Introduction to Surveying and Geodesy

Posted on Dec 2, 2024 in Physics

Surveying is the science concept that aims to implement all necessary linear and angular measurements using mathematical calculations. It also aims to determine the relative position of points, considering the Earth’s surface as a flat surface. Topographic surveying encompasses all operations needed to create a correct planimetric graphical representation of an area, including height differences. This level of detail is essential to begin any desired work and requires a good graphical representation covering both altimetric and planimetric aspects.

Stages of a Survey

  • Recognition of work and work plan
  • Field work
  • Desk work

The efficiency of a survey depends largely on how the land reconnaissance and work plan are handled.

Key Personnel in Topography

  • The surveying engineer: Possesses profound theoretical knowledge of geodesy and surveying, combined with administrative and field work expertise.
  • The topographical technician: Led by the civil engineer, runs fieldwork and has knowledge of algebra, geometry, and trigonometry.

High-tech equipment in acceptable condition is crucial for topographic work.

Classes of Surveying

  • Cadastral surveys: Define the boundaries of areas and properties.
  • Construction surveys: Locate points, lines, and levels to guide the construction process.
  • Road surveys: For linear works such as canals, roads, and railroads.
  • Underground surveys: Mine location and tunnel positioning.
  • Hydrographic surveys: Determine the flow and volume of water and map the relief of lakes and seas.
  • General surveys: Determine terrain composition and locate natural and artificial objects.

Checkpoints in Topography

  • Permanent: Fixed reference points, monuments, antennas.
  • Temporary: Items created specifically for job performance.

Geodesy

Geodesy studies the Earth’s shape and dimensions, as well as large land extensions through cartography.

Coordinates

  • Sphere: The set of points in space equidistant from a fixed point (center).
  • Circle: Intersection of a plane dividing a sphere into two equal parts.
  • Small circle: A circle that does not pass through the sphere’s center.
  • Spherical triangle: A portion of a sphere’s surface surrounded by three arcs of great circles.

Key Geographic Terms

  • Poles: The Earth’s axis of rotation.
  • Equator: Divides the Earth into two hemispheres.
  • Meridian: Perpendicular to the Equator; the Greenwich Meridian is the origin.
  • Longitude: Angle from the prime meridian to any other meridian.
  • Vertical: Plumb line.
  • Latitude: Angular distance north or south of the Equator.
  • Parallel: Lines of latitude (e.g., Tropic of Cancer, Tropic of Capricorn).
  • Earth’s Orbit: Rotation (24 hours) and revolution (365.25 days).

Altimetry and Leveling

  • Eclimeter: Determines slopes and percentages for preliminary work.
  • Altimetry: Studies height differences to determine field volumes and flow direction.
  • Leveling: Mathematical calculation of level differences.
  • Mean Sea Level (MSL): Average height of high and low tides.
  • Elevation: Height of land relative to a reference level.
  • Benchmark: Altitude of a point relative to a specific level (e.g., IGN in Peru).
  • Slope/Gradient: Tangent of the angle between two points.
  • Indirect leveling: Determines level differences using barometers (atmospheric pressure) or trigonometry (theodolite measurements).
  • Direct leveling (Geometric leveling): Determines height differences between two points using a horizontal line of sight.
    • Simple leveling: Used for large slopes where intermediate points are needed.
    • Composite leveling: Used when simple leveling is not feasible due to terrain; multiple readings can be taken.

Leveling Accuracy

Accuracy depends on various factors and the specific project requirements.

  • Preliminary surveys: a = 0.15 * b (backsight and foresight distances not exceeding 500m)
  • Ordinary leveling: a = 0.02 * b (roads, pavements; sight distances not exceeding 150m)
  • Precise leveling: a = 0.01 * b (reference points, sewers; sight distances not exceeding 100m)
  • Geodetic leveling 1: a = 0.004 * b (sight distances not exceeding 90m)
  • Geodetic leveling 2: a = 0.008 * b (sight distances not exceeding 110m)
  • Geodetic leveling 3: a = 0.012 * b (sight distances not exceeding 125m)