What is Leveling?
The process of calculating the height of one level in relation to another is known as levelling. It is used in surveying to determine a point’s elevation relative to a datum or to determine a point’s height relative to a datum.
- The goal of levelling is to find a horizontal line of sight from which to calculate the vertical distances between locations above and below it.
- The goal of levelling is to determine the elevation of a particular place in relation to a predetermined reference line known as the datum.
- To locate a point at the necessary height with relation to a datum.
Types of Levelling Instruments used in Surveying
The following instruments used in levelling during surveying
- Dumpy level
- Wye or Y level
- Cooke’s reversible level
- Cushing’s level
- Tilting Level
The most often used device in levelling is the dumpy level. The dumpy level is an optical device that is used for surveying and levelling. It is made up of a telescopic tube that is securely kept in place by two collars and adjustable screws. The vertical spindle controls the entire instrument.
The telescope may be rotated in the horizontal plane when it is positioned on the dumpy level. The dumpy level can be used to estimate the relative height of survey locations on the ground.
The telescope is prohibited from moving in its horizontal plane at this height, and it is fastened to its support. On the telescope’s top, there is a bubble tube. The levelling head, on the other hand, maybe rotated horizontally with the telescope.
The internal focusing telescope is a metal tube with four major components, which are listed below.
- Objective lens
- Negative lens
Willian Gravatt came up with the dumpy level in 1832. As a civil engineer, he used a Y (Wye) level instrument to invent the dumpy level.
The dumpy level works on the idea of creating a visual connection between two or more places using an integrated telescope and a bubble level. Steps can be taken to obtain the desired level of precision.
It’s also known as Surveyors Level, Builders Level, Dumpy Level, and its prehistoric form, “Y(Wye) Level.” The precision of the Dumpy level is the main argument for choosing it over other levelling devices.
For most Tacheometric techniques, a dumpy level is renowned for having high accuracy values. A dumpy level’s accuracy might be as high as 1:4000 for every 100 metres.
Uses of Dumpy Level
- In the surveying of a building site, the use of a dumpy level is important. The precision and portability of the dumpy level have made it a popular option among surveyors.
- The primary goal of levelling on a building site is to create a level and smooth surface.
- To find the height differences between the two locations.
- Using the theory of relativity, estimate the height and distance of several surveying land places.
- To determine the distance between several points on the surveyed land.
- Creating building levels and sloped surfaces.
- To trace the outlines of a piece of land.
How to Record Dumpy Level Readings?
Taking the backsight readings
- The backsight should be your first point of reference. By doing this, you’ll be able to figure out the instrument/level (height, )’s from which all other levels can be determined.
- The person in charge of the staff should set the bottom of the staff level on the BM or TBM as vertically as feasible.
- Using a precise adjustment, have the person at Level rotate the telescope such that the central line/cross hairs are aligned with the numbers on the staff. You may need to use the eyepiece first to focus to view the cross hairs, and then switch to using the telescope focus to see the numbers.
- When using a telescope, the reading is taken to the nearest centimetre at the point where the central and stadial cross hairs meet.
- figuring out the instrument’s overall height To figure out the instrument’s height, add the value you just took to the known value of the BM or TBM you’re using.
Taking Foresight readings
- Reducing levels To demonstrate changes in height and construct profiles, start by taking height measurements of everything you wish to illustrate on the site top of the slope, bottom of slope, break of slope.
- Record the beginning or maybe next accessible number on the plan for your levels if you’re returning to a survey, then write off each height reading into a separate notebook.
- Make sure you write carefully and keep track of the date, the location of the survey, the BM or TBM, and the initials of the survey participants.
- You must spin the telescope each time, sight on the staff in its new place, focus, and take the reading carefully, double-checking that you have read the number accurately.
- After you’ve taken all of the levels you desire, subtract each one from the instrument height to get the real height values, or reduced levels (RL). This will give you the ‘true’ ground height at the base of the staff.
Also Read: Traverse Surveying
Wye or Y Level
The Y level, also known as the wye level, is the oldest and most bulky of the earlier optical instruments. A low-powered telescope is installed in a pair of clamp mounts, then levelled using a spirit level set parallel to the main telescope. The stage of this instrument has two “wye” supports in which the telescope is mounted.
Two equal-diameter hanging collars are attached to the telescope’s body. The “Wyes” is where these collars lay. The telescope may be adjusted around its longitudinal axis or removed and put in the wyes end-to-end.
A clamp and a tangent screw are included to aid in precise object sighting. This is a fragile instrument with a significant number of loose and unattached components. Because of the telescope’s reversibility, it may be easier to test it for permanent modifications.
Cooke’s Reversible Level
The Cooke’s Reversible Level, built by Cooke, Troughton & Simms, Ltd., Buckingham Works, York, is designed so that the telescope may be levelled for each sight by manipulating a very sensitive setting screw. The ability of a reversible spirit level to select a mechanical axis and a collimation axis in a horizontal plane determines the features of this level.
The spirit level, has both top and bottom surfaces observable, is connected to the edge of the telescope, and therefore is designed in such a way that whenever the bubble takes up an equivalent position relative to index marks for either situation, the instrument is in adjustment, — in other words, whenever the telescope is genuinely level, the bubble is in the central location.
