[Proj] etmerc original author Knud Poder turns 90.
dcesari at arpa.emr.it
Thu Oct 29 03:17:14 EST 2015
As described in a recent thread, for the next release, proj.4 will
switch the default transverse mercator implementation from tmerc to etmerc.
This 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.
Poder 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  for details).
The code was based on theoretical foundations published a decade
earlier, by KÃ¶nig & Weise (, building on prior work by KrÃ¼ger, 1912
Poderâs 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 polynomial evaluation).
Also, 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.
During 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.
In 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 sharing.
Poder 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 computational geodesy.
In celebration of Knud Poder!
/Thomas Knudsen, Danish Geodata Agency
 Thomas Knudsen, Simon L. Kokkendorff, Karsten E. Engsager (2012): A
Vivid Relic Under Rapid Transformation, OSGeo Journal vol. 10, pp.
 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
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