User Tools

Site Tools


remote_sensing_methods:normalized_difference_vegetation_index

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision
Previous revision
remote_sensing_methods:normalized_difference_vegetation_index [2009/12/28 15:20]
jasonkarl
remote_sensing_methods:normalized_difference_vegetation_index [2013/01/31 13:32] (current)
jgillan
Line 1: Line 1:
-<​sup>​[[:​bug_reporting|Report a bug, broken link, or incorrect content]]</​sup>​+<​sup>​[[:​bug_reporting|Report a bug, broken link, or incorrect content]]</​sup>​[[http://​methods.landscapetoolbox.org|{{ field_methods:​methodsguide3.png?​220x120|}}]]
 ====== Normalized Difference Vegetation Index ====== ====== Normalized Difference Vegetation Index ======
-[[http://​www.rangelandmethods.org/​about/​get_involved|{{:​abstract_dne.gif|}}]]+[[http://​www.landscapetoolbox.org/​about/​get_involved|{{:​abstract_in_dev.gif|}}]]
  
 ===== Also known as ===== ===== Also known as =====
Line 7: Line 7:
  
 ===== Description ===== ===== Description =====
-The Normalized Difference Vegetation Index (NDVI) is an index of plant "​greenness"​ or photosynthetic activity, and is one of the most commonly used vegetation indices.+The Normalized Difference Vegetation Index (NDVI) is an index of plant "​greenness"​ or photosynthetic activity, and is one of the most commonly used vegetation indices. Vegetation indices are based on the observation that different surfaces reflect different types of light differently. Photosynthetically active vegetation, in particular, absorbs most of the red light that hits it while reflecting much of the near infrared light. Vegetation that is dead or stressed reflects more red light and less near infrared light. Likewise, non-vegetated surfaces have a much more even reflectance across the light spectrum.
 \\ \\ \\ \\
-{{:​remote_sensing_methods:​ndvi_eq.png?​width=250|}}+{{:​remote_sensing_methods:​veg_spectral_response.jpg?​width=450|}}\\ 
 +<​sub>​Data for this graph courtesy of the Idaho Chapter of The Nature Conservancy</​sub>​\\ 
 +**Reflectance of sunlight from four different land cover types in Hells Canyon, Idaho as measured by a field spectrometer.** 
 +\\ \\ 
 +By taking the ratio of red and near infrared bands from a remotely-sensed image, an index of vegetation "​greenness"​ can be defined. The //​Normalized Difference Vegetation Index (NDVI)// is probably the most common of these ratio indices for vegetation. NDVI is calculated on a per-pixel basis as the normalized difference between the red and near infrared bands from an image:\\ 
 +{{:​remote_sensing_methods:​ndvi_eq.png?​width=150|}}\\ 
 +where NIR is the near infrared band value for a cell and RED is the red band value for the cell. NDVI can be calculated for any image that has a red and a near infrared band. The biophysical interpretation of NDVI is the fraction of absorbed photosynthetically active radiation.  
 +\\ \\ 
 +Many factors affect NDVI values like plant photosynthetic activity, total plant cover, biomass, plant and soil moisture, and plant stress. Because of this, NDVI is correlated with many ecosystem attributes that are of interest to researchers and managers (e.g., net primary productivity,​ canopy cover, bare ground cover). Also, because it is a ratio of two bands, NDVI helps compensate for differences both in illumination within an image due to slope and aspect, and differences between images due things like time of day or season when the images were acquired. Thus, vegetation indices like NDVI make it possible to compare images over time to look for ecologically significant changes. Vegetation indices like NDVI, however, are not a panacea for rangeland assessment and monitoring. The limitations of NDVI are discussed below.
  
 ===== Similar Methods ===== ===== Similar Methods =====
Line 93: Line 101:
  
