Contour Interpolation Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Enter Values
How to use this calculator
- Enter the reduced level (RL) of two spot heights and the horizontal distance between them.
- To find where a contour crosses, enter the contour level; to find the height at a point, enter the distance from A.
- Read the interpolated distance or RL, plus the gradient.
How it works
Between two spot heights the ground is assumed to fall evenly, so position and height are proportional: the distance to a contour is (target RL − RL_A) ÷ (RL_B − RL_A) × distance.
The same straight-line relationship gives the RL at any distance along the line, and the gradient as a percentage and as 1-in-X.
Worked example
RL 12.4 to RL 15.8 over 25 m. The 14.0 m contour crosses (14.0 − 12.4) ÷ (15.8 − 12.4) × 25 ≈ 11.8 m from A. At 10 m from A the RL is about 13.76 m.
Tips
- If the contour level is outside both spot heights the tool extrapolates and flags it — pick points that straddle the contour.
Frequently asked questions
When is linear interpolation valid?
Only where the slope between the two points is reasonably even. Across a ridge, gully or break of slope it will be wrong — interpolate between points that genuinely bracket a uniform grade.
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.