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Go Back   SailNet Community > Out There > Destinations > Great Lakes > Great Lakes Boaters, Polyconic charts, mph
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Thread: Great Lakes Boaters, Polyconic charts, mph Reply to Thread
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Topic Review (Newest First)
10-04-2012 10:56 AM
jackdale
Re: Great Lakes Boaters, Polyconic charts, mph

Quote:
Originally Posted by Hartley18 View Post
So, as a Yachtmaster Instructor, you're telling me that the bit I circled doesn't represent a distance of ten nautical miles??

From wonderful wikipedia: "The nautical mile (symbol M, NM or nmi) is a unit of length that is about one minute of arc of latitude measured along any meridian... By international agreement it is 1,852 metres (approximately 6,076 feet)."

Perhaps it's a good thing I'll never have to sail in that part of the world.
That works well on a Mercator projection. On a polyconic chart the distance is distorted as you move away from the central meridian.
10-04-2012 01:48 AM
Classic30
Re: Great Lakes Boaters, Polyconic charts, mph

Quote:
Originally Posted by jackdale View Post
It is the latitude scale, used to measure distance on a Mercator projection.
So, as a Yachtmaster Instructor, you're telling me that the bit I circled doesn't represent a distance of ten nautical miles??

From wonderful wikipedia: "The nautical mile (symbol M, NM or nmi) is a unit of length that is about one minute of arc of latitude measured along any meridian... By international agreement it is 1,852 metres (approximately 6,076 feet)."

Perhaps it's a good thing I'll never have to sail in that part of the world.
10-04-2012 01:44 AM
jackdale
Re: Great Lakes Boaters, Polyconic charts, mph

Quote:
Originally Posted by Hartley18 View Post
Righto.. What's this then?? Looks like nautical miles (and tenths) to me:

Attachment 12380

I understand that the Polyconic nature of the chart means that lines of lattitude and longtitude aren't necessarily straight up that part of the world, but don't get me wrong, I'm happy to learn something new.
It is the latitude scale, used to measure distance on a Mercator projection.

Polyconic projections are common in the Great Lakes of Canada and the US.
10-04-2012 01:40 AM
Classic30
Re: Great Lakes Boaters, Polyconic charts, mph

Quote:
Originally Posted by jackdale View Post
The OP is correct; on a polyconic or conic chart distances are measured with a distance scale, not the latitude scale. The distance scale on both charts is statute and some of the cautionary notes notes indicate m.p.h., not knots.
Righto.. What's this then?? Looks like nautical miles (and tenths) to me:

Attachment 12380

I understand that the Polyconic nature of the chart means that lines of lattitude and longtitude aren't necessarily straight up that part of the world, so don't get me wrong, I'm happy to learn something new.

Over here anyways, it's only a "chart" if it's in NM - otherwise it's a "map".
10-04-2012 01:17 AM
jackdale
Re: Great Lakes Boaters, Polyconic charts, mph

Quote:
Originally Posted by Hartley18 View Post
Came across this old thread whilst poking around. Looks like an interesting part of the world to sail in - you're lucky!

Polyconic projection is an interesting subject and certainly not something I've come across before, but I thought I'd correct an error in the statements below in case anyone else happened by and came to grief as a result:



The charts listed above (and presumably most others you own) are all clearly marked in Nautical Miles along both edges in the standard fashion of nautical charts of every other corner of the globe. NOAA have helpfully added Statute mile scales and a host of other relevant info for you to use if you want to.

Over such a small area of the planet you're free to use miles for your navigation if you want to - but to categorically state that the charts used are in Statute miles and "Nautical mile just makes no sense" simply is not correct.

FWIW...
The OP is correct; on a polyconic or conic chart distances are measured with a distance scale, not the latitude scale. The distance scale on both charts is statute and some of the cautionary notes notes indicate m.p.h., not knots.
10-04-2012 12:22 AM
Classic30
Re: Great Lakes Boaters, Polyconic charts, mph

Came across this old thread whilst poking around. Looks like an interesting part of the world to sail in - you're lucky!

Polyconic projection is an interesting subject and certainly not something I've come across before, but I thought I'd correct an error in the statements below in case anyone else happened by and came to grief as a result:

Quote:
Originally Posted by BryceGTX View Post
I have dozens and dozens of charts covering Lakes Erie, Huron, Michigan, Georgian Bay, Green Bay and St Clair. Many of these charts are Canadian charts. I have every blow up of every harbor of every lake. After checking every one of my charts, I do not own a single chart that has the units of nautical mile except for a chart I bought last year for the western end of Lake Erie. To this day, the latest Lake St Clair and Lake Huron chart at NOAA still do not have nautical mile as a scale, even though the newest update was Feb 2011.

Lake St Clair: Chart 14850
Lake Huron: Chart 14860

.....

Given that my charts that I use are all Statute mile, and given that they are all polyconic projection charts, it is clear why I use the Statute Mile. Nautical mile just makes no sense. It would be like insisting on driving my car based on nautical miles even though every distance sign and map is in statute miles.
The charts listed above (and presumably most others you own) are all clearly marked in Nautical Miles along both edges in the standard fashion of nautical charts of every other corner of the globe. NOAA have helpfully added Statute mile scales and a host of other relevant info for you to use if you want to.

Over such a small area of the planet you're free to use miles for your navigation if you want to - but to categorically state that the charts used are in Statute miles and "Nautical mile just makes no sense" simply is not correct.

FWIW...
09-20-2011 08:25 AM
Stearmandriver I've always liked cartagrophy. Cool posts, thanks.
09-20-2011 05:52 AM
BryceGTX This also gives us insight into why the distance may be in miles. Surveyors generated information that resulted in maps using miles for land based areas including the coastline. They then apparently just continued surveying across the lakes to complete the survey.

