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)
In his NavList posting Greg Rudzinski explains how you can construct your own plotting sheet using an updated version of the T-Plotter, that is equipped with a protractor. The orientation of the T-Plotter in the following image is for latitude of 34º.
(first published on December 1, 2012)
In a recent posting to NavList, Greg 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)
The selection of available T-Plotters now includes smaller, “Compact” models. For additional info visit:
http://www.t-plotter.com
T-Plotter Compact
T-Plotter Compact Blank
(first published on May 8, 2012)
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)
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):
Input:
UT: 2014 May 5, 13:15:18
Output:
GHA = 58º 58.0’
Dec = S 0º 14.1’
2) Sextant altitude corrections (Venus, p. 259)
alt_corr_xls
Input: Hs = 4º 32.6’
Output: Ho = 4º 17.6’
3) The calculated altitude and azimuth (pp. 279-280)
intercept.xls
Input:
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.
Output:
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)
In a recent NavList posting Jeremy provided a set of high-quality real-life observations that can be reduced to a celestial fix for his vessel at a specified moment in time. The application of Navigation Spreadsheets to this data set results in a very good fix, both by direct computation as well as by plotting.
The following table shows the computed ephemerides, corrected sextant altitudes, line-of position (LOP) characteristics for two choices of an assumed position (AP), and the accounting of vessel motion through dead reckoning (where the mini-spreadsheet time.xls was used to express the time intervals in decimal hours).
The fix at 19:00:00 local time (= 11:00:00 UT, since ZD = -8) is computed to be:
Latitude: N 8º 49.0′
Longitude: E 109º 45.2′
from the many_body_fix.xls spreadsheet:
This location is marked by a black square on the two subsequent plots.
The first plot uses the VSOP plotting sheet (scale 20 nautical miles per inch) with AP at N 9º and E 110º. The LOPs were drawn with a T-Plotter. The LOPs in this plot are not shifted by DR.
The second plot “zooms in” with the scale of 1 NM per centimeter to get a more accurate look. The reference AP is N 8º 50′ and E 109º 40′, which has been individually shifted for each observed celestial body along the vessel’s track (037) in order to account for the motion of the vessel.
The results are excellent, with all LOPs running within a mile of the computed fix.
In the above procedures all LOPs were treated as equally valid. The fact that there are pairs of LOPs that run nearly parallel to each other complicates matters somewhat. The GPS fix given by Jeremy:
Latitude: N 8º 48.4′
Longitude: 109º 45.0′
is recovered almost exactly (both by computation and by plotting) if only Jupiter, Rigel, and Fomalhaut LOPs are used.
For comparison, Jeremy’s position is marked by the square in Google Earth at the top of this post.
(first published on March 8, 2013)
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 1, post 2)
(first published on July 7, 2012)
Welcome to our blog, which accompanies the main site:
http://www.navigation-spreadsheets.com/
Here you will find examples showing how to use our Excel spreadsheets and T-Plotters to establish and track your position at sea independently of GPS using traditional astronomical observations.
You can also follow us on Facebook by joining the “Celestial navigation with Excel” group and by becoming a fan of the “Navigation Spreadsheets” page.
Navigation Spreadsheets 