NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The Sun does not stop for anyone
From: Stan K
Date: 2016 Dec 30, 16:08 -0500
From: Stan K
Date: 2016 Dec 30, 16:08 -0500
I've been following this thread, and decided to stick in my two cents, for whatever they're worth.
It seems to me that the only "perfect" Mercator chart is really a projection of the lines of latitude and longitude on a cylinder tangent to the equator (for the "normal" Mercator) from an Earth that is an oblate spheroid, not a sphere, where even the smallest unit of latitude increases continuously as you move further from the equator. Any other "Mercator" chart is derived mathematically.
In the United States Power Squadrons advanced courses, we use three kinds of plotting sheets:
1) The Constant Latitude Scale Small Area Plotting Sheet: This is usually set up for one degree of latitude (to be chosen by the navigator), with the longitude scale set for the mid-latitude (marked for 0 to 55, but could be extended). As the name implies, each minute of latitude is the same size. Not a true Mercator chart
2) The Universal Plotting Sheet: This covers four degrees of latitude (to be chosen by the navigator), with the longitude scale set for the mid-latitude (marked for 0 to 70, but could be extended). This is also a constant latitude scale sheet, with each unit of latitude being the same size. Not a true Mercator chart.
3) A 900 Series Position Plotting Sheet (the difficulty obtaining these has been discussed in the past): These are printed for a particular range of latitude. For instance, the N.O. 973 sheet covers the three degrees of latitude from 38º to 41º, and four degrees of longitude (to be chosen by the navigator). It can be easily measured that the 40-41 degree latitude range is larger than the 38-39 degree latitude range, but within each degree of latitude it was impossible for me to measure any difference between 0-10' and 50-60', so, within each degree of latitude, it may or may not be constant latitude scale I choose to believe that it is not). As far as longitude goes, the N.O. 973 sheet has "1º Long. = 4 inches" printed on it. Measuring the middle degree of latitude (39º to 40º), I get about 5.125 inches, so I would expect one degree of longitude to be about 5.125" x cos(39.5º) = 3.955". Allowing for printing tolerance and my measuring error, this seems right. This is the closest thing I have to a true Mercator chart. FWIW, it has "MERCATOR PROJECTION" printed on it.
-----Original Message-----
From: Gary LaPook <NoReply_LaPook@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Fri, Dec 30, 2016 12:55 pm
Subject: [NavList] Re: The Sun does not stop for anyone
From: Gary LaPook <NoReply_LaPook@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Fri, Dec 30, 2016 12:55 pm
Subject: [NavList] Re: The Sun does not stop for anyone
The UPS has the same Latitude scale ( NM per inch ) at 10 degrees north and at 50 degrees north. A Mercator chart does not.
John H.
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That's true, if you want to make a chart that covers the entire range from 10 to 50 degrees, but if you just want to make a small area plotting sheet covering 10 to 11 degrees or 49 to 50 degrees then each of those separate small area plotting sheets will have constant scales.
gl
