Thursday, February 20, 2014

survey instruments 02-Dumpy level and Tripod

Dumpy level and Tripod

In 1832, English civil engineer William Gravatt, who had worked with Marc Isambard Brunel and his son Isambard on the Thames Tunnel, was commissioned by Mr. H.R. Palmer to examine a scheme for the South Eastern Railway's route from London to Dover. Forced to use the then conventional Y level, during the works Gravatt devised the more transportable and easier to use dumpy level.  

What is dumpy level
 A dumpy level is used to establish or check points in the same horizontal plane. It is used in surveying and building to transfer, measure, or set horizontal levels. But Dummy level is the most commonly used level.  The dummy level is simple compact and stable. It is cheaper, and easier to maintain in adjustment.
      The term dumpy level endures despite the evolution in design. A dumpy level is an older-style instrument that requires skilled use to set accurately. The instrument requires to be set level (see spirit level) in each quadrant, to ensure it is accurate through a full 360° traverse. Some dumpy levels will have a bubble level ensuring an accurate level.

A variation on the dumpy and one that was often used by surveyors, where greater accuracy and error checking was required, is a tilting level. This instrument allows the telescope to be effectively flipped through 180°, without rotating the head. The telescope is hinged to one side of the instrument's axis; flipping it involves lifting to the other side of the central axis (thereby inverting the telescope). This action effectively cancels out any errors introduced by poor setup procedure or errors in the instrument's adjustment. As an example, the identical effect can be had with a standard builder's level by rotating it through 180° and comparing the difference between spirit level bubble positions.

Setting the Level
The level instrument is set up on a tripod and, depending on the type, either roughly or accurately set to a leveled condition using foot screws (leveling screws). The operator looks through the eyepiece of the telescope while an assistant holds a tape measure or graduated staff vertical at the point under measurement. The instrument and staff are used to gather and/or transfer elevations (levels) during site surveys or building construction. Measurement generally starts from a benchmark with known height determined by a previous survey, or an arbitrary point with an assumed height.

Parts of dumpy level

1. Telescope
2. Eye-piece
3. Ray shade
4. Objective end
5. Longitudinal bubble
6. Transverse bulb tube
7. Bubble tube adjusting screws
8. diaphragm adjusting screws
9. diaphragm adjusting screws
10. foot screws
11. upper parallel plate
12. foot plate

Dumpy Levels can be used for
Determining the height of a particular point.
Determining differences in height between points.
Drawing contours on a land.
Providing data to calculate volumes for earthworks.
Setting out level surfaces for construction.
Setting out inclined surfaces for construction.

Measuring height using Dumpy Level 

The place of which height is to be measured is called Station.
Height is always measured with reference to sea level.
Survey of srilanka established benchmarks (B.M) at several places.
Ideally the distances should be taken from the benchmark.
If it is not available then we can select point on the map whose distance from sea level is known as the reference.
We can fix any suitable point as Temporary bench mark and all heights can be measured from that point. We can fix any temporary bench mark , but if its MSL( at temporary BM) is not sure , then before starting the surveying permanent BM reading should be clear.
                 HI= BM+BS BACKSIGHT(BS)
      In the picture Bench mark is setup at a height of 100 ft.As a first step we need to know the height of the place from where instrument is placed. In the picture, a boy is standing on a place from where survey will begin.To know the height of the instrument place a staff or level rod at bench mark location. And from the instrument take the reading on the staff. In this case reading on the staff is 5.5ft . This reading is called as backsight reading (BS).
                Height of the instrument = BM + BS = 100 + 5.5 = 105.5 ft.
Now surveyor can go ahead and determine heights of other places. Look at the following figure to take further reading. These readings are called as foresight reading (FS):
Now staff or level rods are put on the point (D) height of which is to be measured. The reading on the staff is recorded through telescope of dumpy level. In present case the reading shows 2.3ft.This reading is called Forward site ( FS) Therefore height of point D is calculated as follows Height of point D = Height of Instrument (HI) – FS = 105.5 – 2.3 = 103.2 ft. Distance between the instrument and the station D can be measured using meter tape or from the difference between upper & lower readings on the telescope.

