[OSRS-PROJ] Datum Shifting

[Gerald I. Evenden]

More detailed comments imbedded below.

__________________________________________________
Jerry Evenden and the Low Riders, Katie and  Daisy May
gevenden at capecod.net  http://www.capecod.net/~gevenden
----- Original Message -----
From: Frank Warmerdam <warmerda at home.com>
To: <osrs-proj at remotesensing.org>
Sent: Tuesday, July 04, 2000 10:33 AM
Subject: [OSRS-PROJ] Datum Shifting


> Folks,
>
> I am finally getting around to incorporating datum shifting support into
> PROJ.4, and I would be interested in some feedback on approaches.

Got plenty.

A hypercritical note here.  We are talking about the PROJ.4 library system.

> Currently the PROJ.4 access can be summarized in via the API:
>
>        PJ *pj_init(int argc, char **argv)
>
>        projUV pj_fwd(projUV val, PJ *proj)
>
>        projUV pj_inv(projUV val, PJ *proj)
>
>        void pj_free(PJ *proj)
>
> Note that it is only support translation back and forth between a
projection
> and geodetic (lat/long) coordinates.  There is no support for elevations,
and
> or for translating between different ellipsoids, or datums.

Correct!  PROJ.x system is purely designed for cartographic projection
operations.
Same as NMD's GCTP.

> I propose the following changes to the coordinate system definitions:
>
>  o Add a +towgs84=a,b,c[,d,e,f,g] option for providing the 3 or 7
parameter
>    relationship between this coordinate systems datum, and WGS84.
>
>  o Adding a +datum=<datumname> option which will basically lookup the
datum name
>    in an internal table, substituting the +towgs84= and the ellipse
definition
>    that is contained in the table.
>
>  o Adding a +ld[id] command option for listing the datum table with
definitions
>    in a manner similar to the handling of ellipsoids and projections.

Other than supplying the ellipsoid parameters, datums have nothing to do
with
cartographic projections.  I suggest adding the above to PROJ.4 is counter
to
the purpose of the package.

>  o Adding +proj=latlong as a legal "projection" meaning that the
coordinates
>    are in geodetic coordinates rather than a projection system.

This one makes no sense to me.  Dummy argument?

> We also need a way of driving coordinate system to coordinate system
conversion
> from the command line.  I torn between extending the ``proj'' command with
> some methodology of defining a second coordinate system, or implement a
new
> command which operates coordinate system to coordinate system.  Perhaps a
> proj2proj command or something like that.  I am open to suggestions on
this.

You already have it.  It is called a "pipe."  See other emails.

> I propose to implement several new API entry points, while leaving the
> current API essentially unchanged.  The only changes to the current API
> would be that pj_init would understand the new options (+towgs84, +datum)
> and would store the additional information in the PJ structure.

Strongly disagree as the suggested data has nothing to so with the process
of cartographic projection.

> The new API entry point would likely be:

    ...  deleted as nothing to do with cartographic projection

>    function call overhead.
>
> Unrelated to the datum shifting changes, I also propose to add a variation
on
> pj_init() which takes a single string argument with the coordinate system
> arguments, and will take care of parsing it into internal tokens in the
usual
> way.  Perhaps this could be pj_init_1().

Not sure what you're talking about.

You have two basic problems: 1) cartographic projections and 2) datum
shifts.
All the factors involved are very nicely separated and can be similarly
treated
with separate pieces of software.  Problem 1 seems to be niecely solved and
problem 2 seems easily solvable in a similar manner.  Why throw everything
into one stew pot?  It just makes the stirring more difficult.

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