Intersection Survey

Definition

Intersection survey is a fundamental surveying technique used to determine the location of an unknown point by observing angles from two or more known points (stations). This method is widely employed in surveying, mapping, and navigation applications where direct measurement to a target point is impractical or impossible.

Principles

The intersection survey operates on basic trigonometric principles. When angles are measured from at least two known reference points to an unknown point, the intersection of the lines of sight from those points establishes the location of the unknown point. The accuracy of this method depends on:

The precision of angle measurements

The distance between known points (baseline)

The angle of intersection between the sight lines

Types of Intersection Surveys

Forward Intersection

Forward intersection involves measuring angles at two known stations to sight an unknown point. The unknown point's position is determined by the intersection of the two lines of sight. This method is commonly used in establishing control points and locating inaccessible features.

Back Intersection

Back intersection, also called resection, determines the position of the surveyor's station by measuring angles to three or more known points. This technique is valuable when the surveyor's position is unknown but visible landmarks are identifiable.

Side Intersection

Side intersection combines elements of both forward and back intersection, useful when measuring from intermediate points.

Equipment Required

Transit or theodolite for angle measurement

Surveying tripod

Measuring tapes or electronic distance measurement (EDM)

Ranging poles or prisms

Notebooks for recording data

Procedure

1. Establish baseline: Identify and mark two or more known control points with verified coordinates. 2. Set up instruments: Position transit/theodolite at each known station. 3. Measure angles: Observe and record horizontal angles to the unknown point from each known station. 4. Calculate position: Use trigonometric formulas or surveying software to compute the unknown point's coordinates. 5. Verify accuracy: Cross-check calculations and confirm results meet acceptable error tolerances.

Advantages

Requires no direct access to the target point

Useful for locating distant or inaccessible features

Cost-effective compared to other methods

Can achieve high accuracy with proper technique

Requires minimal equipment

Limitations

Accuracy decreases with small intersection angles

Requires at least two known reference points

Weather conditions and visibility affect performance

Refraction and curvature of Earth may introduce errors

Not suitable for very long distances without correction

Applications

Intersection surveys are widely used in:

Topographic mapping: Locating peaks, buildings, and landmarks

Cadastral surveying: Establishing property boundaries

Construction surveying: Setting out building positions

Navigation: Determining vessel or aircraft positions

Archaeological surveys: Recording artifact and feature locations

Environmental monitoring: Tracking landscape changes

Modern Developments

Contemporary intersection surveys often incorporate:

Total stations for simultaneous angle and distance measurement

Global Positioning System (GPS) for verification

Laser scanning technology

Surveying software for automated calculations

Digital theodolites with automatic angle recording

Accuracy Considerations

To achieve optimal accuracy:

Ensure baseline points are precisely established

Minimize instrumental errors through calibration

Use multiple observations and average results

Apply atmospheric correction factors

Maintain proper sight line geometry

Document all measurements thoroughly

Conclusion

Intersection survey remains a valuable and reliable method in modern surveying practice. Despite advances in GPS and other technologies, understanding and applying intersection surveying principles is essential for surveyors, engineers, and geographers. The method's simplicity, cost-effectiveness, and proven reliability make it relevant for various surveying applications worldwide.