Newbie GIS question

I know Autocad and its cartesian coordinate system. But I'm wondering
how GIS systems, like ESRI's ArcGIS, represent spatial data.

I'm sure ArcGIS's display is in Mercator projection, but is its base
model of the world a sphere (or ellipsoid)?

When you draw a line from point 'A' to point 'B' it looks like a
straight line on your display, but is it actually a curve on the face of
the sphere?

When you measure from point 'A' to point 'B' is it measuring the
circumfrence of an arc segment or is it measuring a straight line?

I'd like to do some mapping on CAD but in a big way, the whole world.
It only makes sense to draw the data on a sphere, instead of a flat
plane. Do any CAD programs do this?

Thanks for your help.

@newssvr25.news.prodigy.net:


Bruce,

One of the main differences between a gis package and a cad package is the
ability to work with data in (nearly) any coordinate system. Most large
scale (ie regional, national, global) GIS data is stored in geographic
coordinates (latitude and longitude) and then converted to whatever
projected coordinate system you desire upon rendering.

The choice of coordinate system is crucial for large scale mapping such as
you describe. There are lots of good references online describing your
options, like this one:
http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj_f.html .


Good luck,

Marc Pelletier