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Product pages » AVHRR NDVI (BoM) v2

AVHRR NDVI (BoM) v2

Last modified by Ian Grant on 2012/03/06 12:41

AVHRR NDVI

Normalised Difference Vegetation Index (NDVI) derived from AVHRR
Vegetation greenness time series

Overview

NDVI is an index of the amount and greenness of live green vegetation.
The AVHRR sensor series operated by NOAA provides the longest running environmental remote sensing satellite series.
AVHRR NDVI gives a long time series overview of the dynamics of vegetation.

Spatial resolution0.01°, 0.05°
Spatial coverageAustralia, land only
Temporal resolution3 per month, monthly
Temporal coverageApril 1992 – present
CustodianBureau of Meteorology
Sensor & platformAVHRR on NOAA-11 to -18
Algorithm date2008
Production date2008, then near real time
Use limitationSee Specifications section
AlgorithmData up to June 2008 is stitched passes from CSIRO. Subsequent data is unstitched near real time passes from Bureau of Meteorology. CAPS navigation, cloud masking, regridding. Calibration drift detrended and normalised to NOAA-14 by invariant semi-arid IBRA regions. No atmospheric or angular corrections. Compositing by maximum value NDVI.

Datasets

AVHRR NDVI0.01°3 per Monthhttp://opendap.bom.gov.au:8080/thredds/catalog/avhrr_ndvi__1km_one_third_month/catalog.html
AVHRR NDVI0.01°Monthlyhttp://opendap.bom.gov.au:8080/thredds/catalog/avhrr_ndvi_1km_monthly/catalog.html
AVHRR NDVI0.05°Monthlyhttp://opendap.bom.gov.au:8080/thredds/catalog/avhrr_ndvi_5km_monthly/catalog.html

Future development

A new version of these datasets, processed with explicit corrections for atmospheric and angular effects, will begin processing in Q3 2012.

Background

Overview

The Normalised Difference Vegetation Index (NDVI) grids and maps are derived from satellite data. The data provides an overview of the status and dynamics of vegetation across Australia, providing a measure the amount of live green vegetation. The satellite data comes from the Advanced Very High Resolution Radiometer (AVHRR) instruments on board the National Oceanic and Atmospheric Administration (NOAA) series of satellites that are operated by the US (http://noaasis.noaa.gov/NOAASIS/ml/avhrr.html).

What is NDVI?

Live green vegetation absorbs visible light (solar radiation) as part of photosynthesis. At the same time plants scatter (reflect) solar energy in the near infrared. This difference in absorption is quite distinctive to live vegetation and provides a measure of the greenness of the vegetation.
NDVI is an index which measures this difference, providing a measure of vegetation density and condition. It is influenced by the fractional cover of the ground by vegetation, the vegetation density and the vegetation greenness. It indicates the photosynthetic capacity of the land surface cover.
NDVI is calculated from the red and near-infrared reflectances rRed and rNIR as NDVI = (rNIR - rRed) / (rNIR + rRed).

The value of NDVI is always between -1 and +1. Vegetation NDVI in Australia typically ranges from 0.1 up to 0.7, with higher values associated with greater density and greenness of the plant canopy. NDVI decreases as leaves come under water stress, become diseased or die. Bare soil and snow values are close to zero, while water bodies have negative values.

Three-per-month and monthly NDVI

Three-per-month and monthly NDVI are composites of the NDVI values from cloud-free observations in the time period from the operational afternoon NOAA satellite. There are, in the absence of cloud, usually one and sometimes two observations per day. Compositing selects the “best” single observation in the period by an objective criterion. The criterion in this case is the maximum NDVI value, which tends to reject values contaminated by undetected cloud.
The satellite data are processed initially onto a 0.01x0.01 degree grid. The 0.05x0.05 degree data are produced by averaging the 0.01x0.01 degree data after compositing. The data is available within a few days after the end of a month.

Examples

NCCwebndvi200903.gifNCCwebndvi200908.gifNCCwebndvi200912.gif

NDVI for three months of 2009, at reduced resolution, showing seasonal vegetation dynamics. The tropical north is green in February and drier later in the year. The south-east and south-west agricultural regions are greenest in September and dry out by December.

