described in a recent thread, for the next release, proj.4
will switch the default transverse mercator implementation
from tmerc to etmerc.
is probably a good occasion to reiterate the history of the
code for the etmerc implementation - especially since the
original author, Knud Poder, turned 90 on October 19th.
Having his transverse mercator implementation becoming the
proj.4 default is a strikingly proper way of celebrating
Poder, among colleagues and collaborators rightfully
considered âthe Nestor of computational geodesyâ.
wrote the first version of what is now known as etmerc,
around 1961. It was written in Algol-60 and ran on the GIER
computer, built for the Danish Geodetic Institute (see 
code was based on theoretical foundations published a decade
earlier, by KÃ¶nig & Weise (, building on prior work
by KrÃ¼ger, 1912 ).
work was characterized by great care with respect to
numerical precision and accuracy (e.g. by using Clenshaw
summation for recurrence series, and Hornerâs scheme for
Poder was noted for his ingeniously implemented âdual
autochecking methodâ (not used in the proj.4 version),
where the same code was used for forward and inverse
projections and was run both ways and compared, to protect
against both coding- and hardware errors. The latter was
very important at a time where the mean time between failure
for computer systems was much shorter than today.
the 1970s Poderâs student, Karsten Engsager (the âEâ
in etmerc, âEngsager Extended Transverse Mercatorâ) took
over maintenance and eventually extended KÃ¶nig and
Weiseâs numerical series by another term, bringing the
accuracy up to todayâs standard.
2008, through the efforts of a.o. Gerald Evenden, Frank
Warmerdam and Karsten Engsager, etmerc was introduced in
proj.4, while in 2013 Charles Karney provided 3 corrections
- stressing the value and importance of open source code
retired 20 years ago, but has been taking active interest in
the maintenance and development of his code ever since.
Switching proj.4 to use a transverse mercator implementation
based on his work is probably the best conceivable way of
celebrating the 90th birthday of a great Nestor of
celebration of Knud Poder!
Knudsen, Danish Geodata Agency
Thomas Knudsen, Simon L. Kokkendorff, Karsten E. Engsager
(2012): A Vivid Relic Under Rapid Transformation, OSGeo
Journal vol. 10, pp. 55-57, URL https://journal.osgeo.org/index.php/journal/article/download/200/167
R. KÃ¶nig and K. H. Weise (1951): Mathematische Grundlagen
der HÃ¶heren GeodÃ¤sie und Kartographie, Erster Band.
Springer, Berlin/GÃ¶ttingen/Heidelberg, 1951. K
L. KrÃ¼ger (1912): Konforme Abbildung des Erdellipsoids in
der Ebene. Neue Folge 52. Royal Prussian Geodetic Institute,
Potsdam. URL http://bib.gfz-potsdam.de/pub/digi/krueger2.pdf