[Proj] etmerc original author Knud Poder turns 90.

[Davide Cesari]

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 “the Nestor of computational geodesy”.


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 [1] for details).


The code was based on theoretical foundations published a decade 
earlier, by König & Weise ([2], building on prior work by Krüger, 1912 
[3]).


Poder’s work was characterized by great care with respect to numerical 
precision and accuracy (e.g. by using Clenshaw summation for recurrence 
series, and Horner’s scheme for polynomial evaluation).


Also, Poder was noted for his ingeniously implemented “dual 
autochecking method” (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 Poder’s student, Karsten Engsager (the “E” in 
etmerc, “Engsager Extended Transverse Mercator”) took over 
maintenance and eventually extended König and Weise’s numerical 
series by another term, bringing the accuracy up to today’s 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




[1] 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


[2] R. König and K. H. Weise (1951): Mathematische Grundlagen der 
Höheren Geodäsie und Kartographie, Erster Band. Springer, 
Berlin/Göttingen/Heidelberg, 1951. K


[3] L. Krüger (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


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