# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Manual slope plotting**

**From:**Gary LaPook

**Date:**2010 Nov 27, 00:04 -0800

There has been some discussions lately about using the technique of graphing a number of sextant observations on the same body and comparing them with a line having a slope representing the change in altitude of the body over time to allow discarding erroneous observations so as to keep them from distorting an average of the good observations. If you are going to do this then, if you are not taking the sights from the beach but from a moving vessel, you must adjust the slope of the line to account for the movement of the vessel. Back in February I posted a table I computed for adjusting sextant observations for the movement of the vessel and this same table can be used for adjusting the slope of the line for "manual slope plotting" technique. The table is here: http://fer3.com/arc/img/111994.moo.pdf For even more precision, and for speeds in the range of .5 to 9, knots you can use the Motion of the Observer (MOO) table found in H.O. 249 simply by dividing the tabulated speeds, 50 to 900 knots, by 100 and at the same time dividing the tabulated values for the correction by the same factor of 100. There are two tables, one for a one minute period and one for a four minute period. I posted excerpts of H.O. 249 including the MOO tables here: http://fer3.com/arc/img/105924.ho%20249%20extracts%20.pdf You can review my previous posts on how to use these tables at these links: http://www.fer3.com/arc/m2.aspx?i=111885&y=201002 http://www.fer3.com/arc/m2.aspx?i=111994&y=201002 http://www.fer3.com/arc/m2.aspx?i=105937&y=200807 http://www.fer3.com/arc/m2.aspx?i=105938&y=200807 You can also use the "Motion of the Body" (MOB) tables in the excerpts of H.O. 249 for actually plotting the sloping line representing the change in altitude of the body over time. The two MOB tables show this for a one minute period and for a four minute period and are equally valid standing on the beach, sailing at 6 knots or flying at 900 knots. When you study this table you will see that the slope of the line is determined solely for the azimuth of the body and the latitude of the observer. You can simply refer to this table and then plot the sloping line. Think these tables through and you can figure the sign to use for plotting the line, (although this should be obvious, bodies to the east are rising and those to the west are descending.) Give em a try. gl