[OSRS-PROJ] Datum Shifting

thk at kms.dk thk at kms.dk
Tue Jul 4 12:19:44 EDT 2000

> proj +proj=latlong +ellps=clrk80 -to +proj=utm +zone=11 +ellps=clrk80

actually I think it would be a good thing to add a "-from" parameter as
this would make the command easier to read, as it resembles everyday
language more:

proj -from .... -to .....

additionally it becomes evident from the first step that we are using the
"new form".

and with the new +proj=latlong option even plain old style conversions
can be stated in a more readable format:

proj -from +proj=utm +zone=32 -to +proj=stere

perhaps it would also be a good idea to introduce a -files flag (or
like that: often a plain dash is used to mark "end of options") for the
new form: This way filenames and options are clearly separated, so
the "+" in the beginning of the options (which only serves to distinguish
them from file names) may be made optional, as in the pj_init() call.

If, furthermore,  -from and -to are made equivalent to +proj= the syntax
be reduced to:

proj    -from utm zone=32  ellps=grs80    -to utm zone=33    -    infile

(I assume here that  the ellps setting is inherited from "-from" to "-to"
not restated)

I think this form is quite readable, but I may have missed some gotchas...

>This approach ensures we just have one command, and a fairly graceful and
> backward compatible way of going from the current style to the two
> system form.  It would also ensure I don't have to duplicate all the
> argument handling currently embedded in the proj command.  On the
> it might be tricky to make the current proj command support Z optional Z
> coordinates without breaking some existing behaviour.

> Do people prefer an approach like the above instead of a new proj2proj

yes - if the syntax is made sufficiently easy to differentiate, while still
being sufficiently close to the original one to be easily understood.

Also, on a different note, I fully second your previous posting on API
I have really missed the pj_init_1() entry which you suggests. Handling
7 parameter) datum shifts within proj is also a really good thing.


Thomas Knudsen, Research Geophysicist, Ph.D. | http://www.gfy.ku.dk/~tk/
National Survey and Cadastre - Denmark       | thk at kms.dk   tk at gfy.ku.dk
Geodetic Department, Rentemestervej 8        | phone:    +45 35 87 52 64
DK-2400 Copenhagen NV, Denmark               | fax:      +45 35 87 50 52

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