Documentation for hydrologic soil group percent,
geographic (latitude/longitude) coordinates

This dataset contains the percent of each mapunit which is in each of the hydrologic soil group (HSG) classes, as reported in the State Soil Geographic (STATSGO) soils data compiled by the Natural Resources Conservation Service (NRCS) of the U.S. Department of Agriculture.

  1. Overview
  2. Projection and georeferencing information
  3. Description of dataset files


Hydrologic soil group classes are used in the computation of runoff by the SCS (now NRCS) Curve Number Method. Class placement is based on the minimum annual steady ponded infiltration rate for a bare ground surface.

The components of a given STATSGO mapunit are not necessarily all in the same HSG class. Since the location of a component within a map unit is not specified, it is only possible to determine the relative amounts (percent) of the different HSG classes over an entire mapunit. For any given point within the mapunit, these percent values may then be interpreted as the probabilities that the soil at that point is in each of the possible classes. The computation of the percent of each HSG class in each mapunit used the variables HYDGRP (HSG class) and COMPPCT (percent of the mapunit's area covered by the component) from the STATSGO Component table. Possible HYDGRP values include A, B, C, D, A/D, B/D, and C/D. The percentage of each HSG class within each mapunit was calculated by adding up the values of the COMPPCT values for all components in that class; mixed classes (A/D, B/D, and C/D) were converted to HSG D during processing. Since some mapunits contain components which are all water, the percent of water for each mapunit was added as a fifth class, HSG W. If no HYDGRP value was specified for a component and it was not all water, a default HSG class of D was used.

To increase compatability with different types of data analysis software, the dataset is available in several different data file formats. These include both Arc/Info polygon format and a gridded version at 30 arcsecond resolution; the latter is available in both Arc/Info grid format and as a three-dimensional array of 8-bit unsigned binary integers. The description of dataset files provides additional details on formats, the data files asociated with each format, and instructions for file retrieval.

For the gridded version of the dataset, any 30-arcsec grid cell which contains portions of two or more mapunits was assigned the HSG percent values for the mapunit which occupies the largest fraction of the cell. For the Arc/Info grid version, the HSG values may be accessed using the mapunit serial numbers associated with each STATSGO mapunit; the percent of each HSG class has been incorporated into the Value Attribute Table (VAT) entry for each mapunit.

This dataset is in geographic (latitude/longitude) coordinates. Note that the same data are also available in the Albers equal area projection which was used by the NRCS for compiling the STATSGO data and in the Lambert azimuthal equal area projection used by the USGS and the EROS Data Center for a number of other datasets covering the 48 states.

Last change: 2003 Aug 27, R. A. White /