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How do use the Latitude and Longitude Plotting Interpolator?

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So, looking at the chart that I just posted on my wall of my most recent bareboat charter, I see it has a "latitude and longitude plotting interpolator".  I've never used this, never came up in any navigation instruction, and google is providing little, unless you want to get into MatLab (not a MatLab guy).

We're talking paper charts, here, and I realize we're laying out some spherical on a 2D flat rectangular surface.  How would one use this?  Would you need to?



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9 minutes ago, SemiSalt said:

The reason you need it is the distance associated with a degree of longitude depends on the latitude. 

That part I understand.  I just don't know how to use the interpolator.

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I would think it'd used in a similar way to the longitude scale on the celestial "Universal Plotting Sheets." Use dividers to move up the vertical axis an amount proportional to the latitude and strike a horizontal line. (or park your ruler there) Then, use that line as the longitude scale.  On the UPS, the bottom of the scale is 0° Latitude and the top is 70°... here, maybe it represents the latitude range of the chart?  Seems a little extreme?

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"A latitude and longitude plotting interpolator serves as an aid in plotting geographic positions on Great Lakes polyconic charts at scales of 1:50,000 or smaller. The interpolator,  similar to the LORAN-C interpolator and others, is designed to compensate for the converging meridians on polyconic charts. A number of interpolators are available and the proper size should be selected for a given chart depending on the spacing of projection lines. One interpolator is required for each chart. For charts and insets at scales larger than 1:50,000, the convergence of meridians is small enough so that a subdivided plan border will suffice for plotting geographic positions. Interpolators will remain on the designated Great Lakes charts until the charts are reconstructed and converted to Mercator projections."

In English - the lines squish together too much on small scale charts because we used an inferior projection. 


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