Hi pfinnerty,
Given a number of data points, when creating a contour chart, the contour region will be
the largest possible polygon formed by the points (the polygon may only use a subset of the
points) so that the polygon encloses all the points.
For example, if the points form a grid, the polygon will be the rectangle created by joining
the outer most points. If the points are points along the perimeter of a semicircle together
with some points inside the semicircle, the polygon will be the semicircle.
So for your case, it is normal that there are lines at the bottom, as this is the largest
possible polygon created by joining the points and enclosing all the points. It seems the
contours at the bottom accurately reflect your data.
If due to the nature of your application, you do not want any contour in the bottom circle,
one method is to draw a white circle at the bottom above the contour, just to hide the
contour. There are many methods to draw the white circle. For example, you can use a
scatter layer to create a single symbol that covers up the bottom circle. You can also use
another contour layer with points along the bottom circle, and use the white color for that
contour layer.
Hope this can help.
Regards
Peter Kwan |
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(pfinnerty) 
Odd Contouring Observed