of Mapping details using chain surveying1-Reconnaissance: The surveyor should visit the area to be mapped in order to:

a- Notice the shape of the area, the existing details and draws a reasonable sketch of the area showing all details such as roads, buildings, fences, electric poles, ….etc.

in addition to the approximate north direction.

b- Choose the most suitable location for the survey station which form the chain lines,

the following points should be taken into consideration when choosing survey stations:

1- chain lines should form well-conditioned triangles with internal angles is between 30º

and 120º .

2- chain lines should be chosen as close as possible to the mapping details.

3- It should be possible to see at least two other survey stations from each station.

4- The number of chain lines should be kept to a minimum, but enough to locate all details.

5- Survey station should be chosen in away such that check lines is available.

6- Survey Station should be chosen on firm ,easy to reach grounds.

2- Booking the measurements

20cm

.

15cm

1.50cm

3- Plotting the details:

a- Choose the appropriate scale

b- If you are drawing the map manually, using a pencil to draw the chain lines on a transparent paper, in order to center the drawing on the final map sheet.

c- Try to make the north direction pointing towards the top of the sheet.

d- Offsets and ties are plotted systematically in the same order in which they were measured and booked.

e- After finishing the drawing by pencil, the plan is taken to the site and checked.

Accuracy of Measurements:• A good draughtsman can plot a length to within 0.20 mm.

• (0.20mm on a map of scale 1:500 represents 10cm on the ground).

• (0.20mm on a map of scale 1:100 represents 2cm on the ground).

• As a result ,measuring a line to 1 cm accuracy is practically sufficient for such scales.

Chaining Obstacles1-Vision obscured, Chaining possible:

2 -Vision Possible, Chaining Prevented:A- Measurements can be made around the obstacle:

AB = AC + EF + DBAB = AG + KL + HB

B- The width of obstacles is greater than tape length:

Case a:HG = FD .FG /ED

= CG × FG / (EC – FG)

Case b:The unknown distance FHIs equal to JG which can

easily measuredCase c:

EG = CE × EF / ED

3-Both Chaining and Vision prevented:

GD = FC × GA / FA HE = FC × HA / FA

ED is the missing part of AB Length AB = AD + DE + EB

DE = HJ

MN = 2 CD

ERRORS IN CHAIN SURVEYING1- Blunders (Mistakes).2- Systematic errors: a- Temperature correction : Ct = 0.0000116 ( T1 – T0 ) L 0.0000116 is the coefficient of thermal expansion of steel per 1º C T1 = is the field temperature. T0 = is the temperature under which the tape is calibrated L = is the length of the line

b- Sag correction: Cs = -W²L / 24 P² or = - w² L³ / 24 P² W = the total weight of the section of tape located between supports. w = weight per meter of tape L = the interval between supports P = the tension on the tape

Ex : calculate the sag correction for : a- A 100 ft steel tape weighting 2 ib and supported at the ends only with a 12

Ib pull b- A 30 m steel tape weighting 0.0112 kg/m and supported at 0, 15, 30m points

under a tension of 5 kg.Solution

a- Cs = -W²L / 24 P² = - (2² × 100 ) / ( 24 × 12² ) = -0.116 ft b- Cs = - 2 ( w² L³ / 24 P²) = -2 (( 0.0112² × 15² ) / (24 × 5² )) = -0.001 m.

c- Tension correction: Cp = (P1 – P0 ) × L / AE Cp = the elongation of the tape of length L in m.

P1= the applied tension

P0= the calibration tension

A = the cross-sectional area of the tape in cm²

E = the modulus of elasticity of the tape material (for steel E= 29,000,000

ib/in² ).

d- Length correction: Cl = ( la – l0 ) L / l0 Ex: A line is measured with a tape believed to be 100 ft long which gives a

length of 705.76 ft . On checking ,the tape is found to measure 100.02 ft . What is correct length of the line ?

Solution

la = 100.02 ft , l0 = 100 ft , L = 705.76 ft

Cl = ( 100.02 – 100 ) × 705.76 / 100 = + 0.14 ft

correct length = 705.76 + 0.14 ft = 705.9 ft.