University of Arizona

DroughtView


Understand the Data behind DroughtView


  BASE LAYERS

Greenness - NDVI     One of the satellite-derived measures of surface greenness we use for DroughtView is NDVI, or the Normalized Different Vegetation Index. These data come from the MODIS, or Moderate Resolution Imaging Spectroradiometer, instrument, which is aboard both the Terra and Aqua satellites. The MODIS instrument collects information about numerous atmospheric, land, and ocean phenomena through measurements over a wide range of the electromagnetic spectrum. MODIS measures surface greenness by comparing amounts of the visible-light and near-infrared parts of the electromagnetic spectrum reflected from the Earth's surface. Remotely sensed measures of surface greenness are linked to several characteristics of vegetation such as growth, density, and type. For example, NDVI values of broad forested areas typically are higher than those of grasslands. However, as a MODIS pixel, or area of measurement, is approximately 820 feet by 820 feet (250 meters by 250 meters), individual plant types or species at a specific location are not discernible through surface greenness values. Although the coverage of the Terra and Aqua satellites allows for a complete electromagnetic view of the globe every two days, NDVI data in DroughtView represent a biweekly composite. This data product, calculated over predefined spans of 16 days, is an attempt to minimize contaminating effects of things such as clouds on surface greenness measurements. Despite the compositing procedure, data quality issues can still arise due to factors like different scanning paths and viewing angles, as well as sensor degradation. Additional information is available online about remotely sensed measurements of vegetation and vegetation indices.

Greenness - EVI     Another satellite-derived measure of surface greenness we use for DroughtView is EVI, or the Enhanced Vegetation Index. These data also are MODIS based and presented as biweekly composites. The EVI differs slightly from NDVI in its calculation in order to better detect changes in surface greenness for areas with higher vegetative cover. Additional information is available online about remotely sensed measurements of vegetation and vegetation indices.

Data Quality     The Cloud and Snow mask, or Data Quality base layer, also is based on MODIS data. This product identifies where satellite views of the Earth's surface are obstructed by clouds and where snow-covered land and snow-covered ice on inland water are located. The Cloud and Snow mask additionally shows where good data, marginal data (often near cloudy or snow-covered areas), and no data (due, for example, to instrument error) exist. The presence of clouds and snow lowers NDVI and EVI values.

Precipitation     We use two different precipitation datasets for DroughtView. For biweekly composite periods prior to 2005, we calculate precipitation totals from daily PRISM, or Parameter-elevation Regression on Independent Slopes Model, data. For biweekly composite periods since 2005, we compute precipitation totals from daily AHPS, or Advanced Hydrologic Prediction Service, data. More information on how both of these daily gridded precipitation datasets are developed is in Arizona Cooperative Extension bulletin az1704.


  OVERLAYS

BOUNDARIES AND PLACES     Developed and provided by ESRI ArcGIS Online, this data layer shows global place names and country, state/province, and county boundaries. The display of these administrative and cities data is scale dependent, with greater detail appearing as one zooms in to a location. This data layer is turned on by default.

ROADS     Developed and provided by ESRI ArcGIS Online, this layer shows global transportation data including highways and roads. The display of these transportation data is scale dependent, with greater detail appearing as one zooms in to a location.

GRAZING ALLOTMENTS     Made available by the Arizona Game & Fish Department, this data layer includes Arizona State Land Department, Bureau of Land Management, and U.S. Forest Service allotments for the state of Arizona. These data are current through May 2011.

        legend (from HabiMapArizona)
        

FEDERAL/TRIBAL LANDS     Provided by ESRI ArcGIS Online, this data layer shows federal and tribal land areas of the United States symbolized by managing agency. The display of these management areas is scale dependent, with greater detail appearing as one zooms in to a location.

        legend (from ESRI ArcGIS Online)
        

CURRENT USDM     This data layer shows the current U.S. Drought Monitor, a weekly mapping of drought conditions based on climatic, hydrologic, and soil moisture information along with related impacts. The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the U.S. Department of Agriculture, and the National Oceanic and Atmospheric Administration.

        legend (from the U.S. Drought Monitor Classification Scheme)
        

CURRENT YEAR FIRES     Made available by GeoMAC, or the Geospatial Multi-Agency Coordination, this data layer shows perimeters of active and inactive fires of the current calendar year. Clicking on the boundary of a fire perimeter polygon will bring up a dialogue box with more information about fire year, name, and acres.

        legend (from GeoMAC)
        

HISTORICAL FIRES     Made available by GeoMAC, or the Geospatial Multi-Agency Coordination, this data layer shows perimeters of inactive fires from years prior to the current one since 2000. Clicking on the boundary of a fire perimeter polygon will bring up a dialogue box with more information about fire year, name, and acres.

        legend (from GeoMAC)
        

WATERSHEDS     Made available by the USGS through The National Map, this layer shows the Watershed Boundary Dataset that defines perimeters of drainage areas formed by local and regional terrain. The display of watershed boundaries is scale dependent, with greater detail appearing as one zooms in to a location. In this context, the Watershed Boundary Dataset has six levels of detail that include region, subregion, basin, subbasin, watershed, and subwatershed. Depending on the zoom level, individual watersheds are labeled by a 2-, 4-, 6-, 8-, 10-, or 12-digit HUC, or hydrologic unit code, respectively.