Impact of Climate Change and Land Use on the Southwestern United States
Population Growth of the Southwest United States, 1900-1990

A brief description of the geographic data

The maps for this project were created from US Geological Survey (USGS) 1:2,000,000-scale Digital Line Graph (DLG) boundary data. These DLG data originally were collected from The National Atlas of the United States of America, published in 1970. Extensive revisions have been made and no data source more than five years old was used in this update. A USGS AML program was used to convert the DLG to Arc/Info coverages. An USGS translator program was used to code the arcs using the Major/Minor DLG polygon codes. The state and county boundaries were retained and the rest of the boundaries were deleted. The six-state area was appended together to make the regional coverage. Area points were added and polygon attribute information entered for each county. The map projection is Albers Equal Area conic and the map units are in meters. This modern coverage was copied for each of the decennial years and is the source for all the historical coverages.

Starting with the modern data set, the historical boundary changes were cartographically interpreted from "Map Guide to the U.S. Censuses, 1790-1920" (Thorndale and Dollrahide, 1983). Boundaries were deleted and area points corrected. Some of the boundary changes that occurred after 1920 were not fully explained in the reference book. This data was collected from state historical societies, state land boards, county assessors and historical USGS published maps. The historical boundary positions are not based on legal land descriptions and may not be accurate on a large scale.

The county population data was retrieved from the US Census web site as plain ASCII text for the six states in the region. A Unix shell script program was used to compile tables of the entire region for all the decennial data sets. The population tables were joined to the Arc/Info polygon attribute table using the state and county FIPS code as an Info relate item.

The square miles for each county was calculated based on the area field in the polygon attribute table. The population density was calculated by dividing the total population by the square miles for each county. The square miles and population density data may not be accurate due to cartographic generalization of the boundaries and map scale.

The classification scheme and visualization graphics was compiled using ArcView GIS software. The classifications used are exponential and slightly modified from natural breaks. Postscript files from ArcView layouts were converted to GIF images for the web page.

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