I suspect current opinion is that triangulation using a Mercator chart except at high latitudes results in minimal error. And navigating at the latitudes of the Great Lakes using Mercator charts does not introduce large errors. So I suspect Mercator charts probably will replace the Polyconic charts in the Great Lakes as time goes on. Already, all large scale (numerical) charts are Mercator except Lake Huron. Lake Erie was converted from Polyconic to Mercator sometime in the last 30 years or so. However, until all small scale (numerical) charts convert over or at least add nautical mile as a scale, it is unlikely that the nautical mile will become the predominant unit of measure in the near future.

I have read some really interresting opinions as to why Great Lakes boaters use statute miles and mph. Hopefully this gives others insight into why many Great Lakes Boaters navigate in statute miles.
09-20-2011 05:43 AM
BryceGTX So now that I know why I use statute miles, I am curious as to why all my charts are Polyconic charts and not Mercator projection charts. If they were, I would probably use nautical miles. For this answer, I have to dig back in history.

From the book “A Textbook onCoast and Lake Navigation” Copyright 1902 by International Textbook Company. Can be downloaded free of charge as a PDF from “Google Books”
From page 38:

The polyconic “projection is therefore advantageous for the representation of a coast line that runs north and south, or in the direction of the meridian, and is for this reason extensively used by the United States Coast and Geodetic Survey in the preparation of working charts of the coast, and also by the United States Engineer’s Office, War Department, in the preparation of charts used by mariners on the Great Lakes.”
From page 43:

“Charts for Use on the Great Lakes….. Great Lakes charts published by the US Engineer Office are constructed on the polyconic projection… on the US Engineer chart only statute mile is used.”

From these two statements we know now why Polyconic charts were used and why the scale has traditionally only had Statute mile as a unit of measure. Polyconic charts were used because of predominant north-south coastlines of Lakes Huron and Michigan. The scale of statute miles was just decreed. I read probably a dozen old navigation books and found this same justification in a number of them.

Now, two more questions came to mind. First, why is Polyconic useful for north-south oriented coast lines? Secondly, are statute miles associated with Polyconic charts?

From “United States Hydrographic Office” “Great Lakes Pilot, Volume II” 1921 Government printing office. Can be downloaded free of charge from Google Books.
From Page 3:

“On a polyconic chart, since a straight line represents (within the limits of 15 or 20 degrees of longitude) nearly the arc of a great circle, or the shortest distance between two points, bearings on the chart are identical with observed bearings. All Lake Survey charts are polyconic projections. The Mercator projection is unsuited for surveying purposes.”

This is an incredibly powerful statement and answers the reason for polyconic charts used in north-south coastlines. The reasoning is that navigating along a coast invariably meant shooting bearings to objects on the coast to determine position based on triangulation.

Great circles define the bearing to an observed object. Great circles are straight lines on polyconic charts. Great circles are curved lines on Mercator charts. If we want the most accurate triangulation, use a polyconic chart which explains why surveyors cannot use a Mercator chart. The error in triangulating on a Mercator chart is greatest in the east-west direction; that is facing a north-south coast.

And following visual aids causes us to follow a great circle, so plotting the course on a polyconic chart shows the obstacles that might be crossed during the transit.
09-20-2011 05:41 AM
BryceGTX
Great Lakes Boaters, Polyconic charts, mph

I use miles and mph exclusively for speeds and distance for navigation. On another thread, other posters were surprised I use mph and miles. I have been navigating the great lakes for 30 years on my own boats and an additional 10 years in the early 60s on my dad’s boat. It has always been the only way to do it for me. Understanding the significance of the nautical mile, I was curious also why I use miles instead of nautical miles.

For cruising, my wife and I travel extensively in the Great Lakes. We have been over all of Lake St Clair, Lake Huron, Georgian Bay, Most of Lake Michigan, Green Bay and Lake Erie. This year alone, we made three trips into Lake Erie and two into Lake Huron.

I have dozens and dozens of charts covering Lakes Erie, Huron, Michigan, Georgian Bay, Green Bay and St Clair. Many of these charts are Canadian charts. I have every blow up of every harbor of every lake. After checking every one of my charts, I do not own a single chart that has the units of nautical mile except for a chart I bought last year for the western end of Lake Erie. To this day, the latest Lake St Clair and Lake Huron chart at NOAA still do not have nautical mile as a scale, even though the newest update was Feb 2011.

Lake St Clair: Chart 14850
Lake Huron: Chart 14860

My charts are mostly 1980 to 1995. I have two charts from 1962 that are from early years on my dad’s boat.

The only Mercator projection chart I own is of Lake Michigan. So it requires no nautical mile scale. However, this chart has statute mile, meters and yard scales. This chart I do not use for navigation as I have smaller scale partial charts that show more detail and are much more useful. All these charts are polyconic projection charts, which have only Statute mile no nautical mile.

Interestingly enough, Superior, Michigan, Erie and Ontario are now Mercator Projection charts. Lake Huron is still a polyconic projection chart. The Lake Erie chart I own is Polyconic, not Mercator. So sometime in the last 20 years, Lake Erie was converted to Mercator.

Much of our boating is in Lake St Clair and the bays. It now seems apparent why I have never used nautical miles. I don’t have any charts that I use that have nautical mile as a unit of measure. I have one chart that I could use nautical mile (Lake Michigan) because it is Mercator Projection chart, if I ever used the chart.

Given that my charts that I use are all Statute mile, and given that they are all polyconic projection charts, it is clear why I use the Statute Mile. Nautical mile just makes no sense. It would be like insisting on driving my car based on nautical miles even though every distance sign and map is in statute miles.

 
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