[Proj] Question about change in epsg file from named datum to defined form...

[Carl Godkin]

Hi,

I notice that Trac revision 1874 updated the "epsg" definitions file to use
+towgs84= instead of +datum=
as expressed briefly in this comment:

"epsg update from GDAL - add TMSO and change of named datums to defined form
(#18 <http://trac.osgeo.org/proj/ticket/18>)"

I was wondering where I could read about the rationale for the latter
change.  Ticket #18
seems to be about the addition of TMSO only as far as I can see.

This seems to make a difference for the NAD27 coordinate systems as far as I
understand
how PROJ.4 works.

As I understand it, the +datum=NAD27 form (for example) uses datum shift
files whereas
the replacement +towgs84=-10,158,187,0,0,0,0 form represents an average
shift, right?

Can someone either explain why this change was made or point me at a
discussion
perhaps in another list?

Thank you very much,

carl
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