Intersection Survey

Definition

Intersection survey is a fundamental surveying technique used to determine the position of an unknown point by observing angles from two or more known reference stations. This method is particularly valuable in surveying when direct distance measurement is impractical or impossible, such as across water bodies, valleys, or hazardous terrain.

Principle and Theory

The intersection survey method operates on the principle of triangulation. When an observer at a known station measures the angle to an unknown point, that measurement defines a line of position. When two or more such angle measurements are made from different known stations to the same unknown point, the intersection of these lines of position determines the location of the unknown point.

The method requires:

At least two known reference stations

Accurate angle measurements from each station

Clear visibility between stations and the target point

Proper instrument setup and leveling

Types of Intersection Survey

Forward Intersection

In forward intersection, the surveyor occupies known stations and measures angles to an unknown point. This is the most common form and is used when the unknown point is inaccessible.

Back Intersection

Also called resection, this method involves occupying an unknown point and measuring angles to at least three known stations. The position of the observer is determined by the intersection of lines of position established from the known points.

Side Intersection

This variation combines elements of both forward and back intersection, used when the unknown point and observer positions need to be determined simultaneously.

Equipment and Instruments

Intersection surveys require precise angle-measuring instruments:

Transit or theodolite for measuring horizontal angles

Level for establishing proper instrument orientation

Measuring chains or tapes for baseline distances

Tripods for stable instrument placement

Prisms or targets for sighting

Procedure

1. Establish baseline: Measure the accurate distance between known reference stations 2. Setup instruments: Place theodolites at both known stations and level them properly 3. Measure angles: Observe and record angles to the unknown point from each station 4. Triangulation: Use the measured angles and known baseline to calculate the unknown point's position 5. Verification: Perform redundant measurements to ensure accuracy

Advantages

Eliminates need for direct distance measurement to the target point

Useful in difficult terrain or across obstacles

Provides checks through redundant observations

Relatively quick compared to direct measurement methods

Cost-effective for locating multiple inaccessible points

Limitations

Requires clear line of sight between all stations

Accuracy decreases with small intersection angles

Dependent on accurate baseline measurement

Weather conditions can affect angle measurements

Requires skilled personnel for precise measurements

Accuracy Considerations

The accuracy of intersection survey depends on:

Quality of angle measurements

Length and accuracy of the baseline

Intersection angle between the lines of position

Optimal intersection angles occur near 90 degrees

Poor angles (acute or obtuse) reduce positional accuracy

Applications

Intersection surveys are commonly used for:

Mapping inaccessible landmarks

Determining positions of islands or mountain peaks

Establishing control points for larger surveys

Creating hydrographic charts

Archaeological site documentation

Route surveying and engineering projects

Modern Developments

Contemporary intersection surveys often incorporate:

Electronic theodolites and total stations

GPS and GNSS technology for baseline establishment

Computer-aided triangulation and calculations

Digital data collection and analysis

Integration with GIS systems

Conclusion

Intersection survey remains a fundamental and reliable surveying method, particularly valuable for establishing positions of inaccessible points. While modern technology has enhanced measurement precision and data processing, the underlying principles of triangulation continue to make intersection surveys essential in surveying practice.