Chapter 3: Measurement of Vertical Distances and Leveling Methods

Leveling

It is a crucial surveying process used to measure vertical distances and determine the elevation of points or the difference in elevation between them.

Level Surface

A level surface is a curved surface that is always perpendicular to the direction of gravity, or a plumb line.

Level Line

A level line is a curved line in a level of points of which are normal to the direction of gravity and equidistant from center of the earth.

Horizontal Surface

It's a plane that is tangent to a level surface of a particular point. The horizontal surface is also perpendicular to the plumb line at the same point.

Horizontal Line

A horizontal line is a straight line plane that is tangent to a single point. It is perpendicular to the direction of gravity at that specific point.

Vertical Line

A vertical line is a line that is parallel to the direction of a plumb line of a given point.

Mean Sea Level

Mean sea level is an imaginary reference surface for ground elevations, representing the average height of the sea over a long period, typically 20 years, based on hourly readings at various stations.

Datum

A datum is a common surface that is either coincident with or parallel to mean sea level. It serves as a reference to which elevations in a specific area are referred.

Elevation

The elevation of a point is the vertical distance above or below a chosen datum, such as mean sea level.

Difference in Elevation

The difference in elevation between two points is the vertical distance between the two-level surfaces in which the points lie.

Direct or Spirit Leveling

It is a common method for determining the elevations of points by using a series of setups with a leveling instrument.

Reciprocal Leveling

It is a method for accurately finding the difference in elevation between two points that are far apart or can be seen from the end points when standard leveling is not possible, such as across a wide river or deep ravine. It is a faster and, when done carefully, can be as precise as direct leveling.

Profile Leveling

This method is used to determine the difference in elevation between points at short measured intervals along a line.

Trigonometric Leveling

It is a method used to determine the difference in elevation between two points through trigonometric calculations. It uses either the horizontal or slope distance and the vertical angle between the points.

Stadia Leveling

It is a method that combines elements of both direct and trigonometric leveling. It is a form of trigonometric leveling that is useful for preliminary survey, mapping, and rough leveling when quick measurements are necessary.

Barometric Leveling

It determines elevation differences by measuring changes in atmospheric pressure with a barometer.

Cross-Section Leveling

This type of leveling is used to obtain data for determining cross-section profiles.

Borrow-Pit Leveling

It is a method of determining the relative elevations of points in borrow-pit excavations for the purpose of calculating volumes of earthworks.

Dumpy Level

The dumpy level is the most common instrument used for direct leveling. It features a long telescope that is rigidly connected to a level bar, allowing the telescope to rotate 360 degrees to fix the line of sight.

Wye Level

The wye level is similar to the dumpy level but its telescope is detachable and rests in Y-shaped supports, or "wyes."

Builder's Level

This instrument, often called a construction level or architect's level, is used mainly in building construction where high precision isn't required.

Automatic Level

Automatic levels are popular for standard leveling due to their ease and speed of use, and they feature a self-leveling mechanism instead of a traditional level vial.

Tilting Levels

This type of leveling instrument can be tilted or rotated about its horizontal axis.

Geodetic Level

The geodetic level is a type of tilting level designed for extreme precision.

Transit as a Level

The engineer's transit is often called the "universal surveying instrument" due to its versatility, and it can also be used for leveling.

Laser Level

A laser system is a portable power-supplied unit that can be attached to surveying instruments like levels, transits, and theodolites.

Hand Level

The hand level is a portable, hand-held instrument used for surveys requiring low accuracy and short sightlines.

Leveling Rods

A leveling rod is a graduated rod which is used for measuring vertical distance between the line of sight through a leveling instrument and the point whose elevation is either required or known.

Self-Reading Rod

This is the most commonly used type of leveling rod. It can be read directly by the instrument man through the telescope by noting the apparent intersection of the horizontal hair on the rod.

Target Rod

It features a sliding target that the rodman adjusts and reads at the location specified by the instrument man.

Rods Named After Cities or States

Some leveling rods are named after cities or states, such as the Philadelphia, Detroit, Chicago, Florida, Boston, New York, Troy, and San Francisco rods.

Rod Ribbons

It's an improvised leveling rod where graduations are marked on canvas or metal strips attached to a piece of lumber with staples.

Precise Rods

The precise rod is a type of rod ribbon that features a graduated invar strip permanently attached to a four-meter-long wooden or metal frame.

Geodetic Rod

This rod, which uses a nilvar metal strip instead of invar, is a geodetic rod.

Tape Rod

This type of rod, also known as the automatic rod or tape rod, is rarely used but is advantageous when many elevations need to be determined from a single instrument setup.

Rod Level

The rod level is an L-shaped device used to quickly and accurately plumb a leveling rod.

