Five years of Navigation Spreadsheets

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Navigation Spreadsheets

At the fifth anniversary of our website’s launch we review some of Navigation Spreadsheets functions.  All three examples are taken from the 2014 Nautical Almanac Commercial Edition.

1) Ephemeris (almanac data), Venus GP on 2014 May 5 at 13h 15m 18s (p. 256):


UT: 2014 May 5, 13:15:18

GHA = 58º 58.0’
Dec = S 0º 14.1’



2) Sextant altitude corrections (Venus, p. 259)

Input: Hs = 4º 32.6’
Output: Ho = 4º 17.6’



3) The calculated altitude and azimuth (pp. 279-280)

GP: GHA = 53º    Dec = S 15º
AP: Lat = N 32º    Long = W 16º
to which we add Ho = 30º 30.0’ in order to allow the calculation of the intercept and the plotting of the LOP.



The resulting LOP (intercept 38 NM away, azimuth 223) is plotted with the T-Plotter.




As it was also calculated by intercept.xls this LOP crosses:
the AP’s meridian at 52 NM north of the AP
the AP’s parallel at 56 NM east of the AP



(first published on February 15, 2014)

Celestial line of position with the T-Plotter

The recent addition of a protractor to the T-Plotter makes it a fully self-contained tool for plotting of celestial LOPs, as is shown in this demonstration video.  It is important to note, that the use of the T-Plotter is not limited to the scale of 20 nautical miles per inch printed on the instrument.  It is always possible to read off the equivalent number of tics from the latitude scale of any chart according to the picture below.



(first published on December 20, 2012)

Horizontal sextant angles

The sextant is a device for measuring angles between two lines of sight.  As such, its use is not limited to observations of celestial bodies.  For instance, the angle between two landmarks can be observed, which (via the central angle theorem), confines the vessel to a circular line of position.  In two separate NavList postings Greg Rudzinski recently published his technique of using the T-Plotter in constructing such an LOP. (post 1post 2)


(first published on July 7, 2012)

T-Plotter applications

In a recent posting to NavListGreg Rudzinski has shared his novel idea of using the T-Plotter in conjunction with a square protractor.  He illustrated the steps of his alternative procedure of plotting a celestial LOP obtained by the intercept method with the following photographs:







(first published on June 3, 2012)

T-Plotter Blank

The “Blank” version of the T-Plotter facilitates the plotting of LOPs on charts of any scale.

Distances are marked directly on the plotter with a dry-erase marker (not included).  For more information click here and view the demo video here.

(first published on March 25, 2012)

T-Plotter Basic

According to the intercept method of Marcq St. Hilaire a celestial line of position (LOP) is plotted on a chart as the line perpendicular to the azimuth line at the intercept distance toward or away the geographical position (GP) from the assumed position (AP).  This can be accomplished with the T-Plotter®- a device consisting of two mutually perpendicular arms: the azimuth arm that is lined along the azimuth line, and the plotting arm along which you can plot the LOP.  The use of the T-Plotter reduces the clutter on the chart by eliminating the need to also plot the intermediate (and usually not needed) azimuth line.

The T-Plotter Basic model is imprinted with a grid that fits the VP-OS (Universal) Plotting Sheets, on which 20 nautical miles are represented by 1 inch.  Click here to view a demonstration video of how T-Plotter Basic can be used to plot celestial LOPs. For additional specification and ordering information, visit:

(first published on January 28, 2012)