Resection Geometry Strength Checker
A free, browser-based tool. Runs entirely in your browser — no sign up, nothing stored.
Observed angles
α and β are the angles subtended at the unknown station U between the three control points; θ is the angle at the middle control point.
Geometry strength
The resection is indeterminate when the unknown and the three controls are concyclic — which is near α + β + θ = 180°. The further the sum is from 180°, the stronger the fix. A study aid only.
How to use this calculator
- Enter the two angles subtended at the unknown station (α between P1–P2, β between P2–P3) and the angle θ at the middle control point.
- Read the strength rating and how far the geometry sits from the danger circle.
How it works
A two-angle (three-point) resection becomes indeterminate when the unknown station and the three control points lie on one circle — the danger circle — which is near α + β + θ = 180°.
The further that sum is from 180°, the stronger the fix. Close to 180° and the position is unreliable.
Worked example
Near the danger circle. α = β = θ = 60° sums to exactly 180° → Danger. Angles around 80° each sum to 240° → Strong.
Frequently asked questions
How do I avoid a weak resection?
Choose control with good spread and move the station off the danger circle, or add a fourth ray. Always observe a check.
Is this official course material?
No. It is free study support mapped to surveying course levels — not official North Metropolitan TAFE content or advice. Always follow your lecturer and the official assessment brief, and check your own working.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.