[OSRS-PROJ] Datum Shifting

[Gerald I. Evenden]

The second set of disturbing concepts if the old "lets-build-a-program-to-
do-all-things."  Program *proj* is a program to do exactly what was
sumarized
below: transform coordinates to and from cartesian and geographic space.
Nothing more or less.

Secondly, it was designed in the Unix spirit to be usable as a filter
funtion
so that it could be cobbled together with other functions to perform larger,
more complex operations without having to stike C code.  That is, let the
shell program do the building.

To borrow from my previous email, the problem of transfering data from one
cartesian system to another is *very* simply performed by:

proj -I [source +proj parms] -o   |   proj [dest +proj parms] -i

The pipe, "|", is a wonderful thing, use it.

If datum transformation is also required, something like:

proj -I [source +proj parms] -o   |   datum_shift -b [parms]   |   proj
[dest +proj parms] -i

Where datum_shift is some program to perform appropriate datum transforms.
The whole operation looks like *nad2nad* which was programmed as an example
of a program which could handle multiple projections.  In hindsight, I
should have
distributed it in this manner.

Lastly, PLEASE, if you proceed with this +to/+from concept, change the name
of the transmogrified proj to something else.

*Proj* is not perfect and I am aware of a few improvements that could be
made.
But none of these improvement impact the current execution flavor which I
feel
has withstood the test of time.  The important thing is to keep things
simple and
isolate various aspects of the problem into well defined units that can be
individually work on.

__________________________________________________
Jerry Evenden and the Low Riders, Katie and  Daisy May
gevenden at capecod.net  http://www.capecod.net/~gevenden
----- Original Message -----
From: Morten Hulden <morten at ngb.se>
To: <osrs-proj at remotesensing.org>
Sent: Tuesday, July 04, 2000 12:04 PM
Subject: Re: [OSRS-PROJ] Datum Shifting


>
> Frank,
>
> On Tue, 4 Jul 2000, Frank Warmerdam wrote:
>
> > As I understand it the current ``proj'' command only allows conversion
between
> > a single projected coordinate system and geodetic (lat/long) coordinates
on
> > the same ellipsoid.  In particular, there is no way to convert directly
from
> > one projection to another since it is assume that all projection
definition
> > information on the commandline relates to one definition.
>
> ob_trans projection allows a second projection to be specified e.g. (a
> pole shift)
>
> proj +ellps=clrk66 proj=ob_trans +lon_0=20 +o_proj=tmerc o_lon_p=45
>
> but, you are correct, not a second ellipsoid
> and anyway I think your approach is cleaner:
>
> > proj +proj=utm +zone=11 +ellps=clrk80 -to +proj=utm +zone=11
+ellpse=GRS80
>
>
> Morten Hulden
>
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