Methods of Levelling in Surveying

Introduction

Surveying is a technique used in construction, engineering, and land management to determine the relative positions of points. This is done by measuring angles and distances, typically with a total station or transit.

Leveling is a discipline of surveying in civil engineering that measures the levels of several points in relation to a fixed point, such as the elevation of a building, the height of one point from the ground, and so on.

Surveying is a huge industry that relies heavily on accurate measurements. No one can, or should rely on an individual surveyor to do the work alone. To ensure consistency, there are different ways of levelling surveys.

Uses of the findings of leveling.

To design highways, railroads, canals, sewers, and water supply systems, among other things, with grade lines that best fit to the existing topography.

• To lay out building projects in accordance with the altitudes that have been designed.
• The volume of earthworks and other materials must be calculated.
• To explore the drainage properties of a certain location, etc.

Leveling Methods in Surveying.

They are:

• Direct Leveling (Spirit Leveling)
• Barometric Leveling
• Hypsometric Leveling
• Stadia Leveling
• Indirect Leveling (Trigonometric Leveling)

Direct Levelling

It is the most often utilized leveling procedure. Measurements are taken straight from the leveling tool in this manner. Direct leveling is classified into the following types based on observation points and instrument positions:

Simple Leveling

It is a simple and straightforward method of leveling in which the leveling instrument is placed between the points where elevation is to be determined.

Leveling rods are put at certain locations and seen with a leveling device. It is only performed when the spots are closer to each other and there are no obstructions between them.

Differential Leveling

When the distance between two places is greater, differential leveling is used.  During this process, a number of interstation are identified, and an instrument is moved to each station to measure the height of interstation locations.

Finally, the difference between the initial two positions is calculated. Differential leveling is the process of determining the difference in elevation between two or more sites without consideration for their alignment. It is used when:

• Two points are separated by a great distance.
• The height difference between the two points is significant, and
• Some impediment stands between them.

Reciprocal Leveling

Reciprocal leveling is used when it is impossible to locate the leveling instrument between the inter-visible spots.

This situation arises in the case of ponds or rivers, for example. In the event of reciprocal leveling, the instrument is positioned closer to the first station and sighted towards the second station.

For such long views, the faults in reading the staff, the curvature of the earth, and the instrument’s faulty corrections become apparent. To reduce these inaccuracies, special approaches such as reciprocal leveling should be utilized.

Precise Leveling

Precise leveling is comparable to differential leveling, however the precision is higher in this situation. A serious observation process is used to obtain great precision. It is possible to obtain a precision of 1 mm every 1 km.

Fly Leveling

When the benchmark is a long distance from the work station, fly leveling is used. In such a circumstance, a temporary bench mark is set up at the workstation based on the original benchmark. Even though it is not extremely exact, it is used to determine approximate levels.

Fly leveling is also used when the original benchmark is not accessible.

Reciprocal Leveling

Reciprocal leveling is used when it is impossible to locate the leveling instrument between the inter-visible spots. This situation arises in the case of ponds or rivers, for example. In the event of reciprocal leveling, the instrument is positioned closer to the first station and sighted towards the second station.

Barometric Leveling

The height of a point is inversely proportional to the weight of the air column above the observer, according to the theory employed in barometer leveling.

However, because air is compressible, the connection between pressure and height is not constant. Storms cause sudden changes in temperature, humidity, and weather conditions, all of which effect pressure.

Barometric methods are particularly well suited for operations in difficult terrain where great precision is not required. When measured electronically, these are also used to minimize slope lengths to horizontal.

A barometer is an instrument used to measure pressure. The altimeter is a modified form of a barometer that is used to determine the relative heights of places on the earth’s surface.

It is simple to use but extremely sensitive to variations in atmospheric pressure. A single base method is the method used to measure altitudes with an altimeter.

There are two altimeters required:

One altimeter and a thermometer are put at a known elevation position called the control point, where readings are taken at regular intervals.

