Definition of Resection
Resection is a fundamental surveying technique used to establish the position of an unknown point by measuring horizontal angles from that point to three or more established reference points of known coordinates. Unlike radiation methods that measure from known points, resection works in reverse—the surveyor positions themselves at the unknown point and observes angles to known points. This makes resection particularly valuable when direct measurement or forward positioning is impractical or impossible.
How Resection Works
Basic Principle
The resection method relies on the geometric principle that if you know three reference points and can measure the angles subtended between them from your observation point, you can calculate your exact position. The minimum requirement is three known points, though four or more points are preferable for accuracy verification and error detection.
When conducting a resection survey, the surveyor: 1. Sets up an instrument at the unknown point 2. Measures horizontal angles to each known reference point 3. Calculates the unknown point's coordinates using trigonometric principles 4. Verifies results by comparing calculated positions
Mathematical Basis
Resection calculations typically employ either graphic methods or computational approaches. Modern surveyors use computational resection, solving simultaneous equations derived from the measured angles and known coordinates. The process generates a system of linear equations that yields the most probable position of the unknown point, minimizing angular measurement errors through least-squares adjustment.
Resection in Modern Surveying
Instruments Used
Traditionally, theodolites and transit instruments were the primary tools for resection work. Today, [Total Stations](/instruments/total-station) have become the standard instrument, offering integrated angle measurement and electronic data recording. Many modern total stations include built-in resection software that automatically calculates point positions in real time.
[GNSS Receivers](/instruments/gnss-receiver) can also perform resection-like operations through the RTK (Real-Time Kinematic) method, though traditional angle-based resection remains preferred in environments where satellite signals are unavailable or unreliable.
Practical Applications
Resection is invaluable in numerous surveying scenarios:
Construction Surveys: Establishing equipment positions when direct measurement to control points is obstructed by structures or terrain.
Boundary Surveys: Locating property corners and boundaries when inaccessible from known monuments.
Topographic Surveys: Rapidly positioning survey equipment in mountainous or densely vegetated areas.
Hydrographic Surveys: Determining vessel or platform positions when conducting underwater or waterborne measurements.
Archaeological Surveys: Mapping excavation sites relative to established control networks without disturbing the site.
Advantages and Limitations
Advantages
Limitations
Related Methods
Resection often works alongside intersection, where two known points measure angles to determine an unknown point. Combined resection-intersection surveys provide robust positioning through redundant observations.
Leading surveying software providers like [Leica](/companies/leica-geosystems) Geosystems integrate resection capabilities into their total station firmware, enabling surveyors to solve resection problems instantly in the field.
Best Practices
For optimal resection results:
Conclusion
Resection remains an essential surveying technique despite advances in GNSS technology. Its independence from distance measurement and effectiveness in signal-denied environments make it invaluable for professional surveyors. Mastering resection calculations and understanding its geometric principles ensures surveyors can establish accurate positions in virtually any field condition.