I'll admit to being a complete novice when it comes to map
projections/datums etc (Prof. Cliff - if you're out there, please don't
shout at me ;-) !).
I've always worked on the basic assumption that if I'm using a
longitudinal/latitudanal setup I'm working on a spherical surface, and if
I'm in a map projection (usually AGD or New Zealand Map Projection) that I'm
working on a flat surface.
I've written a few mapbasic programs (mapbasic defaults to Long/Lat) and
have re-set the coordinate system to that of my mapdata (i.e. a map
projection), assuming that by doing so trigonometry/vectors etc wouldn't be
so difficult (i.e. I could discount the entire "maths on top of a sphere"
scenario). However, although small, errors are introduced that I can't
explain. An example:
One of my programs takes a linear object (i.e a two-node line, not a
polyline) and uses vectors to extend it by a factor of 10. I can then open
the layer with the original (short) line and and the layer with the newly
created extended line. From a distance they appear to overlap exactly.
However if I zoom right in I'll find that they're actually parallel to one
another, but with a separation of anything between 0.5 and 5cm. Pretty
small, but its causing problems with the rest of the program. (I'm not just
extending lines for the hell of it!).
Have I reached a limit of precision in MapInfo? Or is my assumption about
using "flat maths" on map projected data incorrect?
Thanks for any help,
Michelle
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