CHAPTER 2 CHAIN SURVEYING Tape Measurements. Mapping Details using chain surveying In chain...

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.