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Old 21-06-2009, 10:27   #1
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Determining Longitude

I'd like to first start off with Forgive me. I'm not nearly as experienced as many of the members appear to be.

I don't currently own a boat, but have been dreaming of owning one for 20 years.

I hope to purchase one within the next couple of years. Wife willing

Anyhow.

I am chartering one this summer for a weekend and have been reading up on navigation. I'm experienced using Topo maps and UTM to navigate, but not Long & Lat

Perhaps someone could clarify something for me. Again forgive me if my description is not technically correct.

In determining Long say of your current position from GPS to Map. Is say each Long section on a map in the northern hemishere not an actual square, but as the lines on Long point north, they converge on the north pole?

So say on a map you are within a Long & Lat grid as you move north does that grid not narrow? So if you use spreaders say to transpose Long from GPS to map. And you go to border of map to determine the distance that 22' 26" is and then you then move into the applicable grid, how is that accurate if as you move north the lines of Long converge on the north pole?

the distance say along the bottom of the map that each 1' or each 1" make up will not be the same as you move to say the top of the map?

Because if it were could you not use Long for distance as well as Lat?

Maybe I missed something in all the books I've been reading
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Old 21-06-2009, 10:58   #2
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As I understand your question, it is an issue of the commonly used 'Mercator projection' that requires you to use the latitude measurements on either side of the chart to measure distance on the chart to get that (22' 26"). That measurement that is held by your dividers are accurate for any location on the chart.

Did that help clarify your question?
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Old 21-06-2009, 15:41   #3
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Nautical charts, topo maps, in fact most maps will use some projection. A projection is a method of "stretching" the curved surface of the earth onto a flat piece of paper. Unfortunately projections distort properties of the map such as distance, direction, area and there are many different projections that minimize the distortion of one property at the expense of others. The mercator projection used on most nautical charts preserve direction at the expense of distance and area - i.e. you can plot a bearing anywhere on your map and it will be accurate, but as you have discovered, distance measurements are not the same at different logitudes. The topo maps you are familiar with use a transverse mercator projection that are more suitable for distance measurements than direction. If you carefully measure a topo map you will see that the are not exactly square. The thing is, a topo map a relatively tiny area so all these errors are negligible, as they would also be on a small-scale nautical chart of a harbor or harbor approach. On large-scale charts those errors are significant so it becomes much more difficult to measure distances. If you are plotting on a chart, you need to draw a line through the closest tic mark at top and bottom of the chart and then measue from that line.
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Old 21-06-2009, 16:07   #4
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When plotting a position on a chart say 22 degrees 23 minutes North you find the nearest line of latitude marked on the chart say 22 degrees 20 minutes. You set your dividers at the 22 deg 20 and spread it to the the 23 minute mark, you transfer this to the approx longitude position.
You repeat this for the logitude position line.
This will give you an accurate position.
When measuring distance take the measure with your dividers and place them on the same general latitude (side of the chart), not further north or south, and you will get an accurate measure of distance. One minute equals one nautical mile.
Never ever use the longitude marks for distance.
I hope that explains it.
Have fun, hope you get that boat and escape
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Old 21-06-2009, 17:43   #5
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Aloha Outdoor,
If you are using a nautical chart just use the left or right margins as a gage (if you have the chart with north facing up). Don't use top or bottom which are longitudinal lines because, as you so rightly pointed out, unless you are on the equator they won't be equal. Latitudinal lines are equal and each degree is 60 nautical miles. Remember, miles = minutes on the chart.
Hope this isn't too confusing an explanation and I know there will be others who will correct me if I've said it wrong.
Kind regards,
JohnL
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Old 21-06-2009, 18:01   #6
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Quote:
Originally Posted by outdoor View Post
Is say each Long section on a map in the northern hemishere not an actual square, but as the lines on Long point north, they converge on the north pole? So say on a map you are within a Long & Lat grid as you move north does that grid not narrow? So if you use spreaders say to transpose Long from GPS to map. And you go to border of map to determine the distance that 22' 26" is and then you then move into the applicable grid, how is that accurate if as you move north the lines of Long converge on the north pole?
You are quite right: On a globe, the lines of longitude converge toward the north and south poles. In a Mercator projection, the curved surface of the earth is flattened so that the lines of longitude are parallel, which means that images of objects on the chart become falsely larger going north or south from the equator relative to objects closer to the equator.

But, one of the cool things about a Mercator projection is that for any given latitude, 1 minute of latitude on the vertical scale on the left and right borders of the chart at that latitude = 1 nautical mile on the chart at that latitude. So, to find nautical miles with a divider, set the divider to the distance you want to measure, and then count minutes of latitude on the side of the chart at the same latitude where you are working.

Over small areas the distortion is small, but the distortion is worse as chart scale gets smaller (area represented on the chart gets larger). This method does not work so well for charts of whole seaboards or oceans, but works fine for harbours, bays and smaller coastal regions.

Marine navigators typically work with units of degrees and decimal minutes of arc, not minutes and seconds of arc. So 22' 26" vertically on the longitude scale at the edge of the chart would be 22.4 minutes of arc or 22.4 nautical miles. The GPS would give 22.433', but that is too precise to plot with hand tools.