Targets

A target is a small, portable device attached to a leveling rod, used when direct reading is difficult or impossible due to extremely long sights or poor visibility.

Telescopes

The telescope of a surveying instrument is a metal tube with a system of lenses used to define the line of sight and magnify distant objects.

Objective Lens

An objective lens is a compound lens made of crown and flint glass, located at the objective end of a telescope. Its optical axis is concentric with the telescope's tube axis.

Eye Piece

The eyepiece is a type of microscope, containing either two or four lenses that magnifies the image and crosshairs, allowing the user to accurately read the graduations on a leveling rod.

Cross Hairs

The crosshairs are a pair of perpendicular lines that define the instrument's line of sight.

Magnification

The magnification of a telescope is the ratio of an object's apparent size when viewed through the telescope to its size as seen by the unaided eye from the same distance.

Level Vial

A level vial is a sealed, graduated glass tube containing a liquid and an air bubble, used to determine the direction of gravity.

Coincidence Bubble

This bubble uses an optical device that splits the bubble's image and then mirrors one half next to the other.

Tripods

A tripod serves as a stable base for a surveying instrument, preventing movement after setup.

Fixed-leg (or high) tripod

The legs are a single, solid piece of wood.

Extension tripod

The legs have a sliding section, making it suitable for setups on rough terrain where the leg lengths need to be adjusted to conform to the ground's contours.

Setting Up the Level

The instrument man should first select a suitable location, prioritizing solid ground and avoiding muddy or wet areas. The instrument is then taken from its box by the level bar or base plate and securely screwed onto the tripod head.

Leveling the Instrument (Four Screws)

Position the telescope over one pair of opposite screws and approximately center the bubble. Then, rotate the telescope 90 degrees to align it with the other pair of screws and again, approximately center the bubble.

Leveling the Instrument (Three Screws)

Position the telescope so the bubble tube is parallel to the line between any two of the screws. Center the bubble by turning these two screws in opposite directions. Next, rotate the telescope 90 degrees so the bubble tube is perpendicular to the line between the first two screws. Use only the third screw to carefully bring the bubble to the center.

Holding the Leveling Rod

To ensure an accurate reading, the rod must be held perfectly plumb (vertical). The rodman should stand beside or behind the rod, facing the instrumentman, and hold the rod at a right angle to the line of sight.

Waving the Rod

The procedure is used to determine whether the rod is plumb when a reading is taken on it. It is accomplished by slowly waving or tilting the top of the rod through an arc, first toward the instrument man and then away from it. The minimum reading observed is considered the correct rod reading at the particular point sighted.

Carrying the Instrument

The level should always be kept in a box when not used. The level does not have to be detached from the tripod when transferring to another nearby station, if it is securely fastened and carried properly.

Arm and Hand Signals

Any gesture or motion that conveys information or gives a command, direction, or warning. Signals are essential in surveying due to difficulty in verbal communication in the field.

Move Right or Left (Signal)

The instrument man uses the forefinger to point out the desired direction of movement to the rodman.

Give a Sight (Signal)

The right or left hand is raised up and held for a moment in a vertical position.

All Right (Signal)

The levelman extends both arms horizontally and moves them up or down. When both arms are brought still and horizontal, it means "hold steady".

This is a Point (Signal)

The rodman raises the rod and holds it in a horizontal position over his head. It could also mean, "Give me a Line".

Move Back (Signal)

He transmits the command by raising his right hand with the palm facing toward the rodman and then moving it into a horizontal position with his palm down.

Pick Up Instruments (Signal)

The chief of party signals the Instrument man by first extending both arms downward then raising them up quickly as though an object is being lifted.

Raise (or Lower) Target (Signal)

The instrument man motions by either raising his arm above his shoulder to raise the rod, or by dropping his arm below his waist to lower the target.

Come In (Signal)

It is executed by moving the arm into a circular motion starting from below the waist to the front of his face.

Plumb the Rod (Signal)

The hand is extended vertically above the head and moved slowly in the direction it is desired to plumb the rod.

Establish A Turning Point (Signal)

The arm is swung slowly in a circle above the hand.

This is A Turning Point (Signal)

The leveling rod (or range pole) is raised overhead in a horizontal position. It is then lowered into a vertical position and held on the point.

Wave the Rod (Signal)

The instrument man holds his arm above his head and continuously waves it back and forth.

Face the Rod (Signal)

It is executed by raising both arms above the head and twisting both hands back and forth.

Reverse the Rod (Signal)

The command is transmitted by extending the arms above the head and slowly rotating both arms in a circular motion towards one side of the body.

Move Forward (Signal)

From a position where both arms are extended horizontally, the arms are slowly bent at the elbows and the hands raised into a vertical position.

Use the Long Rod (Signal)

The instrument man extends both arms downward then slowly raises them over his head.