Re: MI-L: MB: Spherical surface mapinfo versus flat surface mapinfo

[Bob Young - www.bydesignwales.demon.co.uk]

Hi Michelle

I would say this problem is that MapInfo stores coordinates in terms of
a whole number times a step size as well reported on this list.

If you do not have an MBR on your projection then the step size is the
world divided into 2GB. If you cut the size of your MBR to 2000 KM each
step would only be 1 mm and the precision would go up. If you cut it to
only 200 km your precision would be 0.1mm etc etc

Another problem you would have is extrapolating a long distance from a
short distance will introduce additional errors. As any surveyor would
tell you, you would not expect to accurately set out objects a long way
from a theodolite especially when your reference station is close to the
theodolite.

I do not think this particular problem is to do with curvature because
as you say you are assuming its flat in your calcs.


Regards



Bob
By Design



In message <00f101c0c317$f8c99b00$[EMAIL PROTECTED]>, m.e.smith@fron
tiermapping.com.au writes
>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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-- 
Bob Young - www.bydesignwales.demon.co.uk



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