Date: Wed, 22 Feb 1995 18:06:33 +0100
From: "Admin. scarab J.M. ROQUES (5322)" <scarab@cisi.cnes.fr>
To: grassu-list@max.cecer.army.mil
Subject: v.proj more questionsSue Huse , REGIS , UC Berkeley wrote:
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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 mea: 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@charon.er.usgs.gov
voice: (508)563-6766 Postal: P.O. Box 1027
fax: (508)457-2310 N.Falmouth, MA 02556-1027