Methods of Leveling and Grading in Surveying
Methods of Leveling
Concept of Leveling
Leveling is used to determine:
- The difference in height between two or more points.
- The height of a series of points on a map to create a dimensioned drawing or a cross-section of the land.
- Stake out points of horizontal surfaces such as floors, foundations, or flooring, or points on the slope of roads or sanitation systems.
Equipment needed for leveling:
- A level instrument capable of establishing a visual or horizontal plane.
- A level rod.
- A field notebook (leveling pad) to record data, perform error checks, and draw sketches.
- A tape measure to mark points on the plane used as the basis for the survey.
Methods of Grading
Depending on the instrument used, there are three grading methods:
- Geometric Leveling: Leveling by visual horizontal.
- Trigonometric Leveling: Leveling by steep sights.
- Barometric Leveling.
Geometric Leveling
This method uses a topographic instrument to provide a horizontal visual. To find the height difference between two points, the instrument is placed between them. Readings are taken on both points, and the difference between these readings represents the height difference. Placing the level at the midpoint compensates for potential instrument bias.
Trigonometric Leveling
This method obtains the height difference by observing the zenith angle of sight between two points and measuring the geometric distance between them. The vertical distance is calculated using trigonometry. This method is typically used for long distances and often employs total stations.
Barometric Leveling
This is the least accurate method. The gradient is obtained by measuring the air pressure difference between points.
Level Surface, Reference Surface, Altitude, and Cota
While mapping often assumes a flat Earth, this is not feasible in altimetry due to Earth’s curvature. A level surface is an equipotential surface concentric to the Earth. The intersection of a vertical plane with a level surface creates a level line.
The reference surface is the level surface with zero altitude. In Spain, this is the mean sea level at Alicante.
Altitude is the height of a point above sea level. Cota is the height of a point above any comparison surface. The difference between two points’ altitudes or cotas is their vertical distance.
Geodetic Points and Reference Points
Permanent geodetic points are marked by bronze nails and have altitudes provided by the Spanish Geographic Institute.
Temporary reference points are marked using stakes, paint, or building blocks and serve as checkpoints or starting points for new placements or layouts.
Ball and Refractive Errors: Joint Correction
Three concepts are essential for calculating slope:
- The line of constant height (curved due to Earth’s curvature).
- The horizontal line of sight from the instrument, tangent to the level surface.
- The refracted visual, a curved line due to atmospheric refraction.