[Proj] Switch utm from tmerc to etmerc

[Howard Butler]

Charles,

Thanks for the contribution. Hearing no feedback, I have merged your pull request implementing this change. Please see https://github.com/OSGeo/proj.4/pull/323 for more information.

This update will be provided as part of the upcoming 4.9.3 release[1] . I am shooting for an early 2016 target for that release unless something comes up to hurry things up.

Howard

[1] https://github.com/OSGeo/proj.4/issues?q=milestone%3A4.9.3+is%3Aclosed

> On Oct 6, 2015, at 10:19 AM, Charles Karney <charles.karney at sri.com> wrote:
> 
> Reposted from
> 
>   https://github.com/OSGeo/proj.4/issues/316
> 
> Last year, NGA updated its recommendations on the algorithms to use for
> UTM from tmerc to etmerc (in proj.4's terminology). I recommend that
> proj.4 follow suit, i.e., move the definition of utm from PJ_tmerc.c to
> proj_etmerc.c.
> 
> I took a look at doing this and my initial attempts were stymied by a
> lack of knowledge about the macros that proj.4 uses to construct code.
> So it would be better is someone else could tackle this. Any
> volunteers? I'd be happy to help in checking out the result.
> 
> Background:
> 
> The NGA paper is "The Universal Grids and the Transverse Mercator and
> Polar Stereographic Map Projections"
> 
> http://earth-info.nga.mil/GandG/publications/NGA_SIG_0012_2_0_0_UTMUPS/NGA.SIG.0012_2.0.0_UTMUPS.pdf
> 
> The error in tmerc is less then 1 mm if you stick to the UTM zones.
> However there's interest in extending UTM well beyond the 6 degree
> zones. For example the point 83N 0E is less than 400km from the center
> of zone 35n. So the scale of 35n UTM projection 1.0011 at this point.
> However
> 
> echo 0 83 | cs2cs +proj=latlong +datum=WGS84 +to +proj=utm +zone=35 
> +datum=WGS84 -f %.6f
> echo 0 83 | cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc 
> +datum=WGS84 +k=0.9996 +lon_0=27 +x_0=500000 -f %.6f
> 
> both return 145723.827486, 9300924.850170, while
> 
> echo 0 83 | cs2cs +proj=latlong +datum=WGS84 +to +proj=etmerc 
> +datum=WGS84 +k=0.9996 +lon_0=27 +x_0=500000 -f %.6f
> 
> returns the correct result 145723.870553, 9300924.845226; so tmerc and
> utm are in error by about 43 mm.
> 
>   --Charles
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