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.


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