# NAD83 TM COORDINATE TO GEOGRAPHIC?

(@scott-zelenak)
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January 10, 2018 8:00 am

Well, I had to recreate a spreadsheet to convert NAD83 TM coordinates to Geographic and back because the original seems to have "corrupted" itself.  Manual NOS NGS 5 contains all the constants and all the formulae, so it's just a matter of powering through it.  However, I'm getting one little headache using term "n^2" as defined on page 33. When I apply it to the formulae on page 36 with my TM constants I must be doing something wrong.

In Constants B2, B4, and B6 if I  square it I get the wrong answer.

In Constants B3 and B5  if I square it I get the correct answer.

I'm attaching the XL sheet, if anyone would take a look and give some feedback I'd appreciate it.

And the angels, terrible and without pity, carry savage weapons, and their torture is unmerciful.

(@bill93)
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January 10, 2018 10:25 am

It looks like a very minor difference either way.  Is "wrong" a difference in the last place, about 0.3 mm?  That's all I saw when I took out the ^2 from term N15 in cells N18 and N19.

Using your SPC (in meters) through  CORPSCON6 gets 74 02 05.61455, same as I get if I don't square that already squared term (except to get eta^4 at the end).

I think you need to go back to see why the geographic to coordinate calculation gave you the easting 189300.6493 whereas CORPSCON6 gives 189300.650  I haven't gone through those equations.

(@scott-zelenak)
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January 10, 2018 11:33 am

Bill,

You're right. The n^2 term was getting me in the direct conversion as well.

And the angels, terrible and without pity, carry savage weapons, and their torture is unmerciful.

(@bill93)
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January 10, 2018 3:38 pm

I went through both sets of equations looking for the squared term, and have it matching CORPSCON very well.

The scale factor came out slightly different in the two computations, which I fixed by putting the square on Q in the  SPC->Geo computation.

(@bill93)
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January 14, 2018 4:59 pm

I fooled around with the @scott-zelenak spreadsheet a bit.  First, I figured there was no advantage of a spreadsheet over CORPSCON if you couldn't put in columns of values and get them all done at once.  So I cut and pasted the formulas into a single row instead of a column and put the values you are most interested first.  All the supporting calculations can be in hidden columns that don't print.

Then I wanted to see if I could make another zone work.  So I picked one (Indiana East-couldn't use my Lambert state. That's another project) and made another column for its constants.  You can switch zones by copying one column off the first sheet and pasting it on the second sheet.  For a given conversion sheet to make any sense, of course, that master column has to be set for the zone of interest.  When you look at Indiana points with NJ constants set on the 2nd sheet, the results are useless, but I wanted separate sheets for them so I wouldn't lose the numbers typed in.

Hope someone finds it useful, and of course you must do some checking before you trust it.

(@scott-zelenak)
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January 15, 2018 7:52 am

Bill,

Here's the Lambert from the manual.

And the angels, terrible and without pity, carry savage weapons, and their torture is unmerciful.

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