[OSRS-PROJ] Units used in Expressing Projections

Craig Bruce csbruce at cubewerx.com
Mon Jul 29 13:51:53 EDT 2002

Frank Warmerdam <warmerdam at pobox.com> wrote:

> How would you express the projection in PROJ.4 syntax?  I am assuming
> the correct tangental form would be

The following segment of code indicates that either using 'lat_1' only
or both 'lat_1 == lat_2' will work the same way.  (Which is as things
should be.)

-----=----- PJ__lcc.c:68
    P->phi1 = pj_param(P->params, "rlat_1").f;
    if (pj_param(P->params, "tlat_2").i)
        P->phi2 = pj_param(P->params, "rlat_2").f;
    else {
        P->phi2 = P->phi1;
        if (!pj_param(P->params, "tlat_0").i)
            P->phi0 = P->phi1;
    if (fabs(P->phi1 + P->phi2) < EPS10) E_ERROR(-21);
    P->n = sinphi = sin(P->phi1);
    cosphi = cos(P->phi1);
    secant = fabs(P->phi1 - P->phi2) >= EPS10;

The secant 'boolean' is defined based on the lat_1 and lat_2 values being
~equal, and the rest of the code optimizes the calculation based on this.

> +proj=lcc +lat_0=55 +lon_0=0 +lat_1=55 +k=0.999877341
>     +x_0=600000 +y_0=1200000 +a=6378249.200 +b=6356515.000

I'm pretty sure that those 55's are in Gradians, so they should be
translated to degrees.

Which leads to another problem.  The geographic model (specifically
angular units & prime meridian) can be hidden from the outside world
when defining projections, but they can't be hidden when using +latlong.
You suggested adding a +pm parameter, but technically you also need
an angular-units parameter for geographic coordinate systems, to make
them work how EPSG defines them to.  But then, of course, you run into
problems of whether users are expecting to always use Degrees/Greenwich or
whether they are expecting to use Gradians/Paris for the France geographic
coordinate systems.  There are a few other geographic cases that don't
use Degrees/Greenwich.

Dr. Craig S. Bruce        | Tel: 819-771-8303 x205 |             CubeWerx Inc.
Senior Software Developer |   Home: 613-565-1344   |  Gatineau, Québec, Canada
csbruce at cubewerx.com      | http://www.csbruce.com |  http://www.cubewerx.com/
    "Everything should be built from the top down, except the first time."
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