 ===== Limitations ===== ===== Limitations =====
-Sensitivity of NDVI to low-end of the vegetation ​spectrum.+The NDVI is correlated with a number of attributes that are of interest ​to rangeland ecologist and managers (e.g., percent cover of bare ground and vegetation, biomass). It is not, however, a direct measure of any of these things ​it is a measure ​of "​greenness"​ produced by the ratio of infrared and red light that is reflected from the surface. While the biophysical interpretation of NDVI is the fraction of absorbed photosynthetically active radiation ([[remote_sensing_methods:​fraction_of_photosynthetically_active_radiation|see fPAR wiki page]]) absorbed by the surface, there are a lot of factors that influence the strength of the relationship between NDVI and rangeland ecosystem attributes. These can include: atmospheric conditions, scale of the imagery, ​vegetation ​moisture, soil moisture, overall vegetative cover, differences in soil type, management, etc... It is important when using NDVI data in analyses that steps be taken to understand and, to the extent possible, control for factors that might be affecting NDVI values before interpretations of differences in NDVI between areas of within the same area over time can be made.
 \\ \\ \\ \\
 Light from the soil surface can influence the NDVI values by a large degree. This is of concern in rangeland applications because many semi-arid and arid environments tend to have higher cover of bare ground and exposed rock than other temperate or tropical habitats. Heute and Jackson (1988) found that the soil surface impact on NDVI values was greatest in areas with between 45% and 70% vegetative cover. This limitation was the reason for the development of the several different soil-adjusted vegetation indices (e.g., [[remote_sensing_methods:​soil-adjusted_vegetation_index|]],​ [[remote_sensing_methods:​modified_soil-adjusted_vegetation_index|]]),​ and these indices tend to be preferred for rangeland applications. Light from the soil surface can influence the NDVI values by a large degree. This is of concern in rangeland applications because many semi-arid and arid environments tend to have higher cover of bare ground and exposed rock than other temperate or tropical habitats. Heute and Jackson (1988) found that the soil surface impact on NDVI values was greatest in areas with between 45% and 70% vegetative cover. This limitation was the reason for the development of the several different soil-adjusted vegetation indices (e.g., [[remote_sensing_methods:​soil-adjusted_vegetation_index|]],​ [[remote_sensing_methods:​modified_soil-adjusted_vegetation_index|]]),​ and these indices tend to be preferred for rangeland applications.
 \\ \\ \\ \\
-Relationship between ​NDVI and site characteristics ​of interest (e.g., bare ground, shrub cover)+In addition to the influence of soil surface at the low-end of vegetation cover, ​NDVI also suffers from a loss of sensitivity to changes in amount of vegetation at the high-cover/​biomass end. This means that as the amount of green vegetation increases, the change in NDVI gets smaller ​and smaller. So at very high NDVI values, a small change in NDVI may actually represent a very large change in vegetation. This type of sensitivity change is problematic for analysis of areas with a high amount of photosynthetically active vegetationThis could be an issue in rangeland ecosystems if you were interested in assessing changes in riparian areasIn these situationsit may be advisable to use another vegetation index with better sensitivity to high-vegetation ​cover situations like the [[remote_sensing_methods:​enhanced_vegetation_index|]] or the Wide Dynamic Range Vegetation Index. 
  
 ===== Data Inputs ===== ===== Data Inputs =====
Line 122: Line 131:
  
 ===== Existing datasets ===== ===== Existing datasets =====
-NDVI is easy to calculate from a wide range of different image sources. In terms of already prepared NDVI data, MODIS data are processed into several different vegetation indices and made available on 16-day, monthly, and yearly intervals at different resolutions. MODIS NDVI data can be downloaded from NASA's [[https://wist.echo.nasa.gov/%7Ewist/​api/​imswelcome/​|Warehouse Inventory Search Tool (WIST)]] or from the [[http://​glcf.umiacs.umd.edu/​data/​ndvi/​|Global Land Cover Facility]].+NDVI is easy to calculate from a wide range of different image sources. In terms of already prepared NDVI data, MODIS data are processed into several different vegetation indices and made available on 16-day, monthly, and yearly intervals at different resolutions. MODIS NDVI data can be downloaded from NASA's [[https://lpdaac.usgs.gov/get_data|Land Processes Distributed Actice Archive Center]] or from the [[http://​glcf.umiacs.umd.edu/​data/​ndvi/​|Global Land Cover Facility]].
 \\ \\ \\ \\
 Prepared MODIS and AVHRR NDVI datasets are also used in online tools like [[http://​rangeview.arizona.edu/​|RangeView]]. While these NDVI datasets cannot be downloaded directly, they can be viewed and used for simple analyses via the website. Prepared MODIS and AVHRR NDVI datasets are also used in online tools like [[http://​rangeview.arizona.edu/​|RangeView]]. While these NDVI datasets cannot be downloaded directly, they can be viewed and used for simple analyses via the website.
Line 168: Line 177:
  
 ===== Discussion/​Comments ===== ===== Discussion/​Comments =====
-<​sub>​**You must have an account and be logged in to post or reply to the discussion topics below. [[http://abstracts.rangelandmethods.org/​doku.php/​Home?​do=login&​sectok=db3676cff5bcd873b609b4e582432d73|Click here]] to login or register for the site.**</​sub>​+<​sub>​**You must have an account and be logged in to post or reply to the discussion topics below. [[http://wiki.landscapetoolbox.org/​doku.php/​Home?​do=login&​sectok=db3676cff5bcd873b609b4e582432d73|Click here]] to login or register for the site.**</​sub>​
 ~~DISCUSSION| ~~ ~~DISCUSSION| ~~
  
  
  
remote_sensing_methods/normalized_difference_vegetation_index.1262038848.txt.gz · Last modified: 2012/02/22 13:28 (external edit)