[Proj] How many principal digits on line labels for MGRS or USNG grids?

Mikael Rittri Mikael.Rittri at carmenta.com
Sun Nov 3 11:44:03 EST 2013

Hello.

I have a question about MGRS or USNG grid line labels, and how many principal digits should be used.

The background is that the line label is based on the UTM easting or northing for the line, which is a 6-digit or

7-digit number. But only the five last digits would be used in MGRS or USNG notation, so the fifth digit from the

end is the first principal digit, and is written with a larger font than the leading or trailing digits. So far, so good.

Here is my confusion. In the Military Map Reading 201, http://earth-info.nga.mil/GandG/coordsys/mmr201.pdf ,

examples show that when the grid line spacing is 10 000 m, there is one principal digit in each line label,

and when the grid line spacing is 1 000 m, there is two principal digit in each label. There are no examples
for larger scale maps, but my extrapolation has been that there would be three principal digits when the line
spacing is 100 m, four principal digits when the line spacing is 10 m, and five principal digits when the line
spacing is 1 m. (The basic idea is that the trailing digits are all zeros, with as many trailing digits as possible.)

However, I recently found a style guide for USNG (which is nearly identical to MGRS),

which simply says that there are always two principal digits in a line label. Examples are given for

many line spacings, including 10 000 m. (The basic idea must be that when reading the principal digits

as a number, you get a value in kilometers.)

The two documents clearly contradict each other for the case with 10 000 m line spacing. But I am

not sure if my extrapolation of the military convention to dense line spacing is correct - maybe the

military convention is to have one or two principal digits, but never more than that.

I would like to get comments or opinions on this, since I am currently working with MGRS grid visualization.

Best regards,

Mikael Rittri

Carmenta

Sweden

http://www.carmenta.com

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