Hello Tejaswi

The problem is that the method that you are using doesn't fit for
chemical parameters, because those values could be from decimals to
thousands. I Strongly recommend you to use Krigging method, BUT using
the appropiete variogram. The best software could be SURFER or an
Arcview extention for krigging (you can find out in the internet).

Regards

Xavier COELLO-RUBIO
UNESCO-IHE
The Netherlands


--- In hydrologymodel@..., Maribeth Milner
<mmilner1@u...> wrote:
>
> 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
>