Saturday, January 17, 2009

Universal Plotting Sheets and how to set up

Universal Plotting Sheets
In order to plot celestial lines of position you need a Universal Plotting Sheet, the first thing that probably sticks out on the sheet is the compass rose, just like conventional charts. All the compass rose bearings are true bearings. You will notice the vertical line running due north and south through the center of the compass rose. Next, are the horizontal lines which will represent your desired latitudes. And last on the lower right, a longitude scale, which will allow you along with your dividers a way to lay off minutes of longitude.

Your first observation that you make is the scale between any two horizontal latitude lines. It's a scale breaking down one degree of latitude into 60 minutes. If you designate the latitude line 28 degrees north and your indicated position is 28 degrees put your dividers on 28 you now have your latitude.

Now that you have an understanding of how to plot a latitude position without having to do anything to the sheet other than fill in the latitude and scale up the from the vertical compass rose, you only need to set up the sheet for longitude. Your assumed position is 28 degrees north latitude, sixty two degrees thirty six minutes west longitude. The first step is your latitude, whatever whole degree of latitude that you are currently located in or centered on as part of a plot will be indicated as the central latitude line, In this case 28 degrees. Since we are in North latitude the one below will be 27 degrees the one above 29 degrees. If you are in the Southern Hemisphere, it will be reversed. So now all you need to do is to account for your longitude.

Setting up your Longitude Lines
On the edge of the compass rose there are two tick marks just below 30, two tick marks the top thirty, the other tick mark below thirty.This is important the two tick marks below 30 and above 30 on the compass rose are actually on the 28 degree marks, the same 28 as our central latitude. Next, draw a vertical line connecting the two points. Looking at a plotting sheet you will see that this gives you your second longitude line and your longitudinal spacing. To get additional longitude lines, use your dividers one point on the compass rose central line the other on the constructed line and lay them off as needed.

Since the plot on the sheet required a longitude of 62 degrees West, pick one convenient to your position and labeled it.The next one that would have obvious chart significance is 63 degrees West. Your position is 62 degrees 36 minutes West. How do you lay off the minutes? Look to the longitude scale lower right corner. Draw a line between 25 degrees and 30 degrees latitude, approx. 28 with the construction of this line, you put one point of my dividers on 30, extending the other point back to the zero mark would indicate only 30 minutes. The lines to the right are further broken down into increments of two degrees. Use your dividers and walk out 6 more minutes this should give you 36. This sheet is prepared and published by the US Defense Mapping Agency, normally comes in a pad of 100, and you can use both sides. I like to save mine for a reference if I want to go back and check.