Passage Survey Distance Calculator
Enter each leg's tape length and clinometer reading and get the horizontal distance and depth change for that leg, plus running totals across the whole passage.
Survey reduction
| Leg | Horizontal (m) | Vertical (m) |
|---|---|---|
| 1 | 10 | 0 |
| 2 | 8.66 | 5 |
| 3 | 14.14 | -14.14 |
The trigonometry behind the reduction
A tape measures the slope distance along the passage, not the level ground distance underneath it. Multiplying that slope distance by the cosine of the clinometer's inclination angle gives the horizontal run; multiplying by the sine of the same angle gives the vertical rise or drop. Every leg reduces the same way, whether it climbs, descends, or runs dead level.
Adding up each leg's horizontal and vertical components gives the totals for the whole surveyed passage: the horizontal distance travelled, the net depth change from start to end, and — split apart — the total climbed and the total descended along the way.
Frequently Asked Questions
What is a survey 'leg' and why does it need reducing?
A leg is one shot between two survey stations: a tape measures the slope (straight-line) distance, and a clinometer measures the angle up or down. Because the tape follows the slope, not the level ground, you need trigonometry — cosine and sine of the clino reading — to split that slope distance into its horizontal and vertical components, the same reduction used in BCRA-grade cave surveys.
What does a positive or negative inclination mean here?
This tool treats a positive clino reading as downward and negative as upward, which is a common convention, though some surveyors use the opposite — check your own survey notation before entering readings. What matters for the maths is being consistent: downhill legs should carry the opposite sign from uphill legs so they net out correctly in the total depth change.
Why is the total horizontal distance not the same as 'as the crow flies'?
Summing each leg's horizontal component gives the horizontal distance travelled along the surveyed path, not the straight-line distance between the start and end points. A winding passage can rack up a large total horizontal distance while its start and end are much closer together in a straight line — that straight-line figure needs the compass bearings too, which this tool doesn't plot.
Can I use this for a full cave survey?
It's a distance and depth reduction tool for planning and sanity-checking, not a substitute for a proper survey program that also handles bearings, loop closure, and plotting in three dimensions. For anything you intend to publish or submit to a survey database, use dedicated cave-surveying software.
Planning and sanity-check tool only — not a replacement for dedicated cave-surveying software when accuracy, loop closure, and plotting matter.