Course / Tools
Sight reduction calculator
Enter your DR position and the body's GHA and declination from the almanac. You get the computed altitude (Hc), the true bearing (Zn) and, with your observed altitude, the intercept. Every step is shown, worked from the DR by formula and from an assumed position the Pub. 249 way, so you can check your own sight form.
What the calculator does
A sextant altitude puts you on a circle centered on the body's ground position. Sight reduction compares it with the altitude you would have seen at a known position: Hc, the computed altitude. The difference, Ho − Hc, is the intercept in nautical miles, drawn toward or away from the body along its bearing Zn, and the line of position crosses it at right angles.
The tables (Pub. 249 and Pub. 229) list Hc for whole degrees of latitude and LHA, which is why navigators work from an assumed position near the DR. A calculator can work from the DR itself. Both give the same line. This page shows both, so it matches whichever way your sight form is laid out.
It is a learning aid, not a navigation tool: at sea, use the official Nautical Almanac and tables and check every position by other means.
Learn to do it yourself
- The assumed position and LHA: why the tables need whole degrees.
- The intercept method: Hc, Zn and the intercept with Pub. 249, Pub. 229 or a calculator.
- The line of position: plotting the result.
- Work your own sight: from the watch time and sextant reading, with the almanac built in.