To achieve this, the effective area of the spirit level, which is most easily characterized as barrel-shaped, must be circular at all cross-sections along its length as symmetric in longitudinal section as to its axis.
The bubble will fill the same part of the tube with this axis of symmetry horizontal, but the spirit level can be rotated on its own axis or about another but parallel axis. The spirit level is read using a prism device and magnifier, which is the most exact way yet invented and, when combined with the aforementioned, makes this level an exceptionally accurate and easy levelling equipment.
The telescope is non-removable and cannot rotate in its longitudinal axis while Cushing’s level is present. However, the object and eyepiece ends are reversible and interchangeable.
The telescope in Cushing’s level cannot be rotated in its socket around its longitudinal axis, nor can it be removed from its socket and reversed end for end. The eyepiece and diaphragm, on the other hand, may be turned 180 degrees in their fittings, and the eyepiece and object glass can be swapped out, thus reversing the telescope end to end. The fundamental lines of this level, as well as their relationships, are identical to those of Cooke’s level.
The techniques and means of adjustment are the same as in the case of Cooke’s level.
- The first adjustment is to align the collimation line with the telescope’s axis.
- The second adjustment is to align the collimation line with the vertical axis at a straight angle.
- The next adjustment is to align the bubble’s axis with the vertical axis.
Also Read: Complete Guide on Ranging Rod
The telescope, together with its bubble tube, may be levelled by a micrometer screw without utilizing the instrument’s foot screws, and the collimation line can be set horizontal without regard to the vertical axis. As a result, the collimation line may not be perpendicular to the vertical axis.
Tilting levels were originally intended primarily for preliminary work, but they have since grown in popularity and are now used for regular levelling as well. The level is termed a tilting level because the micrometer screw tilts the telescope in a vertical plane. The vertical axis of a tilting level is only roughly fixed vertical by levelling screws when it is used for levelling work.
Difference between Theodolite and Dumpy level
|Only vertical measurements may be taken using a dumpy level.||Theodolite can be used to measure both horizontally and vertically.|
Difference between Dumpy Level and Tilting level
|The Dumpy level is a basic, portable, and reliable tool. The telescope is firmly attached to its mounts. As a result, it can’t be rotated along its horizontal axis.||It’s also referred to as the I.O.P. level (Indian office Pattern). The telescope tilts about its horizontal axis in this level, thus the name tilting level.|
Temporary Adjustment of Dumpy Level
Each time a level instrument is put up, a temporary correction must be made before any staff observation. It entails a series of well-defined activities that must be completed in the correct order.
The temporary adjustment of a dumpy level entails the following steps:
- Assembling begins with setting up the tripod stand, which is adjusted to a comfortable height with its top horizontal through eye estimation.
- The instrument is then secured on the head by twisting the bottom half of the instrument with the right hand and gripping firmly the upper part with the left hand.
- The tribrach and trivet must be brought together before the levelling screws can be installed.
- If there is a bull’s eye bubble (circular bubble), the tripod legs can be adjusted to centre it.
- The instrument is levelled to ensure that the vertical axis of the instrument is properly vertical. It is accomplished by following the steps below:
- By turning the upper section of the instrument, the level tube may be made parallel to any two of the foot screws.
- By turning both foot screws inward or outward, the bubble is brought to the centre of the level tube. The bubble follows the movement of the left thumb.
- By turning the upper section of the instrument, the level tube is then brought back over the third foot screw.
- By turning the third foot screw inward or outward, the bubble is brought back to the centre of the level tube.
- Step 1 is repeated by rotating the instrument’s upper portion in the same quadrant of the circle, followed by Step 2.
- Step 3 is repeated by rotating the instrument’s upper portion in the same quadrant of the circle, followed by Step 4.
- Rep Steps 5 and 6 until the bubble remains centred in both positions.
- The level tube is brought parallel to the first two foot screws in reverse order by turning the upper section of the instrument 180 degrees. If the instrument is constantly adjusted, the bubble will remain in the centre.
- To produce an image through the objective lens at the plane of the diaphragm and observe a crisp image of the object through the eye-piece, focusing is necessary. This is accomplished by removing parallax through correct objective and eye-piece focusing.
- The telescope is first directed towards the sky to focus the eyepiece.
- The eye-ring piece is then turned in or out until the cross-hairs are sharp and unmistakable.
- Because the eyepiece’s focus is dependent on the observer’s eyesight, it must be adjusted every time the observer changes.
- The telescope is initially directed towards the target before the objective is focused.
- The focusing screw is then adjusted until the object’s picture looks clear and crisp, with no movement between the image and the cross-hairs.
- Before doing any observations, this must be completed.
Also Read: Rise and fall method with Examples
Frequently Asked Questions (FAQ)
Levelling is done by using which instrument
• Dumpy level.
• Tilting Level
• Cushing’s level.
• Cooke’s reversible level.
Height of instrument method of levelling as compared to rise and fall method is
For a large number of intermediate sights, it is faster and less laborious.
How to use levelling instrument?
It’s used in combination with a levelling staff to determine the relative heights of items or markings.
Explain the importance of level tube in a levelling instrument
A level tube is used during construction to ensure that all sides of the wall are the same height.