Dear Tejaswi:

Contouring is a "trend" analysis. While we accept interpolation using known
random points, we do not trust extrapolation, that is program calculating
values outside of domain with known values. You will notice that negative
values are generated in places where field values are missing or spacing
between field points with known values is too big. I have seen maps with
storage coefficients being either negative or more than 1.0; both values
clearly physically impossible. Same as getting negative values, you may
expect also "blown up" values to extremely high ranges.

You may solve the problem in several ways. One is to assign minimum and
maximum contour values to be plotted on the map. Of course, the minimum
values would be a positive number. The other method is to exclude the areas
that have no "positive value" points by cropping the map coverage. For
example, in Ground Water for Windows (GWW) package, you draw a closed area
(enclosing "positive values" points) and allow contours to be plotted only
within that area. In other words, you will not accept extrapolation. Still
another way is to experiment with several methods of solution (krieging,
polynomial, minimum curvature, etc.; see SURFER) and find the one that
produces better results. You may also modify "modeling" paramaters used for
gridding.

Hope this helps.

Jasminko Karanjac, PhD
Consultant Hydrogeologist & Professor (ret.)
karanjac@...
www.geocities.com/karanjac
+381-64-3110304