I'd like to set up a UPS based on a distance of 60 seconds and am having a difficult time of it. It's probably because I've been working at it too long and it's late!
Here are my assumptions;
- The scale for minutes in the middle of the UPS can also be considered to be just 60 seconds.
- In setting the correct meridian distance for say, N 35°, is done exactly as one would set the meridian distance for 35°.
Any ideas about this?
Thanks,
Craig
You are absolutely correct that you may set up the plotting sheet using a scale of 60 arc-seconds on the central longitude line instead of the usual 60 arc-minutes. You would find the correct placement of additional longitude lines in exactly the same manner as before. Generally the easiest method is to find the 90° tick on the compass rose of the plotting sheet and to go both up and down along the compass rose to points that are the number of degrees from the 90° tick as is your central latitude line. In your case for a central latitude of 35°N you find the 55° mark on the compass rose (90 - 35 = 55) and the 125° (90 + 35 = 125) Connect these with a straight edge to draft in a new line of longitude.
If you have used arc-seconds on the latitude scale then the line of latitude tangent to the top of the compass rose will be 1 arc-minute north of your central line of latitude; and the one tangent to the bottom of the compass rose will be 1 arc-minute south of the central latitude. Similarly the lines of longitude will now be 1 arc-minute of longitude from the central longitude line. You can still use the wave-shaped longitude scale as you did before but keep in mind that it is now measuring in arc-seconds, not arc-minutes.
By this method you can zoom in on your plots. However 1 arc-minute of latitude that is now the basis for your plots will be somewhat small to be convenient. Celestial is often in error by amounts on that order of magnitude. (One nautical mile is generally considered "about as good as it gets" in celestial, and 1 nautical mile is 1 arc-minute of latitude change.
Instead you may find it more convenient to label you center line of latitude with the latitude of your assumed position (AP) and then label the lines above and below as 1/2 degree difference from the middle. You then proceed to put in your longitude lines the same way you always have and as described above. But now keep in mind that both the latitude scale and the longitude scale indicate double what they are labeled. e.g. in this case 60 arc-minutes of latitude marked on the scale is now only 30 arc-minutes of latitude on your sheet. The same thing will be true of the longitude scale.
You can also go the other way to "zoom out." This time you would label the central line of latitude as the latitude of your AP and the lines of latitude above and below as 2 degrees difference. Again you find the longitude lines the same way. In this case the scales will read half. e.g. 60 arc-minutes on the latitude scale would become 120 arc-minutes of latitude on your plot. The same is true of the longitude scale. This zooming out can be useful if you plan on doing some running fixes.
You could also use any other scale factor you find convenient, but doubling or halving makes the math easy.
If you zoom out by more than a factor of 3 or 4 you might find the longitude scaling and plotting to be a less reliable as you get near to the top or bottom of your sheet. That is because in the real world lines of longitude are not parallel but rather converge toward the poles.