[Proj] lat/long to northing/easting and back again

[Gerald I. Evenden]

On Monday 16 March 2009 8:37:30 am Roger Oberholtzer wrote:
> On Mon, 2009-03-16 at 13:38 +0200, support.mn at elisanet.fi wrote:
> > Hello,
> >
> > how about using pj_inv() in the proj.4 library?
>
> OK. But does that do an inverse of all projections? I seem to recall a
> discussion a few months ago about this. IIRC, not all inverse
> calculations has the same accuracy. In fact, I have some flexibility in
> which forward calculation is done: it will only be used internally.
> Externally, all will be WGS84 lat/longs.

I tremble at the thought of getting into this thread but here goes:

First, interpolating positions from easting/northing values is shaky and 
accuracy of such efforts is highly dependent upon the distance between points 
used in the operation.

RECOMENDATION/REQUIREMENT: Doing this operation requires someone who is 
familiar with projections (or at least the one selected) and their proper use 
and limitations.

Secondly, before starting on this project check if the projection selected has 
an inverse.  Fairly easy to do: 1) do a forward and 2) see if the inverse of 
the just computed forward x/y executes and returns the original lat/lon.

Not too complicated.

> > regards: Janne.
> >
> > -------------------------------------------------
> >
> > Roger Oberholtzer [roger at opq.se] kirjoitti:
> > > Gee, no opinions/suggestions? Especially on the best way to convert
> > > northings/eastings back into wgs84?
> > >
> > > On Tue, 2009-03-03 at 07:56 +0100, Roger Oberholtzer wrote:
> > > > I have always used proj to convert WGS84 lat/long values to
> > > > northing/easting values in various parts of the world. In doing so, I
> > > > usually need to interpolate between points to find specific
> > > > locations. These interpolated locations have always been reported as
> > > > northing/easting values in meters. Job finished.
> > > >
> > > > I now have a use where, after determining the interpolated
> > > > northing/easting, I need to convert the location back into lat/long
> > > > in degrees. This I have never done with proj.
> > > >
> > > > My question is, if you had to locate a position between two lat/long
> > > > values, how would you best do this with proj? The source data in my
> > > > case is WGS84 values from a Trimble GPS. The 'interpolated' lat/longs
> > > > should also be reported in WGS84. In reality, the values will be
> > > > collected throughout Germany. But everything is requested to be
> > > > provided in WGS84.
> > > >
> > > > I use the C API for this. But any pointers are welcome. If I have
> > > > left out any important detail, just ask.
> > > >
> > > >
> > > > --
> > > > Roger Oberholtzer
> > > >
> > > > OPQ Systems / Ramböll RST
> > > >
> > > > Ramböll Sverige AB
> > > > Krukmakargatan 21
> > > > P.O. Box 17009
> > > > SE-104 62 Stockholm, Sweden
> > > >
> > > > Office: Int +46 8-615 60 20
> > > > Mobile: Int +46 70-815 1696
> > > >
> > > > _______________________________________________
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> > >
> > > --
> > > Roger Oberholtzer
> > >
> > > OPQ Systems / Ramböll RST
> > >
> > > Ramböll Sverige AB
> > > Krukmakargatan 21
> > > P.O. Box 17009
> > > SE-104 62 Stockholm, Sweden
> > >
> > > Office: Int +46 8-615 60 20
> > > Mobile: Int +46 70-815 1696
> > >
> > > _______________________________________________
> > > Proj mailing list
> > > Proj at lists.maptools.org
> > > http://lists.maptools.org/mailman/listinfo/proj
> >
> > _______________________________________________
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> > Proj at lists.maptools.org
> > http://lists.maptools.org/mailman/listinfo/proj



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