Essential Definitions in Geodesy and Land Surveying

Fundamental Concepts in Geodesy and Measurement

The following definitions cover essential terminology related to Earth's geometry, measurement techniques, and coordinate systems.

Magnetic Poles

Defined as the points on the surface of the Earth where the magnetic field lines are perpendicular to the surface.

Geographic Poles

Defined as the points on the Earth's surface that intersect with the planet's axis of rotation.

Bounded System (Orthogonal Cylindrical Projection)

A system of orthogonal cylindrical projection is the most appropriate for the representation of land and figures whose vertical dimensions are much smaller than their horizontal dimensions. The height of a point ($q$) is measured relative to a reference plane and can be positive (+) or negative (-).

Range of a Line

The distance between two points on the line whose dimensions (usually elevation or coordinate values) differ by one unit.

Graduation of a Line

A point's projection onto a series of whole elevation points.

Measurement Techniques

Direct Measurement

Measurements carried out by comparing the magnitude directly to a standard pattern, without any intermediate mathematical steps.

Indirect Measurement

A dimension mathematically related to the desired magnitude is measured first. The magnitude of interest is then obtained by applying this mathematical relationship (ratio).

Coordinate Systems

Cartesian Coordinates

Based on two perpendicular straight lines (axes). The position of a point is marked by its distances from the origin, typically defined as left/right (X-axis) and up/down (Y-axis).

Polar Coordinates

Define the position of a point using the length of a line segment extending from the origin to the point (marking the distance) and an angle that the line forms with a reference axis (marking the direction).

Surveying Instruments and Auxiliary Elements

Surveyor's Tape Measure

These tapes are typically constructed only of steel, as the force required to remove slack could produce unacceptable extension if they were built with other materials. They commonly have lengths of 50 and 100 meters. Tapes are classified into three kinds based on precision (material type).

Ranging Poles (Jalons) and Marking Pins (Tokens)

The Ranging Pole (Jalon) is a steel rod used to mark points when measuring long distances.

Marking Pins (Tokens or Chips) are used as auxiliary elements for measuring great distances. The procedure involves:

1. Counting the number of pins held in hand.
2. Multiplying this count by the length of the tape used.
3. Adding the length of the final, partial tape measurement.

Leveling Staff (Rod)

A staff divided into metric system parts. It is categorized into the following classes:

• Silent: Does not have numbers.
• Speaking: Is numbered and can be read anywhere along its length.
• Direct or Inverted: Numbering depends on the reading position (direct or inverted).

Abney Level

This instrument consists of a double curvature level attached to a vernier, which can rotate around the center of a protractor fixed to the eyepiece. The image of the level bubble ring is reflected through a prism onto the visual field of the eyepiece.

The Abney Level is used to:

• Determine slopes and vertical angles.
• Horizontalize the tape measure.
• Calculate heights.
• Launch a visual line at a given slope.