[Proj] proj-4 introduced errors!

support.mn at elisanet.fi support.mn at elisanet.fi
Fri Oct 19 10:59:30 EDT 2007

To: proj at lists.maptools.org

PJ_Transform_Plus() or similar....

Is there any plan to add a new routine to the library (PJ_Transform_Plus or similar), that would do the datum conversion more elegantly? Current version intoduces datum shits to transformations where usually such shits are not seen! Let's have an example:

Datum 1:


Datum 2:


Now, if I take some coordinates in datum 1, like (62.000000N and 32.000000W) and want to do a datum shit, all would  expect that the result in datum 2 will be exactly the same (62.000000N and 32.000000W)! SINCE IF THE DATUM IS THE SAME THERE CAN NOT BE ANY SHIFTS (The whole idea behind different datums)

Put if I use PJ_Transform I will get huge shits (about 12 minutes or so)??? Using any common datum shift program will keep the transormation clean! Frank Warmerdam already adressed this problem and his idea was to use zero shift grid.... but that does not solve the problem generally when using proj-4 as a routine library.

The idea behind the new transformer would be to avoid using any projection related data unless projections are even used. So the user datum and ellps definitions would win over the defaults... and the spehrical to elliptical conversion would not introduce those shifts. The library should also give a warning if it does not use the user definitions!

Compare for example the following program:


The MAIN PROBLEM might be that proj-4 introduces such shifts that other common programs and methods do not! and it should be fixed. Maybe datum shits and projections should be kept separate. Since that is how they are handled commonly. And now they are mixed....

Fixing this outside proj-4 would require to rewrite the whole scanner again or installing a prepresessor that would take the definitions before proj-4 starts to deal with them. Since this is meaningless when proj-4 allready has the scanners, it is more easy to fix there directly.

Best regards: Janne

P.S. I don't know who has done and what... I am just using the package.

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