On 02/24/2014 09:16 PM, Marco Hugentobler wrote:
Hi Leyan
>I struggled quite a lot with the interface of a very famous
commercial GIS on this issue, I believe there is a real possibility to
do better here ! I think it is essential to allow the >user to change
the transformation later easily, maybe somewhere linked from the CRS
choice ? That's where I would go as a user.
He, he, I bet it is the same GIS :-)
That would be such a coincidence ;-)
Do you mean we should strip the towgs84 parameters from
tbl_srs.parameters or do you mean to deprecate/remove entries from
tbl_datum_transform?
I think it would be better to remove the towgs84 parameters for the
ellipsoids where there is no clear choice, and offer to set the datum
transformation, but I agree it is hard to go about it in a user-friendly
way.
In my case, the coefficients offered for Beijing54 ellipsoid come from
different sources and only cover a small area each. This ellipsoid is
only used in China and there is no official list of coefficients to
transform (CRS are kind of an important secret in China, which does not
really make sense ...).
>Also, do you see a need to allow to define a custom datum
transformation, or do you think it is enough that user can create a
custom CRS with different towgs84 parameters?
It is possible to add new entries to tbl_datum_transform to create
custom datum transformations. Yes, it would be more convenient to have
a gui for that, even if I don't know how often it will be used. You
might judge it better since you obviously have to deal often with CRS
issues.
I was thinking of improving the custom CRS entry further by splitting
the proj4 definition in a gui. In that case, datum transformation could
be a part of it, or near it. I agree this will not be used by most
users, but still, it would be better for the few end users who need it.
Anyway, I don't have a lot of time to code so my progress on these
things is very slow ...
Regards,
Marco
Regards,
Leyan
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