Environmental Summaries

IPUMS NHGIS environmental summaries provide land cover and climate data summarized over counties and census tracts.

Overview

The NHGIS environmental summary data files provide land cover data from the National Land Cover Database (NLCD) summarized over counties and census tracts, and climate data from the PRISM Climate Group summarized over counties. This section describes the geospatial processing used to generate the county and census tract summaries.

Land Cover

Land cover describes the visible features that cover the Earth's surface, and land cover data are usually derived from satellite imagery or aerial photography. Each pixel on an image will be assigned to a land cover class (e.g., water, vegetation, bare rock, wetlands) through visual interpretation or, more commonly, automated classification algorithms. Researchers may then use the data to quantify the amount of land covered by a particular class(es) or to track changes in land cover over time.

NHGIS land cover summaries use data from the National Land Cover Database, which is produced by a consortium of federal agencies. The NLCD data have a 30-meter spatial resolution and were derived from a decision-tree classification of Landsat satellite imagery. NHGIS provides environmental summaries from the 2001, 2006, and 2011 versions of the NLCD. These three NLCD versions were created using a consistent methodology and are comparable over time.

The NLCD data contains sixteen land cover classes:

Class valueClass nameDescription
11 Open water Areas of open water, generally with less than 25% cover of vegetation or soil.
12 Perennial ice/snow Areas characterized by a perennial cover of ice and/or snow, generally greater than 25% of total cover.
21 Developed, open space Areas with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses. Impervious surfaces account for less than 20% of total cover. These areas most commonly include large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes.
22 Developed, low intensity Areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 20% to 49% of total cover. These areas most commonly include single-family housing units.
23 Developed, medium intensity Areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 50% to 79% of the total cover. These areas most commonly include single-family housing units.
24 Developed, high intensity Highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses and commercial/industrial. Impervious surfaces account for 80% to 100% of the total cover.
31 Barren land (rock/sand/clay) Areas of bedrock, desert pavement, scarps, talus, slides, volcanic material, glacial debris, sand dunes, strip mines, gravel pits and other accumulations of earthen material. Generally, vegetation accounts for less than 15% of total cover.
41 Deciduous forest Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species shed foliage simultaneously in response to seasonal change.
42 Evergreen forest Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species maintain their leaves all year. Canopy is never without green foliage.
43 Mixed forest Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. Neither deciduous nor evergreen species are greater than 75% of total tree cover.
52 Shrub/scrub Areas dominated by shrubs; less than 5 meters tall with shrub canopy typically greater than 20% of total vegetation. This class includes true shrubs, young trees in an early successional stage or trees stunted from environmental conditions.
71 Grassland/herbaceous Areas dominated by gramanoid or herbaceous vegetation, generally greater than 80% of total vegetation. These areas are not subject to intensive management such as tilling, but can be utilized for grazing.
81 Pasture/hay Areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20% of total vegetation.
82 Cultivated crops Areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20% of total vegetation. This class also includes all land being actively tilled.
90 Woody wetlands Areas where forest or shrubland vegetation accounts for greater than 20% of vegetative cover and the soil or substrate is periodically saturated with or covered with water.
95 Emergent herbaceous wetlands Areas where perennial herbaceous vegetation accounts for greater than 80% of vegetative cover and the soil or substrate is periodically saturated with or covered with water.

NHGIS used the following process to create the land cover summaries:

  1. Download the 2001, 2006, and 2011 land cover data as georeferenced raster files.
  2. Extract each land cover class into its own raster dataset.
  3. For each class-specific dataset, execute the ArcGIS 10.4.1 Zonal Statistics As Table tool with two input datasets: the class-specific raster file and an NHGIS boundary file representing all units of a census geographic level as polygons. For each polygon, the tool calculates how much of its area is covered by the land cover class.
  4. Compute the proportion of the polygon's area covered by the land cover class.
  5. Repeat steps 3 and 4 for each census geographic level of interest.

NHGIS has produced 2001, 2006 and 2011 land cover summaries for the contiguous United States for these areas:

  • 2015 counties
  • 2015 census tracts
  • 2010 census tracts
  • 2000 census tracts

We plan to add land cover summaries for Alaska and Hawaii and for more geographic levels in the future. NLCD provides no land cover data for Puerto Rico or other territories.

Climate

Climate data describe the weather conditions for a given area over a long time period. These data are often published as raster files where each pixel contains an estimate of temperature or precipitation for a given time period derived from data collected at nearby monitoring stations. Scientists use modeling techniques to convert station-level data to raster datasets.

NHGIS climate summaries uses data from the PRISM Climate Group at Oregon State University and the Northwest Alliance for Computational Science and Engineering. The PRISM data have a 4-kilometer spatial resolution and daily, monthly, annual, and 30-year normal temporal resolutions. Available climate variables include precipitation; minimum, maximum, and mean temperature; mean dew point; minimum and maximum vapor pressure deficit; and vapor pressure.

PRISM temporal resolutions and time periods summarized in NHGIS files

Temporal resolutionAvailabilityDescription
Monthly 1895-2014 For each pixel, PRISM reports the total amount of precipitation received during the month and the average maximum and minimum daily temperatures for the month. PRISM takes a pixel's maxiumum and minimum daily temperatures and averages them over the entire month.
Annually 1895-2014 For each pixel, PRISM reports the total amount of precipitation received during the year and the average maximum and minimum daily temperatures for the year. PRISM takes a pixel's maxiumum and minimum daily temperatures and averages them over the entire year.
30-year normals 1981-2010 This resolution describes the average monthly and annual climatic conditions over a 30-year time period. There are 12 time steps in the 30-year normal - one for each month (January - December).

