v.proj more questions

[Gerald I. Evenden]

>Date: Wed, 22 Feb 1995 18:06:33 +0100
>From: "Admin. scarab J.M. ROQUES (5322)" <scarab at cisi.cnes.fr>
>To: grassu-list at max.cecer.army.mil
>Subject: v.proj  more questions
>
>Sue Huse , REGIS , UC Berkeley wrote:
 ...
>----- End Included Message -----
>
>Hello everyone,
>
>after reading these discussions on projection transformation to/from
>UTM from/to State Plane, I realised that I will have to face the same
>problems soon.  Here in France, we generally use a Lambert conical
>conform representation (I am not sure of my english translation), based
>on the 30 parallel (where the projection cone is tangent to the earth)
>and, as far as i know, using the 1880 Clarke ellipsoide.
>
>As I am not sure of the State Plane projections used in the US does
>someone can tell me
>
>a: if "lcc" in the Proj field of State Plane file means
>"lambert conical conform" (or whatever better translated)

        "Lambert Conformal Conic" is the most common US nomenclature

>b: if i may use the "ellps" field with something like clark1880
>or if i will have to put the right values for a and es along with
>my local origins.

I don't know what the GRASS flavor of PROJ.4 usage allows, but
either +ellps=clrk80 or the +a=6378249.145 and +rf=293.4663 may
be used.  Specification of the +a/+es for the US SPCS control is
an artifact of sources for the contol information--- +ellps=clrk66
could have been used.

>c: if there is a mean to "add" a homothetic correcting factor

I am unfamiliar with this factor.  Could you please explain.

>d: finally, if point a: is relevant, what is the exact meaning (or
>where will i find it) of the long_0, lat_0, lat_1, and lat_2 fields;
>(validity zone limits for the lat_0 and lat_2, center for long_0 and
>lat_1 ? or is lat_0 the base parallell ?)

Complete explanation for these terms is in the PROJ.4 documentation
available as PostScript files at charon.er.usgs.gov (128.128.40.24)
under directory pub/PROJ.4 (see README under this directory for
details).  In summary:

lat_1 and lat_2 are the standard parallels of the secant cone
intersection with the globe.  A tangent cone with scale factor k_0
at tangent latitude lat_1 may also be used.

lat_0 and lon_0 are the cartesian coordinate origins (easting=northin=0).
A false origin may be specified by using x_0 and y_0 (a common practice
for coordinate systems so as to eliminate negative numbers).

>more generally where can i find a good documentation about projections
>supported by Grass4.1 (last updates) ?

In addition to aforementioned PROJ.4 documentation, a good reference
is Snyder's "Map Projections---A Working Manual" available from the USGS.

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
voice: (508)563-6766          Postal: P.O. Box 1027
  fax: (508)457-2310                  N.Falmouth, MA 02556-1027