Tejaswi,

There are many algorithms for interpolating data. Some are "exact" (in
that they return the value of the input data) and some are "inexact".

The Surfer (http://www.goldensoftware.com/) documentation provides this
list of exact and inexact interpolators...

Exact:
Inverse Distance to a Power (no smoothing function)
Kriging (no specified nugget effect)
Nearest Neighbor
Radial Basis Function (no R2 specified)
Modified Shepard's Method (no smoothing factor)
Triangulation with Linear Interpolation (TIN)
Natural Neighbor

Inexact:
Inverse Distance to a Power (with smoothing factor)
Kriging (with error nugget)
Polynomial Regression
Radial Basis Function (with R2 specified)
Modified Shepard's Method

Note... the ability to return input values may or may not be related to
surface accuracy. To test accuracy, hold out (don't use) a few well
positioned data points and compare those values with their predicted
values. All interpolators have problems at the edges of data sets, so be
sure to collect extra data (outside of your study area) to avoid this problem.

Also, interpolators generally describe local variability or global
variability - not both. The exceptions are geostatistics and radial basis
functions.

Surfer has been a favorite interpolator package, but ESRI's Geostatistical
Analyst extension (in ArcGIS) is also very good. Besides having both
radial basis functions and geostatistical functions - it provides surface
error analysis tools that produce both graphic and numeric descriptors.

ESRI has a free online course that covers this extension
(http://campus.esri.com/campus/catalog/index.cfm?CFID=17792502&CFTOKEN=22787
414 "Introduction to ArcGIS 9 (or 8) Geostatistical Analyst") that will
get you started. Looks like this month's free online seminar (offered 3
times on Jan 26th) will also cover this extension (http://campus.esri.com/).

Good luck,

Maribeth

At 04:15 PM 1/10/2006 +0000, you wrote:

>Hello members,
> I am using ArcView 3.2a (demo version) to generate contour maps for
> different physico chemical parameters of groundwater samples. The
> contours are being generated by making use of the point interpolation
> method. While generating these maps from the database created, i am
> encountering this problem. The problem is, many of the contours are
> displaying negative values. There are no negative values in the database
> and moreover no field in the database is left blank. The minimum value
> for any of the parametes is zero. Eventhough for some of the parameters
> there are not even zero values, the negative contour values are being
> displayed.
> Any one knowing the solution for this problem is kindly requested
> to help me in this regard.
>
> Thanks in advance
>
> Tejaswi