My gratitude to all who suggested solutions, I am currently working through
them but cannot yet pass a final "verdict".
For 045 striking elliptical buffers the solution provided by Bill Huber
implemented in conjunction with Discover > Data Utilities > Transform
coordinates is a working very well.
Other solutions have not yet been fully tested/implemented.
a reminder of the suggestions.
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Could you create a convex hull around your points and buffer that? This
may
come near to what you want.
Regards
Mark Percival
ICT Client Officer(GIS)
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Geometric anisotropy is a standard way to model spatial correlations among
geologic variables where those correlations vary with orientation. It
works by applying a linear transformation to the coordinates, buffering the
points with circles, then inverting the transformation. In this case, with
the major axis of the ellipse in the NE-SW direction, suppose you want
buffers with major axis A and minor axis B. Let t = B/A (which is between
0 and 1) and apply the linear transformation
x |--> ((1+t)*x + (t-1)*y)/2
y |--> ((t-1)*x + (1+t)*y)/2
to every sample point. (This transformation shrinks the NE-SW direction by
B/A but leaves distances in the NW-SE direction unchanged.)
Buffer the image points with circles of radius B. Then apply the inverse
of the transformation to the buffers:
x |--> ((1+t)*x + (1-t)*y)/(2*t)
y |--> ((1-t)*x + (1+t)*y)/(2*t)
The inverse centers the buffers on the original points and stretches them
out into ellipses of the desired size and orientation.
In general, if the major axis of the ellipse should make a counterclockwise
angle u with the x-axis, compute c = cos(u), s = sin(u), and replace the
"(1+t)/2" terms above by c^2*t + s^2 and the "(t-1)/2" terms above by
s*c*(t-1).
--Bill Huber
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Unfortunately the MapInfo Create Ellipse
command cannot create an oriented ellipse - ie. the major and minor axes
must be EW and NS or vice versa.
If you really want ellipses you'll have to write some MapBasic code to
generate an ellipse by creating a region in the shape of an ellipse for
each
point, but this is probably a lot more work than you want.
Alternatively, if the ellipse minor axis dimension is important, you could
create 4 points for each sample site, the 4 points representing the 4 end
points of the ellipse axes. Then create a convex hull for the four points
for each sample site, and then buffer this convex hull to produce a rounded
figure.
Best Regards,
Martin Higham
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An uninvestigated possibility regarding Martin Higham's suggestion is the
creation of ellipses in a rotated coordinate system.
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Thanks again
I'll sum properly once I get a solution fully functional
Reagards Brendan O'Donovan
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