Hi Bill,
According to the USGS Map Projection manual; "the datum provides the
surface to which ground control measurements are referred. The latitude and
longitude of all control points in a given area are then computed relative
to the adopted ellipsoid and the adopted "initial point".
Does MapInfo not read the ellisoid to determine the lat/long of the control
points?
Susan
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Bill Thoen wrote:
> Here's a really fundamental question for project experts.
> MapInfo's Lat/Long "projection" is really an equi-distant
> cylindrical projection (from pg 419, pp 4 under "Projections
> vs.Coordinate Systems"). But from what I read in Snyder's book on
> projections used by the USGS, this projection requires 4
> parameters: radius of the earth, central longitude and latitude,
> and scale factor along the meridian.
>
> My question is that if this is true, what does it matter what
> datum you use? Note that the variable used in the projection is
> the radius, not an ellipsoid axis. Why is there any choice of
> Datum then? This appears to be a bit pointless unless MapInfo is
> doing something else beyond the basic projection.
>
> And where does MapInfo get its origin of lat and long from for
> this projection? It looks like it might be the map window center,
> but I need to know for sure.
>
> Any ideas?
>
> - Bill Thoen
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