Is it possible to save a local transform to GCS list?

RE: Is it possible to save a local transform to GCS list?

Alain Robert — Wed, 18 Mar 2020 12:29:11 GMT

The Affine tranformation is a generalized casse of the Helmert.

Affine is:

X = xA1 + yA2 + A0

Y= xB1 + yB2 + B0

While Helmert is

X = xA + yB + OffsetX

Y = -xB + yA + OffsetY

So if you set A1 = A, A2 = B, B1 = -B (sign change here) and B2 = A, A0 = OffsetX, B0 = OffsetY it should be equivalent.

I think both apply transformation in the same direction but I may be mistaken so you may have to tweak the parameters a bit.

RE: Is it possible to save a local transform to GCS list?

Martin Mendez — Wed, 18 Mar 2020 00:15:01 GMT

thank you, Alain,

how do the Helmert transform numbers translate to the Affine Parameters? can't find any material that explains this...

again thanks!

RE: Is it possible to save a local transform to GCS list?

Alain Robert — Tue, 17 Mar 2020 23:50:57 GMT

Martin,,

Yes, that's it.

RE: Is it possible to save a local transform to GCS list?

Martin Mendez — Tue, 17 Mar 2020 22:16:21 GMT

Hi Alain, do you mean

Lambert Conformal Conic with Affine processor? I don't see "Lambert Conformal Conic with Affine post-processing"

RE: Is it possible to save a local transform to GCS list?

Alain Robert — Tue, 17 Mar 2020 21:50:49 GMT

Martin,

Unfortunately it is not possible to do so. The local transform is an artefact added by Bentley Systems and stored in the DGN file, while GCS definitions are stored using the Open Source CSMAP format.

As an alternative you can define a GCS based on the projection method "Lambert Conformal Conic with Affine post-processing" that enables to also specify a transform, (Affine instead of Helmert) and since this is part of the Open Source CSMAP format then it could be stored in a user-defined GCS.

Note that in both case the GCS definition is not portable and exporting to other non-Bentley format will likely lose the GCS definition.