The *Inverse Distance to a Power* gridding method is a weighted average interpolator, and can be either an exact or a smoothing interpolator.

With *Inverse Distance to a Power*, data are weighted during interpolation such that the influence of one point relative to another declines with distance from the grid node. Weighting is assigned to data through the use of a weighting power that controls how the weighting factors drop off as distance from a grid node increases. The greater the weighting power, the less effect points far from the grid node have during interpolation. As the power increases, the grid node value approaches the value of the nearest point. For a smaller power, the weights are more evenly distributed among the neighboring data points.

Normally, *Inverse Distance to a Power* behaves as an exact interpolator. When calculating a grid node, the weights assigned to the data points are fractions, and the sum of all the weights are equal to 1.0. When a particular observation is coincident with a grid node, the distance between that observation and the grid node is 0.0, and that observation is given a weight of 1.0, while all other observations are given weights of 0.0. Thus, the grid node is assigned the value of the coincident observation. The Smoothing parameter is a mechanism for buffering this behavior. When you assign a non-zero Smoothing parameter, no point is given an overwhelming weight so that no point is given a weighting factor equal to 1.0.

One of the characteristics of *Inverse Distance to a Power* is the generation of " bull's-eyes" surrounding the position of observations within the gridded area. You can assign a smoothing parameter during *Inverse Distance to a Power* to reduce the "bull's-eye" effect by smoothing the interpolated grid.

For more information, please see the Surfer Help or see the references below.

**References**

- Davis, John C. (1986),
*Statistics and Data Analysis in Geology*, John Wiley and Sons, New York. - Franke, R. (1982), Scattered Data Interpolation: Test of Some Methods,
*Mathematics of Computations*, v. 33, n. 157, p. 181-200.

*Updated November 7, 2018*

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