Quick Summary
More detailed information on these topics and a summary of the 1997 results may be found at the NRCS National Resources Inventory
(NRI) site.
Information collected
The NRI tracks changes at state and national levels for a variety of soil, water, and other natural resources data on non-Federal lands. Included are:
- Broad land cover/use
- Cropland use by irrigated and nonirrigated acres
- Broad land cover/use by land capability class and subclass
- Prime farmland
- Erosion and erodibility
- Wetlands and deepwater habitats
Sample design
The NRI is a longitudinal panel survey comprised of samples located in all counties and parishes of the 50 states and Puerto Rico, the Virgin Islands, the District of Columbia, and parts of the Pacific Basin. The sample design is a two-stage stratified area sample. The first-stage sampling unit is an area of land called a segment. Sample points located within the segment are the second-stage sampling units. Detailed data are collected for the sample points, with some area data collected for the segment.
Data were collected for about 300,000 segments and about 800,000 sample points in the 1997 NRI. In each NRI, data are collected on the same or a subset of the same segments and sample points. Data from 1982, 1987, 1992, and 1997 are included in the 1997 database, allowing analysis of trends. In 2000, the NRI began a transition to an annual survey called the Continuous NRI. About 42,000 segments, nationwide, are included as the "core" sample and these are sampled annually. Additionally, about 30,000 segments are selected from the remaining 258,000 each year to form a supplemental sample. This supplemental panel rotates through the remaining segments.
Data collection
Photo-interpretation and remote sensing, with ancillary materials such as USDA field office records, information from local NRCS personnel, soil survey and wetland inventory maps and reports are used to collect data. The Soil Survey Database provides information on specific properties and characteristics of soil.
Area for the total surface, federal land, and large water bodies and large streams is determined using GIS databases, not estimated from segment or point data.
Computer-assisted survey information (CASI) collection methods, featuring direct entry into hand-held computers and a national database server at CSSM, were first developed for the 1997 NRI. Current photo-interpretation data collection features a desktop CASI.
Field studies are conducted to obtain data not easily obtainable using remote sensing techniques. Currently, a range survey in the western US is underway using a ruggedized handheld device for data collection.
Quality assurance procedures ensure that year-to-year differences reflect actual conditions.
Estimation
The estimation procedure combines information from several sources to produce a final data set composed of records containing information for the years 1982, 1987, 1992, 1997, 2000, and annually thereafter. The estimation weight is attached to each record. Data were collected at the segment level and at the point level in each of the NRI years. In estimation, the areas measured for small streams and waterbodies, roads, and urban land collected at the segment level are converted to point data. Each of these points is given an initial weight based on the area in the segment and the probability that the segment is included in the sample. Imputation is used for unobserved data elements for the created points. Initial weights for created points and for observed points are adjusted during the estimation process using ratioing and small area estimation. Control totals for surface area, federal land, and large water areas, derived from GIS databases, are maintained throughout the process. Finally, the weights are adjusted using iterative proportional scaling (raking) so that areas estimated for major broad cover/uses for historical years (before 2000) in the current survey closely match those earlier estimates.
Contact:
Jim Kienzler, Project Manager
216E Snedecor Hall, Ames, IA 50011
Phone: 515-294-4795 / Fax: 515-294-2456 /
e-mail: jkienzle@iastate.edu
