Well, it is true that the USNG style guide is just a draft, but the following paragraph is from the established USNG standard:

C.1.7.1 Principal digits.

The 10,000-meter and 1,000-meter digits are known as the principal digits and identify USNG grid lines. Preceding and following UTM digits are shown as superscript. Alternatively, only the principal digits for grid lines need be shown, but a sample full UTM value for both the Easting and Northing axis must be depicted at least once on the map, usually near the southeast corner.

http://www.fgdc.gov/standards/projects/FGDC-standards-projects/usng/fgdc_std_011_2001_usng.pdfSo that is exactly two principal digits, regardless of line spacing. (Another thing: I'm not sure if this paragraph allows discarding the final zeros of the UTM digits that come after the principal digits. For example, if the line spacing is 100 m, then the small digits after the principal ones would be 000, 100, 200, ..., or 900, but the style guide would only display 0, 1, 2, ..., or 9 after the principal ones, and discard the two final zeros. This practice is not mentioned in the USNG standard itself, as far as I can see.)

According to "Datums, Ellipsoids, Grids, and Grid Reference Systems", DMA Technical Manual 8358.1, military map grids don't have line spacing denser than 1000 m. But that's a fairly old document.

My problem is also that style guides for printed maps don't always fit GIS systems well.

Regards,

Mikael Rittri

Carmenta

Sweden

http://www.carmenta.com________________________________________

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Sent: Monday, November 04, 2013 3:14 PM

To: PROJ.4 and general Projections Discussions

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

Hello,

without going any deeper into the subject the second pdf document says "DRAFT" and

the first one not .. so if you don't care about the second document at all since it is only

a draft (and could be any fairy tails).

Regards: Janne.

------------------------------------------------------------------------------

Mikael Rittri [

[hidden email]] kirjoitti:

> 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),

>

>

http://www.floridadisaster.org/gis/usng/Documents/USNG_style_sheetV1-11plus.pdf ,

>

> 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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