Hi Leyan
>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.
Yes, it was the original behaviour to always ask the user, however it
was considered as not user friendly.
>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 ...).
Even in Switzerland, where it seems to be a clear choice (one CRS for
the whole country), the default towgs84 parameters of the national crs
are only accurate one digit behind the dot. So choosing the
transformation with an accuracy of three digits behind the dot in the
datum transform dialog makes a well visible difference.
Regards,
Marco
On 25.02.2014 12:56, Leyan wrote:
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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Dr. Marco Hugentobler
Sourcepole - Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
[email protected] http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee
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