How does MapInfo handles muliple Projections in the same window?
Does anyone know how MapInfo handles multiple projections in the same map
window? I.e. if a WGS84 vector table is open into the same window as a
vector OSGB36 table or vice versa, is it possible to calculate the offset?
To give you a short answer, yes.
MapInfo can display layers in different projections (even different
projections based upon "datum"s, that is, different models of the Earth) in
a Map window. This is accomplised by the fact that:
Each mappable MapInfo table has a projection permanently associated with it.
This is in contrast to an ESRI shape file, where you have to specify the
projection every time you use it.
Each Map window has a projection associated with it. Although this is
generally the projection of the first table added to the map as a layer, you
can change it (using Map -> Options).
Whenever MapInfo draws a Map window, it checks each layer to see if it has
the same projection as the Map window. If the projection is different, it
calculates new coordinates for each object as it draws the layer. Because
of this, drawing maps with layers in different projections is slower than
drawing maps with all the layers in the same projection. But that's the
price you pay for it being possible in the first place.
There is one other consideration: The datum transformation the MapInfo uses
most of the time is simple and (relatively) fast, but fairly inaccurate. If
your layers are in projections based upon different datums, they might not
line up *perfectly*. You will have to decide if you can live with the
inaccuracy.
HTH
Spencer
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