Here's what I've found out:
MapInfo does not make use of the datum for calculating coordinate
positions in the lat/long "projection" system. However, it does
use this information when converting coordinates to other
projections that may need it.
MapInfo's lat/long coordinate system (coordsys) is indeed using a
spherical model of the earth and the equi-distance cylindrical
projection. The origin lat and long come from where the center of
the map window was at the last time that window was redrawn. Note
that panning doesn't cause a redraw, so you can set a view and
then drag it all over, and the map will not be reprojected until
something cause a full window redraw.
- Bill Thoen
Susan N Muleme wrote:
>
> 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
> --
> Susan Muleme ([EMAIL PROTECTED])
> Cartographic and Software Support Technician
> Avenza Software Inc.
>
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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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