The other altimeter, known as the wandering altimeter, is brought to the specified elevation sites. Readings from the roving altimeter taken at the target points are later modified to account for variations in temperature and other factors seen at the control point.

The elevation difference between the two points can be calculated using the following formula:

• H = The elevation difference between the two sites.
• h2 = The barometric pressure (in cm) at the lower and upper points, respectively.
• T2 =Air temperature (in 0 C) at the lower and upper positions, respectively.

Barometric Leveling Types

• Mercurial Barometric
• Barometer

Mercurial Barometric

The Mercurial barometer works on the premise of balancing a mercury column against atmospheric pressure, with atmospheric pressure at the location of observation being a function of elevation above mean sea level.

Mercurial barometers are classified into two types: cistern and siphon. Mercurial barometers must be sustained vertically, hence they are frequently suspended by a gimbal mounting attached to a specific tripod.

The mercury in a cistern barometer is contained in a glass tube about 85 cm long, with the upper end covered and the bottom open end immersed in a cistern containing mercury open to the atmosphere.

The tube is devoid of air, resulting in a vacuum above the level of the mercury in the tube.

Because the pressure on the surface of the mercury in the cistern is atmospheric, and there is no pressure on the upper end of the column of mercury in the tube, a column of mercury is maintained in the tube, the height of which is determined by the pressure on the surface of the mercury in the cistern.

The tube containing the mercury in the syphon type of mercury barometer is bent into a U-shape at the bottom end. One of the U-branches tube’s is kept shorter than the other.

The upper end of the short branch has a small aperture to allow air, but the long branch is closed at the top with the vacuum at its top. This type of barometer is inferior than cistern barometers and is rarely used.

Barometer

In aerial photogrammetry, barometers are employed in leveling to make an approximate assessment of heights, a difference in altitudes, and the flying height of aero planes. They are also employed in some types of astronomical observations to calculate the refraction correction.

Because barometer leveling is not extremely exact, it is typically used exclusively for small-scale topographical and reconnaissance surveys when great accuracy in determining heights is not required.

There are two types of barometers: mercurial and aneroid. The former is more accurate, but it is inconvenient to carry and easily breaks.

Hypsometric Leveling

The difference in elevations is calculated using this method by monitoring the temperature at which water begins to boil. This is due to the fact that the boiling point of water falls as altitude increases. The aim of this method is to determine the difference in elevation between a spot at sea level and another spot at a certain altitude above sea level.

Stadia Leveling

Tacheometric surveying is another name for it. Chaining is a typical method for measuring horizontal distances, and differential leveling is a way for measuring vertical distances.

Both of these strategies produce results with the needed precision.

Chaining on rocky ground, on the other hand, does not produce particularly accurate results. When the ground is rough and more observations at a faster rate with ordinary precision are acceptable, the tachometer is the preferred instrument.

The collecting of data to generate contours on a topographic map is an example of how a tachometer can be used for the aforementioned situations.

When compared to chaining on flat ground, the precision of tachymetric distances is lower, but it is higher on uneven and hilly ground.

A tachometer is a means of measuring visual distance. Despite being less exact, this form of surveying is incredibly quick and convenient.

The tachometer is also known as tachymetry or telemetry. The fundamental goal of a tachymetric survey is to create a contoured plan.

It is especially useful for filling in details on topographical maps, conducting preliminary location surveys (for railways, motorways, canals, reservoirs, and so on), and surveying steep grounds, broken boundaries, and water stretches, among other things.

It may also be used to offer a rapid check on distances measured using a chain or tape on higher accuracy surveys. A tachometer is essentially a transit theodolite with stadia wires in addition to cross-wires on the diaphragm.

Indirect Leveling (Trigonometric Leveling)

This is an indirect method of leveling in which the difference in elevation of the spots is calculated using observed distances and vertical angles.