Mercator (not his real name - people gave themselves nicknames in the 1500s) was a fascinating person. He made globes of both the earth and the celestial sphere, as well as maps of places he had never seen. Perhaps take Nicholas Crane's recent biography "Mercator" on your charter. It's a great read.
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Old 21-06-2009, 20:36   #7
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Quote:
Originally Posted by outdoor View Post
In determining Long say of your current position from GPS to Map. Is say each Long section on a map in the northern hemishere not an actual square, but as the lines on Long point north, they converge on the north pole?

So say on a map you are within a Long & Lat grid as you move north does that grid not narrow? So if you use spreaders say to transpose Long from GPS to map. And you go to border of map to determine the distance that 22' 26" is and then you then move into the applicable grid, how is that accurate if as you move north the lines of Long converge on the north pole?
On the globe, the lines of longitude (meridians) converge on the N and S poles - on a mercator projection, these lines are parallel and to compensate for this, the chart is stretched northward as you go further North (opposite in the S hemishpere). On a small scale chart (one that covers a very large portion of the sea, say 1:1,000,000 or smaller scale) you can see this by measuring the latitude at the bottom of the chart and top of the chart - the same relative figure such as 5 degrees will measure 2 inches at the bottom of the chart, but might measure 3 inches at the top. On a large-scale chart, this stretch is imperceptible. In practice, marking out longitude is easy - taken from the scale at the bottom or top of the chart will give the same accurate result.

Quote:
the distance say along the bottom of the map that each 1' or each 1" make up will not be the same as you move to say the top of the map?
As I explained, this is not the case with longitude measurements - but it is a problem when measuring latitude. As I said the latitude scale is larger at the pole-side of the chart. When plotting a position, it is best to take the actual point on the scale or as close to it as possible. Same with measuring distance on small-scale charts - distances measured off the chart should be measured on the scale closest to the latitude where the measurement is taken.

Quote:
Because if it were could you not use Long for distance as well as Lat?
As you pointed out, the meridians are converging - measure the difference on your chart between 5 degrees of longitude and 5 degrees of latitude. The longitude measurement will be smaller everywhere but at the equator. You can use the Long. for distance, but you have to apply a mathematical correction - a minute of Lat. will always give you a nautical mile with no math required.
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Old 22-06-2009, 02:58   #8
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Greetings, and welcome aboard, outdoor.
Hope you find this little sample of resonsive conversation interesting enough to keep you coming back.
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Old 22-06-2009, 06:21   #9
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Thanks

Thank you all for the responses.

You have either clarified or confirmed my understanding.

So I can more accurately plot Longitude by:

1) Using spreaders to measure minutes, seconds on Right/Left of map (latitude) And transposing that to longitude lines

2) using a map ruler measure diangelly from each line of long until line intersects with Lat line and the proper min and sec.

(not sure if I explained this clearly)

But I think I understand

Thanks again
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Old 22-06-2009, 06:45   #10
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Quote:
Originally Posted by outdoor View Post
So I can more accurately plot Longitude by:

1) Using spreaders to measure minutes, seconds on Right/Left of map (latitude) And transposing that to longitude lines
No. Absolutely not. Always measure longitude from the scales on the top and bottom sides of the chart.

Quote:
2) using a map ruler measure diangelly from each line of long until line intersects with Lat line and the proper min and sec.

(not sure if I explained this clearly)
I'm not certain what you're suggesting here, but I can't imagine how you could use your ruler diagonally??? You can use a parallel ruler and mark lat and lon in the same way on a mercator chart - align the ruler with the line (lat or lon) and move it parallel until it matches up with the desired deg/min/sec on the scales sides and top/bottom of chart - mark lines here and where they cross is your position.

Hope that's clear.
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Old 22-06-2009, 07:02   #11
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Hello, outdoor.

If all this good advice is confusing to you, download and read Chapter 3 - The Nautical Chart, from the USCG Auxiliary's Basic Coastal Navigation course. It explains what you need to know in very straightforward and understandable terms.
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Old 22-06-2009, 07:23   #12
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eh?

But I thought that was what everyone was agreeing is that if you use the top or bottom the distance is not the same since as you move north the lines converge.

so the same distance on the bottom that represents a " is not the same distance on the top?
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Old 22-06-2009, 07:25   #13
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see the following method

Measuring and Plotting Lat/Lon Coordinates
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Old 22-06-2009, 08:07   #14
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The method shown in the link (Measuring and Plotting Lat/Lon Coordinates) works. It is a method of scaling. It is not commonly done. It is more of a drafting technique. Nothing wrong with it, it's just not nautical.
The method used by us oldtimers is;
For example 75* 25" West.
Find the line of Longitude closest to your position that extends up and down through the chart. This lin on our hypotectical chart is 75* 20'.
Take your deviders and set one leg either at the bottom or top of the chart, whichever is closer, at 75* 20', move the other leg to the 75* 25'.
Transfer this to the chart, one leg on the 75* 20', and mark it. This is your east west position.
There is no need to play with a ruler, very ugly looking.
Does the above make sense?
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Old 22-06-2009, 09:43   #15
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I'm obviously not getting it :(

[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
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