[Proj] RE: Proj Digest, Vol 50, Issue 22

Wed Jul 23 21:12:13 PDT 2008

When considering "small" areas such as Tennessee or Kansas in comparison to "large" areas such as multiple continents or single continents, the topic of scale variation can be narrowed down to the conventional conformal projections.  For those two states in particular, the Lambert (Secant) Conformal Conic offers the best minimal scale compromise over any azimuthal or equivalent/equal-area/authalic/automecoic projection.  The Postel or Azimuthal Equidistant is only valid for true scale distances from the center of the projection- it's the same as what you see on a radar screen.  The others offer even less.

As you narrow down the size of the area, conformality becomes particularly attractive when considering distance conformity between "grid" and "graticule."  For areal computations, one need only consider the cenroid of the area and then compute the scale factor at that point and square the value to obtain "true" area.  Of course, as the area increases, the generalization deteriorates.

Your discussions (so far) with daan Strebe have been associated with "large" areas.  A whole lot of other considerations come into play when you trim down the size of your area of interest.  

This discussion seems to be covering the universe of projections and is becoming tedious.  Exactly what is your objective?  There's always a university course or two on map projections that might satisfy you if a book or two or three won't do.  Try reading a few of the late John P. Snyder's tomes.  John was a gift to the world of Cartography from the world of Chemical Engineering.

Clifford J. Mugnier, C.P., C.M.S.
LSU Student ASCE Chapter Faculty Advisor
and
Past National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing 
and 
Chief of Geodesy,
CENTER FOR GEOINFORMATICS 
Department of Civil Engineering 
Patrick F. Taylor Hall 3223A
LOUISIANA STATE UNIVERSITY 
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic] 
Voice and Facsimile:  (225) 578-4474 [Research] 
Honorary Life Member of the 
Louisiana Society of Professional Surveyors 
Member Emeritus of the ASPRS 
Member of the Americas Petroleum Survey Group
======================================================
http://www.asprs.org/resources/GRIDS/
======================================================

________________________________

From: proj-bounces at lists.maptools.org on behalf of Michael Ossipoff
Sent: Wed 23-Jul-08 18:19
To: proj at lists.maptools.org
Subject: [Proj] RE: Proj Digest, Vol 50, Issue 22

Sometimes a problem of an unaccustomed form shows someone that they didn't understand the fundamental principles as well as they thought they did. Of course I haven't made any claims about myself, and it's a good thing I didn't. I'm here as an interested member of the public. Now, this will probably really show my ignorance, but my un-verified intuitive impression now is that, when mapping, say, Tennessee or Kansas, in comparison to azimuthal equidistant, oblique width-adjusted Aitoff, even if it has less average scale variation, wouldn't have less maximum scale variation than azimuthal equidistant. Any comments on how the maximum scale variation of azimuthal equidistant, oblique cylindrical equidistant, and oblique width-adjusted Aitoff would be likely to compare when mapping Kansas or Tennessee?

Mike Ossipoff

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