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LandcoverHelp.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta http-equiv="Content-Language" content="en-us">
<meta charset="UTF-8">
<title>USGS Global Visualization Viewer</title>
<link rel="stylesheet" href="../includes/glovis.css" type="text/css">
<style type="text/css">
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font-family: Arial,Verdana,Helvetica }
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<meta name="description" content="USGS Global Visualization">
<meta name="keywords" content="Global Visualization, EROS, Imagery,
Browse Images, Satellite, Landsat, Satellite Data, World">
<meta name="publisher" content="USGS - US Geological Survey, EROS">
<meta name="abstract"
content="The USGS Global Visualization Viewer is a quick and easy
online search and order tool for selected satellite and aerial data.">
</head>
<body style="font-family: Arial; font-size: 10px" bgcolor="#FFFFFF">
<font class="glovistitle2">NDVI FAQ</font>
<p>USGS Global Visualization Viewer: Frequently Asked Questions about the Normalized Difference Vegetation Index (NDVI) time-series curves
<h2>What is the source of the NDVI data?</h2>
<p>The NDVI data are derived from the Advanced Very High Resolution Radiometer (AVHRR) aboard the National Oceanic and Atmospheric Administration's series of daily polar orbiting weather satellites. Every day the AVHRR sensor collects imagery with 1 km spatial resolution covering the entire globe. See <a href="http://noaasis.noaa.gov/NOAASIS/ml/avhrr.html" target="_blank">http://noaasis.noaa.gov/NOAASIS/ml/avhrr.html</a> for further information.
<h2>Why are the NDVI curves not available outside of the coterminous United States?</h2>
<p>The AVHRR data used for these time-series curves are generated by USGS Greenness Mapping and Remote Sensing Phenology projects that focus monitoring efforts on the 48 contiguous United States. See <a href="http://phenology.cr.usgs.gov/index.php" target="_blank">http://phenology.cr.usgs.gov/index.php</a>.
<h2>How are NDVI curves generated?</h2>
<p>The daily AVHRR acquisitions within a two week period are compiled to generate a mostly cloud/haze-free composite image. The latest two-week composite is then added to the previous time-series collection of two-week composites. A "smoothing" algorithm is applied to the collection of two-week composites which "smoothes-out" the "noise", extraneous peaks and valleys, which make the curve harder to interpret. On a pixel-by-pixel basis, the smoothing algorithm evaluates the six previous two-week composites and can slightly change the value of any one of the previous 6 values. See: <a href="http://phenology.cr.usgs.gov/methods_data.php" target="_blank">http://phenology.cr.usgs.gov/methods_data.php</a>.
<h2>How are the land cover-specific NDVI curves derived?</h2>
<p>Each smoothed two-week NDVI composite image is intersected with the National Land Cover Dataset (NLCD), either the 1992 or the 2001 version.
<p><a href="http://eros.usgs.gov/#/Science/Landscape_Dynamics/Land_Cover-Land_Use/National_Land_Cover" target="_blank">http://eros.usgs.gov/#/Science/Landscape_Dynamics/Land_Cover-Land_Use/National_Land_Cover</a>
<p>Then for every Landsat Path/Row footprint, the average NDVI value is calculated for the total area of each land cover category found within the footprint.
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