Intersection Survey

Overview

Intersection survey is a fundamental surveying technique used to determine the location of a point or object by measuring angles from two or more known reference points or stations. This method is particularly valuable in situations where direct measurement is difficult, dangerous, or impossible.

Principles

The intersection survey method is based on trigonometric principles. By establishing sight lines from known positions to an unknown point and measuring the angles formed, surveyors can calculate the exact coordinates of that point. The method relies on the principle that if you know the distance between two reference points and the angles to a third point from each reference station, you can determine that third point's location through triangulation.

Types of Intersection Surveys

Forward Intersection

Forward intersection involves measuring angles from two known stations to an unknown point. This is the most common type and works well when the unknown point is accessible but its exact location needs verification.

Backward Intersection

Also called resection, this method involves occupying an unknown point and measuring angles to three or more known reference points. This technique is useful for establishing positions in areas where access to known stations is limited.

Side Intersection

This variation involves using one known station and measuring angles to multiple reference points to establish the location of an unknown point.

Equipment Used

Modern intersection surveys typically employ:

Theodolites or total stations for precise angle measurement

GPS receivers for establishing initial known points

Prisms and reflectors for establishing sight lines

Measuring tapes for baseline measurement

Field notebooks for recording measurements

Procedure

1. Establish reference stations with known coordinates 2. Set up instruments at each reference station 3. Measure angles to the unknown point from each station 4. Record all measurements accurately 5. Calculate coordinates using trigonometry or surveying software 6. Verify results through check measurements

Advantages

Accurate position determination without direct access to the point

Useful for inaccessible locations such as across water bodies

Relatively quick method for establishing multiple points

Cost-effective for certain applications

Can be combined with other surveying methods for enhanced accuracy

Disadvantages

Requires clear line of sight between stations and the unknown point

Accuracy depends on the distance between reference stations and angles measured

Poor angles can result in weak geometry and reduced accuracy

Atmospheric refraction can affect angle measurements

Requires multiple observers for efficient operation

Applications

Intersection surveys are widely used in:

Boundary surveying for property determination

Hydrographic surveying to map underwater features

Archaeological surveys to locate artifacts and features

Construction staking for positioning structures

Route surveying for roads and utilities

Industrial surveys for positioning equipment

Accuracy Considerations

The accuracy of intersection surveys depends on several factors:

Quality of angle measurements

Distance between reference stations

Geometry of the triangle formed

Atmospheric conditions

Instrument calibration

Weak geometry occurs when angles are too acute or obtuse, resulting in less accurate position determination. Optimal results occur when angles approach 90 degrees.

Modern Applications

While traditional intersection surveying remains valid, modern surveyors often integrate this method with GPS technology and computer-aided design software for enhanced accuracy and efficiency. Integration with GIS systems allows for better data management and analysis.

Conclusion

Intersection survey remains an essential technique in the surveyor's toolkit, particularly for situations where direct measurement is impractical or impossible. Understanding its principles, applications, and limitations ensures accurate and reliable surveying results.