What is dumpy level
A dumpy level is a precision instrument used for measuring angles in the horizontal plane, typically designed to measure vertical angles. It is a type of optical “spirit” level. The name comes from “dumpy level,” which was an instrument with a telescope and crosshairs that was able to measure all three Cartesian coordinates – X, Y, and Z.
A dumpy level has two main components: the telescope and the level vial(s). The telescope is straight with crosshairs at its center. It can be rotated through 360 degrees for use in any orientation or elevation, although telescopes designed specifically for measuring horizontal angles are always oriented along a line parallel to the Earth’s horizon (parallel to the direction of gravity) when in use. The vial(s) are mounted perpendicular to the telescope, so that it forms a cross with the telescope when looking through the vials. A second crosshair is added just below the lower vial at half height for improved accuracy.
The level is zeroed by adjusting the telescope so that both vials read “level.” This is done by turning two knurled screws, one on either side of the level. The telescope and vial assembly is then rotated until it reads “plumb,” at which point a third screw is readjusted to bring the crosshairs into alignment with the gravity vector. The level may be marked in degrees and/or percent grade to show elevation.
Care must be taken when using a dumpy level, as the two vials will not always agree with each other by exactly the same amount. For example, if one is oriented into direct sunlight it may read slightly “low”. Where extreme accuracy is required, the level must be checked for accuracy by a separate instrument such as an agate clinometer.
The dumpy level has been replaced in modern times with the use of total stations and digital levels which are accurate to within 0.2mm/m (0.01 degree) compared to some hand levels that can have an error of 4 degrees for a complete grade.
Components of Dumpy Level
A dumpy level is an instrument that has been used for centuries to help surveyors make precise, accurate measurements of vertical angles. Dumpy levels are now replaced with the use of digital levels.
The primary components of a dumpy level are:
Carrying tray (or base): This provides support for the instrument’s components.
Beam (or body): This consists of an upright rod, or “tape,” that is used for leveling the instrument. The primary function of the tape is to check depth of water in a well; it can be extended and retracted by turning a screw mechanism on top of the main housing.
Level vials (or windows): These are placed at a right angle to one another on the horizontal crossbar of the instrument. The vials contain a bubble level that is used for leveling the instrument and a plumb line that can be adjusted by turning a screw on the housing.
Ocular telescope: This piece of equipment has a small hole, called a “peepsight,” used for sighting an object. It is mounted on the instrument’s crossbar and can be moved by turning a screw or knurled nut.
Main housing: This contains a needle on a circular scale, which indicates the angle of the line on level vials at full depression of the leveling screws. The scale is in degrees, minutes, and seconds.
Leveling screws: These are located at the top of the main housing. They allow the user to level the instrument when adjusting for differences in height between objects by turning clockwise or counterclockwise until both bubbles are centered in their respective levels. The leveling screws may also be adjusted to account for the width between two objects or a gradual decline in ground.
Plumb bob: This is located on the vertical crossbar of the instrument, and it allows for precise alignment with another object. A plumb bob has an adjustable attachment to allow it to be moved up or down on top of the main housing by turning a screw or knurled nut.
Measuring tape: This is a metal tape with a small magnet attached to its end. The tape is used for measuring the depth of water in a well. It can be extended and retracted by turning a screw mechanism on top of the main housing that engages or disengages the gears that pull the tape into or out of the housing. This can be done before or after leveling the instrument.
Carrying case: This protects the components of the dumpy level when not in use.
Tripod stand: Sometimes called a “tripod head,” this provides support during operations. The tripod stand is usually made of wood, but some are made of aluminum.
Laser: Some dumpy levels include a laser that projects a line from the instrument to the target object. This allows for precision alignment of two objects, such as during construction or repair work where vertical alignment is critical. It can be used with reflective tape for nighttime use.
Advantages of Dumpy Level over Auto Level
The main advantages of dumpy level are as under –
• The transit is easy to carry which makes it possible to carry it along during field measurements without any hindrance.
• It gives accurate results with the help of simple reading directly from the vial.
• It can be used by both professional surveyors or ordinary farmers.
• The lens can show an object at a distance up to 600 ft and there is no need of focusing the lens while using it.
• It gives quick results and thus reduces the time taken.
• The close fitting boxes protect the level from dust and damage while not in use.
• There are fewer chances of breakage or leakage of vial in the transit.
• It is easy to understand and operate.
• The lenses are safe in case of accidental breakage or damage because they can be replaced easily.
• When compared to an ordinary level, this instrument provides more accuracy even in difficult situations like in a dark room or when viewing objects in dense trees.
• The use of this instrument is not restricted to any particular class of people.
• It is inexpensive and the lenses do not require frequent replacement which makes it an economic investment.
