[Proj] a problem about polyconic inverse translation

Clifford J Mugnier cjmce at lsu.edu
Wed Nov 3 00:18:10 EST 2004


I believe the only country where it's still currently used in large scale
topographic mapping is India.  More than a couple degrees from the central
meridian is absolutely a misuse of the projection.  Rarely used as a
Quadrillage, it was originally intended to be used no more than a couple of
arc minutes of longitude on either side of a single plane table sheet's
central meridian.  Ferdinand Hassler invented it as a facile mechanism for
drafting a graticule in a tent with a candle for illumination for
topographic mapping by planetable and alidade in the early 1800s.

I developed it as a Quadrillage in the middle 1970s for the New Orleans
District, Corps of Engineers - but in recent years it has fallen into
disuse in favor of the simpler and friendlier Louisiana State Plane
Coordinate System.

In general, the Polyconic is a really lousy projection.



Clifford J. Mugnier
Chief of Geodesy and
Associate Director,
Department of Civil Engineering
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic]
Voice and Facsimile:  (225) 578-4474 [Research]

American Polyconic will not inverse project when longitudes are more
than 90 from the central meridian.

Polyconic was normally used for large scale mapping where the longitude
extent was typically less than a degree.  I do not believe it is currently
common use as it is neither conformal nor equal area.

On Nov 2, 2004, at 9:25 PM, ÕÅÁ¢ wrote:

   I am doing some work on polyconic inverse translation using proj4.
   and I find  a minor  problem. What I have done is:

$proj +proj=poly +lon_0=0 <<EOF
70e 3dN
7776455.16  580425.56

$invproj +proj=poly +lon_0=0 <<EOF
7776455.16  580425.56
70dE 3dN

All is ok,but when I do the followed:

$proj +proj=poly +lon_0=0 <<EOF
160e 3dN
17723619.31  1629185.73

$invproj +proj=poly +lon_0=0 <<EOF
17723619.31  1629185.73
*   *
I got nothing .

I also tried some other data , if the latitude is small and longitude is
large ,I can't get correct result  or got not noting.

I look at the source file PJ_poly.c, and find that the inverse translation
is a kind of iteraor.

Is it possible that there is a minor bug in the polyconic inverse
translation algorithm ?

      Zhang Lili
      Institute of Geography and Nature Resource Reseach,
      Chinese Academy of Sciences
    Building 917, Chaoyang District, Beijing, 100101, China
    E-Mail: zhangli at supermap.com
    Http://www.igsnrr.ac.cn, www.gischina.com
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