## line of Position Worksheets
This section has the step by step instructions of how to construct a line-of-Position
starting from a Sextant (Altitude) measurement.
This process is rather tedious and prone to making errors.
This is why the calculations involved should be organized in some kind of logical
scheme, which is fixed in a worksheet that can serve as a guide through the process
of elaborating the data to construct the line-of-Position. - Record the Sextant and Chronometer data
- Adjust the recorded Sextant and Chronometer data
- Get the coordinates for the sighted celestial object at the time of observation
- Do the Sight-Reduction process
- Determine the Altitude difference between calculated and measured Altitude
- Create a Mercator Plotting Sheet or find an appropriate nautical chart
- Construct the line-of-Position in the prepared chart
Some worksheets are available to go through the above process in a structured and intuitive way. The worksheets are an attempt to avoid errors while dealing with tables and doing simple manual computations:
While on deck to do an altitude measurement with the sextant, it is useful to carry a pen,
some scratch paper and a watch synchronized to UTC time shortly before (e.g. with a precision chronometer).
Immediately after the Altitude (object on the Horizon) is fixed on the sextant, the time is recorded.
To check the results of the sight reduction it's not a bad idea to record the estimated compass direction in which the sight was done.
Also useful is making a small sketch of the view seen through the Sextant, which documents which
limb was used and for the Moon it should show the orientation of the visible part of the Moon. ## Altitude-Intercept Worksheet Instruction Guide
This finishes the "calculation" part of finding a line-of-Position. To continue the graphical evaluation for the LoP in the Mercator Plot, the following information is needed: **Assumed Latitude**(AssLat)**Assumed Longitude**(AssLon)**Altitude Difference**(Hd)**Azimuth Angle**(Zc)
## Sight-Reduction Worksheet Instruction GuideThe following instruction are for the Sight-Reduction Worksheet using the Altitude-Azimuth Tables. This method requires the specification of an Assumed Position, which has to be chosen close to the Estimated Position but such, that the resulting Latitude and Local Hour Angle are integral degree values.
## Mercator Plot Worksheet Instruction GuideFrom the Altitude-Intercept Worksheet the following information to construct a line-of-Position on an chart is obtained: **Assumed Latitude**(AssLat)**Assumed Longitude**(AssLon)**Altitude Difference**(Hd)**Azimuth Angle**(Zc)
In order to plot a line-of-Position an appropriate chart must be used or a sheet of
paper with a suitable Mercator Grid valid for the Assumed Latitude of the observer must be
prepared (use the proper page from the
Mercator Plotting Sheets Book
or follow the section on Plotting Sheets).
If a nautical chart is used, it must be large enough so that both the Estimated Position
and the Assumed Position can be plotted in it.
In the following section some examples are worked out according to the above instructions. |

Cover << Sail Away << Celestial Navigation << . | last updated: 15-Mar-2012 |