[Proj] NAD27 to WGS84 ?

[Clifford J Mugnier]

Octavio,

At that scale (1:1,000,000), it is NOT significant.  I'd say the choice of
ellipsoid is quite "iffy."  Forget about Datums; they are meaningless at
that scale.

C. Mugnier
LSU
------------------
The data I have is in the scale of 1:1000000. Do you think it will be
significant?

The proyect I'm working on will be a free/able to modify/improve GPS base
map of Mexico: roads, water bodies, state limits, main streets in bigger
cities, etc. The only vector data freely available is in the scale I
mentioned, but it is no too far off, actually it is more accurate/detailed
than the Garmin base map the unit comes with. The idea is for the users of
the map to improve it over time.

Thank you,

Octavio

On Wed, 22 Feb 2006, Clifford J Mugnier wrote:

>
>
>
>
> Octavio,
>
> Note that using those published 3-parameter values will yield no better
> than 12-meter accuracy.  What I earlier suggested with respect to the
> National Geodetic Survey has the potential accuracy improvement of more
> than ten times better - IF such a data file exists for their software.
>
> If accuracy to no better than 12 meters is sufficient for your needs,
then
> there's no need to contact NGS.
>
> C. Mugnier
>
> --------------------------------
>
> Thank you Eric and everybody that contributed to answer my questions. It
> is a good thing that a list like this one with such nice people
> posting exists.
>
> Octavio
>
> On Wed, 22 Feb 2006, Eric Miller wrote:
>
> > There are published "3-parameter" datum transformation values for NAD27
> (Mexico) at:
> >
> >
>
http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryNorthAmericaTable.html#NASL
> >
> > You can use the DX,DY,DZ values via the +towgs84 parameter for the
source
> projection.  Then, use the pj_transform() function to convert from
> LCC/NAD27 to GEO/WGS84.
> >
> > Something like:
> >
> > src = pj_init_plus("+proj=lcc <your parameters here>
> +towgs84=-12,130,190");
> > dst = pj_init_plus("+proj=latlong +datum=WGS84")
> >
> > There's a fair amount of error in those conversions, but they're okay
for
> small-scale mapping.
> >
> > >>> opo at hp.fciencias.unam.mx 2/22/2006 9:30 AM >>>
> >
> > Hello,
> >
> > I'm new to the list, and to proj, and to Geography. I'm working in some
> > maps for GPS receivers (Garming). One of the tools (graphical
> > interface) for map creation (visualization) assumes the data is in
> > lat/long datum WGS84.
> >
> > All the vector data available for Mexico is in lcc NAD27. The data is
> > available in shapefiles for which I implemented a C program that
> > successfully extracts the coordinates and translate them to lat/long
> > in the NAD27 datum using the proj library.
> >
> > The questions are: Is it possible to shift (?) the data to WGS84 using
> > proj?
> >
> > Which command/parameters should I use to do it?
> >
> > Any help will be really appreciated,
> >
> > Thanks,
> >
> > Octavio
> >
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