Wednesday, February 19, 2014

Traditional survey - chain survey -02

  Chain Survey

Chain survey is the simplest method of surveying. In this survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. This is most suitable adapted to small plane areas with very few details. If carefully done, it gives quite accurate results. The necessary requirements for field work are chain, tape, ranging rod, arrows and some time cross staff.

Principles of Chain Survey
A triangle is a simple figure which can be plotted from the lengths of three sides even if the angles are not known. In chain survey, the area to be surveyed is divided into a framework consisting of triangles.
Selection of survey station and Survey lines
I. The main station should be mutually invisible so that ranging can be done easily.
II. Survey lines should be as few as possible so that framework of triangles can be plotted easily.
III. Survey lines should pass through a level open ground as far as possible so that chaining can be done easily.
IV. The frame should have at least one long baseline that runs through the middle of the area.
V. The main survey lines should form well conditioned triangles with no angles less than 30 degrees or more than 120 degrees.
VI. The survey lines should be selected such that there are no obstacles in chaining and ranging.
VII. The survey lines will be close to the details to avoid long offsets.
VIII. If required, number of subsidiary stations should be selected and subsidiary lines run to avoid long offsets.
IX. Each triangle should have a check line to detect mistakes in measurements and plotting.
X. While selecting main stations and survey lines, the basic principles of surveying of working from whole to the part should be followed.

Main Stations
Main stations are the end of the lines, which command the boundaries of the survey, and the lines joining the main stations re called the main survey line or the chain lines.

Subsidiary or the tie stations
Subsidiary or the tie stations are the point selected on the main survey lines, where it is necessary to locate the interior detail such as fences, hedges, building etc.

Tie or subsidiary lines
A tie line joints two fixed points on the main survey lines. It helps to checking the accuracy of surveying and to locate the interior details. The position of each tie line should be close to some features, such as paths, building etc.

Base Lines
It is main and longest line, which passes approximately through the centre of the field. All the other measurements to show the details of the work are taken with respect of this line.

Check Line
A check line also termed as a proof line is a line joining the apex of a triangle to some fixed points on any two sides of a triangle. A check line is measured to check the accuracy of the framework. The length of a check line, as measured on the ground should agree with its length on the plan.

These are the lateral measurements from the base line to fix the positions of the different objects of the work with respect to base line. These are generally set at right angle offsets. It can also be drawn with the help of a tape. There are two kinds of offsets:
The measurements are taken at right angle to the survey line called perpendicular or right angled offsets.
The measurements which are not made at right angles to the survey line are called oblique offsets or tie line offsets.

survey instruments 01-Tapes


Tapes are used in surveying to measure horizontal, vertical, and slope distances. They may be made of a ribbon or a band of steel, an alloy of steel, cloth reinforced with metal or synthetic materials. Tapes are issued in various lengths and widths and graduated in a variety of ways.
I. Cloth or Linen Tape
II. Metric Woven Metallic Tape
III. Metric Steel Tape
IV. Invar Tape
V. Synthetic Tape

I. Cloth or Linen Tape
It is made of varnished strip of woven linen 12 to 16 mm wide with a brass handle at zero ends, whose length is also included in the length of the tape. The tape is attached to a spindle and is wound in a leather case. It is very light and handy, but not so accurate. For very precise measurements, it is not used. The linen tape may be used for taking subsidiary measurements such as offsets. It is easily affected by damp. When the tape gets wet, it shrinks and care should be taken that it is not wound up until is cleaned and dried. It stretches easily and is likely to twist and tangle. It is therefore little used in surveying.