Limitations of Dumpy Level
Limitations of dumpy level are as under
1.Correction of dumpy level is done by trial and error method for precise measurements.
2.A dumpy level cannot be used to determine the height of tall buildings or high mountains, where high accuracy is required because of its inherent limitations as discussed below:
3.It is difficult to maintain a constant height between the telescope when the horizontal cross hair is viewed in both the levels.
4.Dumpy level consists of two water filled reservoirs which are connected by a capillary tube, but it is difficult to fill all the air bubbles completely out of the capillary tube hence producing error due to air bubble.
5.Leveling rod used with the dumpy level is not absolutely free from flexure.
6.It will take more time to find out the height of object by using dumpy level as compared to electronic levels because in the latter case, digital display is available which shows direct reading of height, but in cased of dumpy level, it is read through a table.
7.It is difficult to lay out the level line and set out stations properly with dumpy level as compared to electronic levels because in case of latter, digital display provides direct reading of slope distance, but in cased of dumpy level, it has to taken from the table provided along with dumpy level.
8.A dumpy level can be used to determine the height of a building or a mountain only in case if it is situated in an open place away from any obstruction within 100 m radius and with clear sky view between the observer and object whose height is to be determined.
9.The use of dumpy level requires considerable skill for leveling.
10.The process of adjustment of dumpy level is complicated and time taking as compared to the use of electronic levels.
11.In case of a dumpy level, observer has to move along with telescope from place to place, but in the case of an electronic level, it can be fixed at one place where leveling rods are also fixed, so that observer can see the digital display of height from observation post.
12.If the bubble is not properly centered in A level when it is read, then the reading obtained will be reduced to half value when it is read again with the bubble in the small vial.
13.When the reading is taken from a dumpy level, it should be noted that no water must be allowed to come into contact with the delicate parts of instrument.
14.Dumpy level is not adjusted for measuring height of a building or a mountain whose height is more than 100 m and telescope of dumpy level is not properly adjusted for finding out the difference in height between nearby objects and that of remote object, then it may produce wrong results when the angle is measured in open place.
15.In case when angle is to be measured from observation post and dumpy level is not available at the observation post, then angle is to be measured by using conventional methods by selecting suitable station point and observatory point.
16.It is difficult to find out prominent objects from those which are very small in distance as compared to those which are far away.
17.The height of tall buildings, high mountains and other similar objects can be determined only through astronomical methods.
18.Observer should not place his head inside the telescope while using dumpy level or electronic levels, otherwise he may sustain serious injuries due to air pressure inside the instrument.
19.While working with a dumpy level, the instrument should be kept on a smooth horizontal platform and it must not be placed at such place where there is possibility of air draft.
20. It is difficult to make use of dumpy level in case of heavy winds or very cold weather.
21. A dumpy level should be protected from shock, dust, dirt, extreme heat or cold and direct sunlight.
Accuracy of Dumpy level over other levelling instruments:
The prime reason for using Dumpy level over other levelling instruments is its accuracy.
Dumpy level is the most accurate of all the levelling instruments available. This device can be used to test any of them for accuracy or calibration. Accuracy of this instrument is 0.2mm/km at three significant figures, which means that you can measure earth surface to within 0.05mm over 100km.
Dumpy level is accurate because it has very stable bubble of about 10cm of diameter inside the telescope. Bubble does not move up or down along with temperature change, humidity, atmospheric pressure changes etc., Hence after careful leveling on both ends of the baseline, ane can read off the length of the baseline with great accuracy.
The telescope has its own light source (a pin hole) which enables us to see an inverted image of reference mark given that the telescope is leveled correctly. This reference mark gives you exact elevation below, so one can actually go there and measure the distance accurately with tape measure.
By looking at this inverted image of the reference mark through telescope, one can actually see if the instrument is level by observing whether or not the bubble in telescope appears to be inside or outside edge of the fibre rod.
Leveling of Dumpy (or any other levelling instrument) is time consuming process; accuracy is directly proportional to length of your base line. If your base line is short you will get less accuracy.
Recommendation: If you operate in a place where there are frequent fluctuations in temperature/humidity etc., then Dumpy level should be calibrated timely. This can be done at any Government run Survey of India office anywhere in the country.
You should never lend / borrow / hire / purchase a Dumpy level, unless you are satisfied that it is calibrated correctly.
You should never try to adjust the bubble inside the telescope by yourself, as this will damage the instrument beyond repair.
If you find that accuracy of your instrument does not match specification mentioned above, then take it immediately for calibration at Survey of India office where you got it.
You can read more about Dumpy level (illustrated with photographs) at the following link: http://www.avonmaps.com/articles_details.asp?sid=101&cid=1&scid=11