Vertical danger angle.

A vertical sextant angle can be used to ensure the safety of a ship in the vicinity of dangers. If the danger is close to high land or a lighthouse, the height of the land or lighthouse can be used as a reference for determining the sextant angle. Decide what distance from the danger you wish to remain. Add that to the distance between the danger and the reference point and use it as a radius to draw a circle around the reference point. Find the height of the reference point (remember to add the difference in the tide between MHWS and the tide level. Use Nories tables to determine the angle subtended by the height of the reference point at the safety distance you have selected. Set this angle on your sextant. As long as the image of the reference point remains below the water level, the ship will be outside the circle and in safe water.

Horizontal safety angle.

The horizontal angle between two objects on shore may be used in a similar manner to that of the vertical sextant angle. The objects selected should be approximately the same distance either side of the danger to be avoided. Plot the point on the chart at the distance you wish to remain clear of the danger. Join that point with a line to each of the objects ashore that you have selected. Measure the angle and use it as the horizontal danger angle. If the angle between the two points remains smaller than the danger angle, the ship will be outside the circle in safe water.