II. Metric Woven Metallic Tape
They are available in 2, 10, 30, and 50 meters. The tape is made of yarn and metal wire. A metal ring is attached to the outer end of tapes. The length of the tape includes the metal ring. At every centimeter a black line 8 to 10 mm in height is drown. Every 5 centimeters is marked with an arrow in black. Every decimeter and meter is marked with a back line extending over the full width of the tape the graduation marks at every decimeter and meter are numbered with black and red figures, respectively.

III. Metric Steel Tape
Tape is available in 1, 2, 10, 30, and 50 meters. The tape is of steel or stainless steel. The outer end is provided with a ring. The length of the tape includes the metal ring. The tape is marked with a line at every five millimeters, centimeters, decimeters, and meter. Every decimeter and meter shall be marked with Hindu Arabic numerals in bold. When the button release devised is pressed, the tape automatically rewind in to the case.

IV. Invar Tape
For highest precision work the invar tape in used. Invar tapes are made of an alloy of nickel (36%) and steel (64%) having very low co-efficient of thermal expansion (0.000000122 per 1 C) These are 6mm wide and are available in length of 30m, 50m and 100m, Invar taps are used mainly for high degree of prevision required for base measurements.

V. Synthetic Tape
The tapes are manufactured of glass glass fiber having PVC coating. They are graduated every 10 mm and figured every 100 mm. Meter, figures are shown in red. They are convenient for measuring shorts lengths.

Traditional survey - chain survey-01


The chain is a instrument and unit, using for surveying and measure distances. The chains used in surveying are generally of the following types.
I. Gunter’s chain
II. Engineer’s chain
III. Metric chain
IV. Steel Band Chain
V. Revenue Chain


I. Gunter’s chain
surveyor’s chain, also called Gunter’s chain, measuring device and arbitrary measurement unit still widely used for surveying in English-speaking countries. Invented by the English mathematician Edmund Gunter in the early 17th century, Gunter’s chain is exactly 22 yards (about 20 m) long and divided into 100 links. In the device, each link is a solid bar. Measurement of the public land systems of the United States and Canada is based on Gunter’s chain. An area of 10 square chains is equal to one acre.
II. Engineer’s chain
This steel engineers’ chain would have been used by a surveyor measuring out lands and plot sizes.  It is different from a surveyor’s chain as it is larger in length however it would have been used for the same purpose, measuring out horizontal distances.  The chain measures 100 ft and contains 100 links, each measuring 12 inches.  The links each consist of a thin round steel rod joined with an oval ring on each end.  Tags mark the chain at every ten links.  It was made by W. & L.E. Gurley of Troy, New York.  Brothers William Gurley and Lewis formed the company in 1852; they were both graduates of Rensselaer Polytechnic Institute in New York.

III. Metric chain
A metric chain is generally divided into 100 or 150 links. The links are composed of pieces of galvanized mild steel wire 4mm in diameter. The ends of each link are bent into loops and connected together by means of three oval shaped rings which afford the flexibility to the chain. The joints of the links are usually pen but in good quality chains, these are welded so that true length of the chain does not alter due to stretching. The ends of the chain are provided with brass handles with swivel joints so that the chain can be turned round without twisting. The outside of the handle is the zero point or the end point of the chain. The length of a chain is measured from the outside of one handle to the outside of the other. The length of a link is the distance between the centers of the two consecutive middle rings. The end links also include the length of the handle. Metallic tags of different patterns are fixed at various important points of c chain.

IV. Steel Band Chain
It consists of a ribbon of steel with bras handle at each end. It is 20 or 30 long and 16 mm wide. It is wound on an open steel cross or on the metal reel in a closed case. The graduations are etched as meters decimeters, centimeters on one side and 0.2 m links on the other. Brass tallies are fixed at every 5 m length of the band.
V. Revenue Chain
It is commonly used for measuring fields in cadastral survey. It is 33 ft long and divided into 16 links. Each link is 2.0625 ft long.

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