Interactive Spreadsheets for Celestial Navigation
T-Plotter
T-Plotter® is a plotting aid consisting of two mutually perpendicular arms: the plotting arm and the azimuth arm.  The main purpose of this device is to simplify the plotting of celestial lines of position (LOP) by eliminating the need for the drawing of the azimuth line from the assumed position (AP) to the intercept.  Also, with the help of the T-Plotter, distances can be both marked and measured without dividers. Customers' comments are at the end of this page.

NOTE:
ALL T-PLOTTER PRODUCTS WERE DISCONTINUED ON
MAY 15, 2016.





T-Plotter (Basic): design updated January 19, 2013 



This latest version of the "Basic" model has a shortened azimuth arm (6 1/4" vs. the original 8 1/2"), which allows the flipping of the T-Plotter by 90º simply by using a straightedge, as shown in this video:



Another enhancement is the addition of a protractor with lines fanning out from the top of the azimuth line in 10º increments, with the additional two longer lines marking the 45º positions.  The first 10 tics along the scales are less conspicuous in order to allow better visibility of the underlying chart near the top of the azimuth line.  The two 45º lines extend from edge to edge to facilitate the plotting of the symmedian point in the "cocked hat" LOP triangles (see NavList postings by Herbert Prinz, Gary LaPook, and others for details).



We thank Greg Rudzinski and Örjan Sandström, for the above suggestions for improvement.

The images below show the original T-Plotter Basic that has been superseded by our newer version.

The "Basic" model is intended to be used primarily with Universal Plotting Sheets (VP-OS) on which the distance of 1 inch represents 20 nautical miles on the Earth's surface.



T-Plotter Basic is particularly useful if the AP of the St. Hilaire method coincides with the center of the compass rose on the plotting sheet.  This choice for AP is always available if an electronic sight reduction method, such as the spreadsheet intercept.xls, is used.  Then, if the intercept is AWAY, the T-Plotter can be positioned on the plotting sheet by simultaneously using the intercept distance and the azimuth.  If the intercept is TOWARD, the T-Plotter position can be obtained using the same intercept distance but AWAY from the reciprocal azimuth. 





Alternatively, the grid marked along the plotting edge can be used to find the locations at which the LOP intersects the AP's parallel and meridian (as computed by the intercept.xls spreadsheet) which allows the plotting of the LOP simply by connecting those two points without referring to any angular information.

The following image shows the spreadsheet intercept.xls containing data illustrated in the above T-Plotter image.  For more information about this spreadsheet, click here.




Azimuth transfers from the compass rose to a non-centrally located AP can be accomplished with the help of a straightedge by first sliding the correctly oriented T-Plotter along the plotting arm direction until the azimuth line covers the AP, followed by sliding along the azimuth arm direction into the prescribed intercept distance.

Click here to view a T-Plotter Basic demonstration video.

T-Plotter Basic can also be used on charts of any scale, with the extra step of having to identify the requisite number of T-Plotter tics to represent the required distance according to the latitude scale of the given chart.




There are dry-erase markers (not included here) with which one can mark on the T-Plotter as well as erase.  This is useful if the intercept distance comes out longer than what the azimuth arm allows. In such a case that distance has to be partitioned into pieces, each one shorter than the azimuth arm.  The LOP is then arrived at in stages, in which each piece of the intercept distance is covered by the requisite additional sliding and marking along the azimuth arm direction.




T-Plotter Basic specifications:

Material: acrylic
Thickness: 1/8"
Width (plotting arm): 11"
Height (azimuth arm): 6 1/4"
Arms' widths: both 1 1/2"



T-Plotter Blank 

We also offer a simpler, lower-cost version called T-Plotter Blank equipped with only the central azimuth line but no additional grid or lettering.




Intercept distances can be marked on the T-Plotter by dry-erase markers (not included), as shown in the demo video here.

T-Plotter Blank specifications:
Material: acrylic
Thickness: 1/8"
Width (plotting arm): 11"
Height (azimuth arm): 8 1/2"
Arms' widths: both 1 1/2"



T-Plotter Compact 

T-Plotter Compact is a smaller version of the older "Basic" model, i.e., it does not include the protractor lines (see image below).



T-Plotter Compact specifications:
Material: acrylic
Thickness: 1/8"
Width (plotting arm): 8 1/2"
Height (azimuth arm): 4 3/4"
Arms' widths: both 1 1/2"



T-Plotter Compact Blank 

T-Plotter Compact Blank is a smaller version of the "Blank" model.



T-Plotter Compact Blank specifications:
Material: acrylic
Thickness: 1/8"
Width (plotting arm): 8 1/2"
Height (azimuth arm): 4 3/4"
Arms' widths: both 1 1/2"



 

I have put the compact T-Plotter to the test and this plotting tool can do more than just plot azimuth and intercepts on a standard plotting sheet. The scaled version is ideal for the construction of a standard plotting sheet. When a pair of Ts are used they act just like parallel rulers. Put them in an opposing position to lay down an extended course track line. What I like best is that they fit neatly in a Pub. 208 or Weem's Line of Position table.

I'm happy to have them at my chart table :)

Greg Rudzinski, California, USA

(Click here for the original NavList posting which includes illustrating photographs.)


Well, I got my very own T-Plotter for Christmas! I like it. Using only the plotter I was able to construct a plotting sheet and plot 3 LOPs on a blank sheet of 8 1/2 x 11 paper with very nearly the same precision as if I had used a pre-printed sheet, protractor, ruler and dividers.

The design of the plotter makes it easy to lay out the central parallel and meridian. One method is to start with the paper in landscape orientation and align the top edge of the page with the 100 mark on one end of the plotting arm. (See linked photos.) Align the bottom edge of the page with the 70 mark on the opposite end and either the left or right edge of the page (depending on the orientation of the plotter) with the 110 mark on the azimuth arm. Now the plotting arm will be running down the center of the page and the top edge of the azimuth arm will be running across the center. Mark these and extend the parallel across the page.

Next, all you have to do is measure an angle from the center parallel corresponding to your AP Latitude (as explained elsewhere on the list) and mark that. (In the photo below, the angle is set at 40°.) Voila! One plotting sheet!

I was even able to estimate the azimuths of the bodies fairly accurately using the built-in protractor. (But, to be honest, if it had 1° or even more 5° divisions...I would buy another!) It's fast, convenient and well designed for the task. All in all, I'd say it's just about the perfect plotting tool! Thanks Peter!

Regards,

Sean C, Virginia, USA

(Click here for the original NavList posting which includes illustrating photographs.)




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