[Proj] Re: Comments of tmerc, etmerc and ftmerc errors

strebe strebe at aol.com
Sat Jun 14 20:32:06 EDT 2008

On Jun 14, 2008, at 8:08:47 AM, "Gerald I. Evenden" <geraldi.evenden at gmail.com> wrote:
This is the place where we will apparently never agree. I would never use 
small scale maps for distance/azimuth measurements and anyone who does is ill 
You know, Mr. Evenden, you are the one who touted the superiority of equal-area maps for such purposes (quoting from your previous e-mail):

>Extended geographic range usage of any conformal projection is a contentious 
>issue as any resultant grid system has sufficiently large scale errors as to
>make the Cartesian usage of the grid very questionable.
>Your comment:
>>This is true of any projection. It is true that conformal projections carry
>>an inevitable disproportion of areas, but that's not related to scale error.
>My sentense had nothing to do with area and by Cartesian usage I was denoting 
>measuring distances, azumuths, etc. from the Cartesian data and it certainly 
>seems that scale error greatly affects these measurements.

You're not arguing with me. I did not bring up using maps for arbitrary distance/azimuth measurements. You did, and you did so in order to prove the superiority of equal-area projections over conformal projections for small-scale usage. So now you are arguing with yourself. This is a matter of public record. So is the rest of your weaseling over the course of this discussion.

As far as your tirade below goes, I dare you to find anyone who thinks we should use maps when something else is better. I dare you to find someone who thinks most thematic mapping should be on anything other than equal-area maps. Those are straw men. Meanwhile, because you apparently can't imagine any use for small-scale maps that is not tied to area mensuration or estimation, you call for the elimination of conformal maps. You might ponder that it's dogma and your own limited imagination that lead you to such conclusions, not logic, and not real-world needs.

-- daan Strebe

I feel it is fair to say most small scale map usage is in the realm 
of thematic mapping and and in such usage I feel that the principal factor of 
concern is the sense of extent of a feature and its *area* of scope. A kind 
of "my area is bigger than yours" attitude.

The best example of worst case usage of a conformal map is the classic 
Mercator map of the world. One of its few practical uses is to demonstrate 
the foible of measuring with this map and the rhumb line (loxodrome) and its 
relation to the prefered measurement of a great circle or geodesic. It is 
even rediculous to call it a map of the world when it can't even show the 
poles due to the ultimate distortion of a singularity.

Lastly, I can put a dime on an equal-area map and the area that the dime 
covers---in terms of real acreage on the ground---is the same anywhere on the 
map, If I am looking at comparative areal extents of oil fields I have a 
reliable method of comparison with the equal-area map. As for actual 
measurements I can use a planimeter on the map and get a direct error-free 

If I were making a map of the areal extent of oil reserves in the Gulf of 
Mexico I would definitely use an equal area map. This is the *only* class of 
projection that properly displays the intent of the map and I can drop a dime 
anywhere on the map and know that it covers the same number of square meters.

Lastly, I cannot think of a practical use of a conformal map of the Gulf 
because all practical problems of distance/azimuth determination are micro 
computer (or possibly shirt pocket calculator) functions with accuracy far 
exceeding any scale layed on a map---and I do not mean using a cartographic 
projection as an intermediate step unless it is to get data into the proper 

In fact, lets abolish conformal mapping altogether after we make sure all 
coordinates of interest are stored as either geographic or geocentric x-y-z.
No more UTM, bastardized UTM, state/national plane coordinate system with wild 
a wooly projections. Ahhh! Utopia! Unfortunately, when pigs fly.

> Regards,
> -- daan Strebe

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist

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