Application of the Equal-Area Spline Function to Legacy Soil Data.

Splines have often been used to approximate continuous depth functions of soil properties. Mathematical approaches have often fit polynomial curves through the mean value of a given soil property at pedogenic soil horizons, where the mean value is assumed to occur at the mid-point of the given horizon. The fit of such curves are highly sensitive to small variations in the soil property values at the soil horizons.

Recently, a new tool that utilizes the equal-area spline has become available. The equal-area spline fits quadratic curves piecewise through soil horizons down the soil profile; these quadratic curves are joined at the soil horizon boundaries. The equal-area spline is so-called because within a soil horizon, the area between the mean property value and the portion of the curve above the mean value, and the portion of the curve below the mean value, respectively, is equivalent, which preserves the mean soil property value across the entire soil horizon as a whole.

The splines are a rapid, effective way of estimating soil property values at user-specified standard depth increments, and do not require profiles to be sampled at fixed depths in order to do so.

We have applied the equal-area spline function to SSURGO and STATSGO2 map unit components, and present preliminary results of mapping selected soil properties across the contiguous United States at standard depths.