Vertical angles are measured via transit, while distances are measured directly or trigonometrically. In topographical work, trigonometrical leveling is widely used to determine the elevation of the tops of buildings, chimneys, church spires, and so on.

It can also be used to advantage in challenging terrains such as mountainous locations. There could be a plethora of scenarios depending on the field conditions and the measurements that can be made with the devices available.

A few situations have been attempted to be solved, and many more can be solved by the reader.

In Summary

The methods of levelling in survey can be applied in a number of fields. As the surveyors improved their skills, the method of levelling in survey diversified as well. It has been around for quite some time already.

In modern times, it is still widely used in surveying work both in fieldwork and office work. Surveying is a job for the surveyors, and levelling is undeniably one of their core abilities to get the job done.

FAQs

How many methods are used in levelling?

The main methods for performing leveling are four. Direct levelling, trigonometric levelling, barometer levelling, and stadia levelling. Other methods like hypsometric leveling and Tacheometric surveying are considered subgroups.

What is the definition of levelling?

This refers to the process of establishing horizontal positions (or heights) of physical points on the ground or above it.

Whether it is done by hand or machine, the basic procedure will be driving a survey benchmark in a location as accurately as possible and then measuring to that mark from other locations using a measuring device.

What is direct method of levelling?

Direct levelling is simply a form of levelling that is not conducted by the use of a transit. It is done by the height of the Height-altitude table or height measures which are fixed on a base datum. It is also called topographic levelling.

In direct levelling, there will be two stations involved with reference marks defined in advance between them, and the heights of these stations will be recorded using direct descendant methods.

What is the method of leveling?

The methods of leveling are:

• Direct Levelling
• Stadia Leveling
• Barometer Levelling
• Hypsometric Levelling
• In direct Levelling

What is Traverse method?

Traverse is a surveying approach for establishing control networks. Geodesy makes use of it as well. Traverse networks are constructed by installing survey stations along a line or course of travel and then using the previously surveyed sites as a base for surveying the following point.

What does re-measurement mean?

In survey re-measurement is done by providing updates or corrections. It also refers to double or multiple measurements of a single object made at different times and recorded by different instruments, after allowing for any change in position of the object between observations.

How do you measure the elevation?

There are two steps to determining the elevation. First is to observe the height of the object and second is to determine the local vertical dot-error. In order to estimate vertical error, you need to use a trigonometric method or LVD (Least Vertical Deviation).

This is a trigonometric procedure that can be used to estimate the vertical error. This method is used to measure the elevations of objects by a process called “stratification”, but it can also be defined as “the use of weighted averages”.

What is Special Leveling?

Special leveling are surveys that are performed for special purposes, rather than for general surveying needs. For example: levelling for the purposes of constructing and maintaining maps and for field reconnaissance in disaster areas.

What is Fly Leveling?

The most accurate leveling method is fly leveling because it uses a balance scale to perform the weighing. There are many methods used for Fly leveling, including torsion, triangulation, bore sighting and the Laser.

What are the advantages of levelling?

Levelling is an accurate and timely survey technique that can be performed by any skilled surveyor. Calling in a professional surveyor ensures that the job is done right. Also, levelling work is a very important step in map making.

Why is elevation important in surveying?

In surveying, elevation or height is an important characteristic of the surveys because it determines the area that can be viewed. Elevation also helps in determining the height of certain construction and other structures, and it will help determine any level changes of the topography.

What is a Vertical Section?

A vertical section as it sounds is a cross-section that represents the vertical dimension, or depth of a scene.

Challenges of Levelling?

Depending on the terrain, both horizontal and vertical errors can occur. The main challenge of levelling is to ensure that the topography is surveyed very accurately so as not to leave any errors.

How do you obtain an accurate survey?

For an accurate survey, you need a level line which is a horizontal datum to base on and use it to lay out the base for levelling. This ensures that you are placing your levels on an even surface.