User's guide of Proceessing Modflow 5.0

Processing Modflow 197
< To assign a value to a digitized point
1. Click the Digitize button
2. Move the mouse cursor to a point.
3. Press the right mouse button once.
The Digitizer shows a dialog box.
4. In the dialog box, type a new value then click OK.
5.2 The Field Interpolator
Numerical groundwater models require areally distributed parameters (e.g. hydraulic
conductivity, hydraulic heads, elevations of geological layers etc.) assigned to each cell or
element in the model domain. Usually, the modeler obtains a parameter distribution in the form
of scattered (irregular) data points (x , y , f ), i=1, ..., N. N is the number of measurement points,
i i i
x and y are the coordinates and f is the parameter value at point i. A basic problem is how to
i i i
estimate the parameter values for each model cell from these data.
A number of interpolation (or extrapolation) methods for solving this kind of problems have
been proposed. Some of the methods are used by commercial contouring software, e.g.
® EM
GEOKRIG, GRIDZO, SURFER or TECKON . A common approach done by many modeler
is that contour maps are firstly created by these software then overlaid on the model grid for
assigning parameter values to model cells. The process is indirect and somewhat cumbersome.
The Field Interpolator provides a more direct way for assigning cell values by using the
Kriging method and methods developed by Shepard (1968), Akima (1978a, 1978b) and Renka
(1984a, 1984b). The interpolation programs take measurement data and interpolate the data to
each model cell. The model grid can be irregularly spaced. Interpolation results are saved in the
ASCII Matrix format (see Appendix 2), which can be accepted by the Data Editor. Depending
on the interpolation method and the interpolation parameters the results may be slightly different.
With the Data Editor, you can create contour maps of the interpolation results and visually
choose a "best" result.
Theory is not emphasized in this description, because it is introduced in an extensive
literature. For example, Watson (1992) presents a guide to the analysis and display of spatial
data, including several interpolation methods. Franke (1982) provides a brief review and
classification of 32 algorithms. Hoschek and Lasser (1992) give a comprehensive discussion of
theories in geometrical data processing and extensive references in the area of data interpolation
5.2 The Field Interpolator