[GRASSLIST:1873] Re: Importing USGS SDTS DLG maps from differingdatums? I'm confused. [very long]

Roger S. Miller rgrmill at rt66.com
Thu May 24 23:50:22 EDT 2001

"Eric G. Miller" wrote:

> Shortcoming actually.  Don't think GRASS actually can handle datum
> transformations yet (though there's no reason it shouldn't).  I believe
> Andreas added the ability to specify a datum, and some functions that
> will do datum transformations, but these haven't been tied into existing
> routines for inverse and forward projections.  I think do_proj.c
> (libes/proj/) will need to be hacked to do datum transformations when the
> input/output datums are different.  It simply doesn't look, and libproj
> does not do datum shifts.

Thanks for the insight, Eric.  I see you're right.  When pj_do_proj is
asked to convert lat-long to lat-long it simply returns an error.  It
doesn't check for the datum at all.

OK, I can accept that as a shortcoming in the code, but it's a pretty
big shortcoming when you consider what's going on in North America right

Although it isn't necessarily a bug in the code, it's definitely a bug
in the documentation.  The user is prompted for the datum when he
creates a location and g.region shows the datum as part of the location
definition.  The documentation for v.proj says that the v.proj converts
between mapsets in different locations so I think most GRASS users will
logically assume that v.proj converts between different datums.

At a minimum the documentation needs to be changed.  Probably the code
should be changed to detect when a user is trying to convert between
datums and return an appropriate statement.

I have to wonder how many people have already used GRASS for that
conversion and now have erroneous data as a result.  I'm pretty sure
that I've done it.

Roger Miller

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