[Proj] Scale factor for Transverse Mercator

Charles Karney ckarney at sarnoff.com
Wed Sep 10 11:39:31 EDT 2008

Gerald I. Evenden <geraldi.evenden at gmail.com> wrote:
> Let me pose the question: why are all of the state plane zones in the
> US established to have very small scale errors (factors) over their
> extent?

Because an important application for state maps is defining land
ownership, building roads, etc.

> Why did DOD create the 6 degree zone if it were not to minimize the
> scale error.

DOD also wants bullets to go predicable distances in straight lines on

However, both these applications are concerned with a subclass of human
activities close to the surface of the earth.

Google maps' use of Mercator is different class of application.  The
requirements here are conformality and seamless scrolling over most of
the globe with the normal application being navigating on roads rather
than plotting great circle routes between continents.  Here the
much-maligned Mercator seems to be an appropriate choice.

I also read with interest Dozier's remark in NOAA Technical Report NESS
81 that the TM projection (extended beyond +/- 3deg) is useful for
analyzing data from the NOAA A-G series of satellites.  I don't know why
this might be beyond my ascertaining that these satellites are in polar
orbits and TM can at least handle a single such orbit seamlessly.  So
here is an application unanticipated (probably) by Gauss for the TM and
beyond the narrow question of how to get a good map of Hanover.

I guess my point is that a library offering general-purpose projection
capabilities should being agnostic on questions of what is a "good"
projection.  Such issues are really in the application domain.

Charles Karney <ckarney at sarnoff.com>
Sarnoff Corporation, Princeton, NJ 08543-5300

URL: http://charles.karney.info
Tel: +1 609 734 2312
Fax: +1 609 734 2662

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