Urban Dynamics: Source Materials

Patuxent River Watershed
Agricultural Land Use Change

Acreage statistics for agricultural land use were derived from the U.S. Census of Agriculture, the National Resources Inventory, and a variety of digital land use datasets derived from remotely sensed images.

U.S. Census of Population

The U.S. Census of Population provides counts of the national's population. Statistics are aggregated to the national, state and county level making this data useful in correlation to land use or other human activities. The U.S. Census commenced in 1790 and has generated population totals for every decade since that time. Population totals for years in between the decennial years of the census are also estimated and checked by the Census Bureau. For this study decennial population counts were used for 1850-1990. Population counts for 1991, 1992, 1993, and 1994 were also used. The estimated population counts for 1997 were also included to bring the representation of population change to the near present.

Table and graph of population data by county, 1850-1992

Methods for using population data

U.S. Census of Agriculture

The U.S. Census of Agriculture provides tabular data related to agricultural land use, production and operations. While the current Ag Census is conducted every 5 years (in years ending in 2 and 7), previous Ag Censuses have varied in date; from 1850 to 1950 agricultural census statistics were gathered as part of the decennial U.S. Census. Beginning in 1954, the Census of Agriculture has been conducted separately from the Census of Population. The Census of Agriculture has been conducted for 1954, 1959, 1964, 1969, 1974, 1978, 1982, 1987, 1992. The Ag Census for 1997 is still being compiled, therefore data for that time period was not included in this study.

Only one of the many categories represented by the Census of Agriculture was used in this study. "Total acres of land in farms," a category represented in every Ag Census since 1850, was used to represent agricultural land use temporally.

Table and graphs of ag census data, 1850-1992

Methods for using Ag Census data

National Resources Inventory (NRI)

The NRI is a statistical survey conducted every five years (in years ending in 2 and 7) by the USDA Natural Resources Conservation Service (NRCS). The collected information is compiled into a database and contains information on soils, water, agricultural lands, and a variety of related resources. It is said to be "the most comprehensive database ever assembled on the non-Federal lands of the United States." It documents data for 74% of the nation's land area.

Methods for using NRI data

Landsat Data

A Landsat Multispectral Scanner (MSS) image from 1972 was used to reconcile the GIRAS land use/land cover data set. Landsat Thematic Mapper (TM) scenes from 1982 and 1992 were used to reconcile CCAP and MRLC land cover data sets.

Digital Land Use and Land Cover Data (GIRAS)

The USGS GIRAS land use and land cover (LULC) data provided a digital source of LULC maps at 1:250,000 scale. GIRAS data from 1972 was used for this study.

Methods for using GIRAS data

Multi-Resolution Land Characteristic (MRLC) Data

MRLC data sets are derived from Landsat TM data and reconciled with intermediate scale spatial data. An unsupervised classification algorithm is used to classify areas according to their land use/land cover characteristics. The data is then checked for accuracy and revised to reflect one of fifteen different land cover classes such as urban, forested, agricultural or bare land.

For purposes of this study MRLC data developed from 1993 imagery was used to build a spatial representation of agricultural lands in the Patuxent River Watershed.

Methods for using MRLC data

Coastal Change Analysis Program (C-CAP) Data

The C-CAP program, conducted by National Oceanic and Atmospheric Administration (NOAA), is designed to monitor change in the coastal environments of the United States. The data sets are compiled from Landsat TM data and reconciled with aerial photography and field data.

C-CAP data for Maryland from 1984 and 1988/89 were used in the creation of ag data layers for this project.

Methods for using C-CAP data

Digital Raster Graphics (DRG)

The USGS-produced DRG data product is a useful raster data source because it is georeferenced and contains useful spatial information in the form of colors. Basic image processing software can be used to select and extract geographic areas according to color. Once extracted, the color can be vectorized to represent polygons of a land use/land cover.

For example, the green tint on DRGs (and the corresponding paper quadrangles) can be extracted to determine a representation of forested lands in an area. Agricultural land areas were selected by subtracting urban (pink and/or purple) areas, forested (green) areas, transportation networks and hydrographic features from the DRG.