Determining Longitude

[Dockhead]

Originally Posted by outdoor

[IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot-1.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot-2.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot-3.jpg[/IMG]Over dramatization of Convergence of lines of Longitude

so lets assume I use spreaders to measure once space of longitude on the 44 14' line of latitude

If I then transpose this further up towards 44 15', the same space will not represent the same number of Min and sec

No, you are getting it. You are correct -- you will NOT get minutes of longitude based on your scale -- minutes and degrees of longitude are equal to minutes and degrees of latitude ONLY at the equator.

The point is not degrees and minutes, but MILES. A mile or a number of miles you get by matching the minutes on your left or right scale with your dividers will give you that mile or miles anywhere on the chart.

To get minutes or degrees of longitude, use the top or bottom scale.

[mesquaukee]

I looked at your attached file. You are right of course, there is a small difference.
This difference does not matter on a chart as it is very little, as long as you are measuring the position along the bottom or top of the chart whichever is closer this error is negliable for coastal charts. If you are using a chart showing the whole North Atlantic it would matter but then you have a lot of sea room so it doesn't matter.
Does that make sense?
Miles has nothing to do with it, it realy is spherical geometry simplified to a flat plane. There will be errors.

[David M]

I hate to make a simple concept any more complex than this has become, but you can always use a Gnomonic chart for small scale charts. For large scale charts Mercators are fine. Remember, small scale charts cover large areas. Large scale charts, the opposite.

Map projection - Wikipedia, the free encyclopedia

[Dockhead]

Originally Posted by outdoor

[IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot-1.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot-2.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/sgraham/LOCALS%7E1/Temp/moz-screenshot-3.jpg[/IMG]Over dramatization of Convergence of lines of Longitude

so lets assume I use spreaders to measure once space of longitude on the 44 14' line of latitude

If I then transpose this further up towards 44 15', the same space will not represent the same number of Min and sec

Oops, that's right of course -- the question was more subtle.

But however subtle is the question, the answer is the same -- degrees of longitude slowly contract as you go north or south from the equator. The bottom scale shows them exactly the way they are at that point on the chart, likewise with the top. Anywhere in between the top and the bottom, the degrees and minutes will be proportionately in-between the degrees and minutes shown .

[David M]

Minor point...dividers, not spreaders. Also, fenders, not bumpers. Sorry, those are two words that sound like fingernails on a chalkboard.

[outdoor]

Ok. First I'd like to make a comment

I appreciate all the responses.

There have been other discussion forums that the members don't seem to bother answering some questions that are obvious to the majority

So I think I understand. So what I'm saying is correct, but the error is so small it is negligible

For distance use Latitude 1' = 1 nautical mile

for Longitude coordinates use spreaders (dividers) at top or bottom

[mesquaukee]

You got it. It's simple once you see it. It's just a different way of thinking, it is time tested and foolproof.

On Gnomonic Charts.
Gnomonic chart are great for ploting great circle routes, they are used by pilots crossing long distances near the poles. They are useless though for distances as there is still a lot of distortion. You need all sorts of tables or use spherical geometry to calculate distance

[David M]

True, Gnomonic charts are becoming less useful with the more common use of electronic plotters. Still though, electronics are not 100% bullet proof either. I would still keep a few around as backups for ocean crossings.

[Lodesman]

Originally Posted by outdoor

So I think I understand. So what I'm saying is correct, but the error is so small it is negligible

Sorry you're being steered wrong - in your attached file, the projection is conic, not mercator. For the most part you will navigate on mercator charts, where the meridians are drawn parallel. The measurement of Lon at the top of the chart is identical to the measurement of Lon at the bottom of the chart.

For distance use Latitude 1' = 1 nautical mile

for Longitude coordinates use spreaders (dividers) at top or bottom

Yes.

I now understand what you meant by measuring diagonally, but I can't understand why you would want to greatly complicate a simple task.