From an observation of the sun's altitude and azimuth and an assumed latitude a position is computed and plotted and a line drawn on the chart through this position at right angles to the azimuth of the sun as taken from the azimuth tables and laid off from a meridian. Another method is to compute positions with two assumed latitudes and plot the two resulting positions and draw a line through them. The vessel must be somewhere on the resulting Sumner line. A good determination may be obtained by the intersection of two Sumner lines obtained from two observations of the sun with sufficient interval so that there will be a change of azimuth of as much as 30 degrees to give a fair intersection. Allowance must be made for the movement of the vessel between the two observations by drawing a line parallel to the first and at a distance equal to the distance made good. An excellent intersection may be obtained by observation of the sun, and before or after it of a star in the twilight at a different azimuth.
COURSE TO ALLOW FOR SET, GRAPHICAL SOLUTION
FIG. 38.
POSITION BY INTERSECTION OF SUMNER LINES
FIG. 39.
POSITION BY COMPASS BEARINGS
FIG. 40.