The 30-year normals resolution may be unfamiliar to some users, so we want to break it down in more detail. Normals are baseline datasets describing the long-run average climatic conditions for an area. There are typically 13 times steps in a normals dataset: one for each month and one annual. For example, to create a January 30-year normal maximum temperature, PRISM computes the average of the 30 January monthly average maximum temperatures. To create a July 30-year normal precipitation, PRISM computes the average of the 30 July monthly total precipitation measurements. Finally, to compute an annual normal precipitation, PRISM computes the average of 30 yearly total precipitation measurements.

PRISM climate variables summarized in NHGIS files

VariableUnitsDescription
Precipitation Millimeters Total precipitation (rain + melted snow) received by a pixel in a month or year
Minimum temperature Degrees Celsius Average daily minimum temperature in a pixel over a month or year.
Maximum temperature Degrees Celsius Average daily maximum temperature in a pixel over month or year
Mean temperature Degrees Celsius Mean of the minimum and maximum temperature variables

Process NHGIS used to create the climate summaries

  1. Download monthly and 30-year normal PRISM climate data as georeferenced raster files.
  2. Resample the original PRISM raster files from 4-kilometer to 2-kilometer spatial resolution so that every county in the contiguous U.S. had at least one pixel fall inside its boundary.
  3. For each resampled PRISM raster file, execute the ArcGIS 10.4.1 Zonal Statistics As Table tool with two input datasets: the raster file and an NHGIS boundary file representing all units of a census geographic level as polygons. For each polygon, the tool calculates summary statistics (minimum, maximum, mean, standard deviation) from the pixels that fall inside the polygon.

Areas covered in NHGIS climate summaries

  • 2015 counties (excluding Alaska, Hawaii, and Puerto Rico)

We plan to add climate summaries for more geographic levels in the future. PRISM monthly, annual, and 30-year normals are generally restricted to the contiguous U.S.

Summary statistics provided for each climate variable for each county

  • MIN - this statistic is based on the county's pixel that has the smallest value for a climate variable. For example, the minimum precipitation value in December 2014 in Autauga County, Alabama, is 120.5 millimeters. One pixel in Autauga had that estimated amount of precipitation in December 2014.
  • MAX - this statistic is based on the county's pixel that has the largest value for a climate variable. For example, the maximum precipitation value in December 2014 in Autauga County, Alabama, is 162.1 millimeters. One pixel in Autauga had that estimated amount precipitation in December 2014.
  • MEAN - this statistic is the average of all pixels in a county for a climate variable. For example, the average precipitation value in December 2014 in Autauga County, Alabama, is 132.7 millimeters. The mean is computed by summing the precipitation value of all pixels in Autauga County for that month and dividing by the number of pixels in the county.
  • STD - this statistic is the standard deviation for a climate variable among all pixels in a county. For example, the standard deviation for precipitation in December 2014 in Autauga County, Alabama, is 9.2 millimeters.

Technical Details

The environmental summaries are provided through the links below as comma-separated values (CSV) files within Zip archives.

Each Zip file includes a "README" text file that describes the contents of the data files.

Environmental summaries can be downloaded in any of three layouts:

  • Time varies by column: Data for different times are placed in separate columns within one file. The rows correspond to geographic units, and the columns correspond to particular time - land cover combinations. E.g., one column reports deciduous forest area in 2001 and another column reports deciduous forest area in 2006.
  • Time varies by row: Data for different times are placed in separate rows within one file. Each row represents a geographic unit - time combination (e.g., Autauga County, Alabama - 2001 NLCD, or Autauga County, Alabama - 2006 NLCD). Each column corresponds to a environmental data summary.
  • Time varies by file: Data for different times are placed in different files. Within each file, the rows correspond to geographic units, and each column corresponds to an environmental summary at a single time.

Climate summaries are available only in the time varies by row layout.

The unique idenfier for each geographic unit is the GISJOIN, which matches the identifiers used in NHGIS data tables and boundary files. A county GISJOIN concatenates these codes:

ComponentNotes
"G" prefix This prevents applications from automatically reading the identifier as a number and, in effect, dropping important leading zeros
State NHGIS code 3 digits (FIPS + "0"). NHGIS adds a zero to state FIPS codes to differentiate current states from historical territories.
County NHGIS code 4 digits (FIPS + "0"). NHGIS adds a zero to county FIPS codes to differentiate current counties from historical counties.

A census tract GISJOIN concatenates these codes:

ComponentNotes
"G" prefix This prevents applications from automatically reading the identifier as a number and, in effect, dropping important leading zeros
State NHGIS code 3 digits (FIPS + "0"). NHGIS adds a zero to state FIPS codes to differentiate current states from historical territories.
County NHGIS code 4 digits (FIPS + "0"). NHGIS adds a zero to county FIPS codes to differentiate current counties from historical counties.
Census tract code 6 digits

Download

Land cover summaries:

Climate summaries (available only in time varies by row layout):

Citation and Use

Use of the NHGIS environmental data summaries is subject to the same conditions as for all NHGIS data. See NHGIS Citation and Use.

Supported By

Eunice Kennedy Shriver National Institute of Child Health and Human Development National Science Foundation University of Minnesota