Intersection Survey

Overview

Intersection survey, also known as resection or angular surveying, is a fundamental technique in surveying that establishes the position of an unknown point by measuring angles from two or more known reference stations. This method is particularly valuable when direct measurement to a point is impractical or impossible.

Basic Principles

The intersection survey operates on the principle that if you know the distance between two reference points and can measure the angles from each reference point to an unknown point, you can mathematically determine the unknown point's location. This relies on basic trigonometry and geometric principles used throughout surveying applications.

Types of Intersection Surveys

Forward Intersection

Forward intersection involves occupying known survey stations and measuring angles to an unknown point. The surveyor stands at established positions and sights toward the target point, recording the horizontal angles. This method works best when the unknown point is visible from multiple known stations.

Back Intersection (Resection)

Back intersection, or resection, reverses the process. The surveyor occupies the unknown point and measures angles to three or more known reference points. This method proves valuable when you need to establish your position in the field without direct access to established stations.

Trilateral Intersection

Trilateral intersection combines distance and angle measurements. When three known points form a triangle around the unknown point, measuring angles from the unknown point to each known station provides positional information.

Equipment and Instruments

Intersection surveys require precise angle measurement equipment:

Theodolites - Traditional instruments for measuring horizontal and vertical angles

Transit Levels - Used for angle measurement in conventional surveying

Total Stations - Modern instruments combining electronic angle measurement with distance capabilities

GPS Receivers - Contemporary tools for establishing reference points

Binoculars - For sighting distant reference points

Methodology

The typical intersection survey process involves:

1. Establishing Reference Points - Identifying and marking known survey stations 2. Setting Up Instruments - Positioning surveying equipment at known stations 3. Taking Measurements - Recording angles to the target point from each reference station 4. Calculations - Using trigonometry to determine the unknown point's coordinates 5. Verification - Cross-checking results using multiple angle measurements

Advantages

Intersection surveys offer several benefits:

Useful for inaccessible or dangerous locations

Requires minimal equipment at the unknown point

Provides accurate positioning when executed properly

Cost-effective for establishing point locations

Can work over considerable distances

Limitations and Considerations

Surveyors must consider potential challenges:

Requires clear line of sight between stations

Accuracy depends on precise angle measurement

Weather conditions can affect visibility

Intersecting lines at poor angles reduce accuracy

Requires at least two reference points, preferably three or more

Applications

Intersection surveys are employed in:

Establishing control points for mapping projects

Locating inaccessible features

Surveying across water bodies

Archaeological site documentation

Building and property surveys

Construction stakeout operations

Modern Developments

Contemporary surveying has enhanced intersection techniques through electronic total stations and GPS technology, enabling automated angle recording and real-time calculations. However, traditional intersection methods remain fundamental to surveying education and practice, particularly in situations where electronic equipment is unavailable.

Conclusion

Intersection survey remains a cornerstone technique in surveying practice, combining geometric principles with practical field methods to determine point locations efficiently and accurately when direct measurement is impractical.