Algorithm

The NDVI anomalies are produced from data from the AVHRR instrument carried by the series of polar orbiting satellites operated by the US NOAA. The NOAA-11, -14, -16 and -18 satellites are used, which are all in afternoon orbits. The AVHRR images each location in Australia in daytime at least once daily, in five or six spectral bands. Bands 1 and 2 make the measurements of red and near-infrared reflected sunlight, respectively, from which NDVI is calculated, while other bands measure the Earth’s thermal emission.
The AVHRR data from April 1992 to June 2008 were generated by CSIRO (http://www.eoc.csiro.au/cats/). Data from July 2008 onward were received and processed by the Bureau of Meteorology.
For key processing steps, CSIRO and the Bureau both use the Common AVHRR Processing System (CAPS) software developed by CSIRO. CAPS Modular Processing applies geolocation, calibration, cloud masking, sea masking and regridding. No atmospheric or angular corrections are applied.
While the AVHRR instruments are calibrated before launch, their calibration changes in orbit, typically with a rapid change immediately after launch and then drifting during their mission lifetime of several years. Furthermore, the AVHRR carries no on-board calibration system for the reflective channels. Therefore, consistency of calibration over the length of the NDVI time series was achieved by adopting the procedure developed for use in Australia by the Environmental Resources Information Network (ERIN). This procedure adopts the calibration published by Rao and Chen of NOAA in 1999 using a stable Libyan desert site for the NOAA-14 segment of the time series, and then assumes the reflectance stability of a set of Australian arid sites to detrend the calibration of the other satellites and match them to NOAA-14.
The cloud mask applied by CAPS is a modification of the CLAVR-1 scheme developed by NOAA, and uses the reflective and thermal bands. The data are regridded using nearest neighbour interpolation to a 0.01° geographic grid spanning the Australian continent. NDVI is calculated from the red and near-infrared (band 1 and band 2) reflectances rNIR and rRed as NDVI = (rNIR - rRed) / (rNIR + rRed).
A single satellite orbit over Australia is typically received at more than one ground station, as two to four swaths with substantial overlap. These are merged into a single file: by CSIRO as data in a low-level format ("stitching") before CAPS processing; and by the Bureau of Meteorology as regridded data after CAPS processing. The data from January 2006 to June 2008 comes from the Alice Springs reception station only.
Orbits are composited into three periods in each calendar month - the 1st-10th, 11th-20th and 21st-end of the month - by the NDVI Maximum Value Composite (MVC) method. Besides greatly reducing data volumes, compositing improves spatial completeness and temporal consistency at individual locations. After rejecting pixels with solar zenith angle exceeding 80°, these three sub-monthly composites are further composited by maximum NDVI value to produce the one-month composites on a 0.01°×0.01° grid. These are averaged to a 0.05°×0.05° degree grid, excluding water pixels. 

Quality

Quality and limitations

NDVI is not an absolute measure of primary production. Also, due to the short period used for calculating the statistics, the NDVI standardised anomaly analysis may be less representative of normal conditions than analyses using rainfall, which use more than 100 years of records, compared to the 17-year NDVI record.
The use of the arid-site detrending technique to establish the long-term calibration consistency implies that while the temporal stability of these NDVI data is expected to be comparable to the best available to date for AVHRR NDVI time series covering Australia, the data cannot be relied upon to make absolute statements about long-term trends.
While the US NOAA satellite program aims for continuous quality coverage of the afternoon orbit, contingencies in the operation of the spacecraft and in the processing requirements have introduced periods of no coverage or reduced data quality. Specifically:

  • Data commences in April 1992, and is contiguous, except for a gap from October 1994 to January 1995 due to the lack of a sensor following the unexpected end of NOAA-11 operation.
  • Coverage and quality are reduced during the winters of 1993, 1994 and 2000 due to very low sun elevations (and consequent shadowing on the surface) during the last year or so of operation of some satellites.
  • Maps for September 2003, and possibly October, November and December 2003, display some artefacts due to a NOAA-16 sensor scan-motor problem.

Data completeness

Data from April 1992 are available, except for a gap from October 1994 to January 1995.
There are no data at locations which are sea or inland water bodies. Other no-data regions appear transiently due, for example, to persistent cloud cover or very low sun elevation.

Validation

No validation information is available.

Validation plans

The AVHRR surface reflectances from which the NDVI is calculated will be validated

  • Against higher quality surface reflectances derived from other satellites, in conjunction with the improved processing leading up to the dataset update.
  • Against top-of-canopy reflectances measured by AusCover validation activities.

Product variants

A section might be included to describe derived products and related products.
These two Bureau-only non-AusCover datasets are mentioned here only to illustrate this section of this model product user page..
The following product variants are available from the Bureau of Meteorology (www.bom.gov.au). They may be made available through AusCover depending on priority and demand.

Multiple-monthly NDVI

Unavailable from AusCover
The 3-month and 6-month grids are averages of the one-month grids. They are generated monthly and are available within a few days after the last month of the averaging period.

NDVI standardised anomaly

Unavailable from AusCover
The NDVI standardised anomaly is the departure of NDVI from the long-period average, normalised by the long-period variability. It indicates whether the vegetation greenness at a particular location is typical for a particular compositing or averaging period of the year, or whether the vegetation is more or less green.
Monthly anomalies are generated from the monthly NDVI dataset by subtracting the long-period mean and dividing by the long-period standard deviation for that month of the year, for each grid cell. The reference period is 1992 to 2008, excluding the poor quality months of April to September 1994 (due to low sun elevations, resulting in excessive shadowing on the ground) and September 2003 (due to satellite instrument scan anomalies).
For the 3-month and 6-month anomalies the NDVI values are averaged over the three or six months in question, and normalised by the mean and standard deviation calculated over all instances of those months over the reference period except for the poor quality months listed above.

NCCwebndvianom200903.gifNCCwebndvianom200908.gifNCCwebndvianom200912.gif
NDVI standardised anomaly for the three months of 2009 shown above

Specifications

CitationAVHRR Normalised Difference Vegetation Index 1-Month Australia 0.05degree GEO Grid v20100421
Data sourceCSIRO Marine and Atmospheric Research & Bureau of Meteorology
Geographical coverage110°E–155°E, 45°S–10°S
Coordinate systemGeographic; Datum: WGS84
Positional accuracy0.01°
AccuracyUnvalidated. See validation plans in the Quality section.
CompletenessApril 1992 to most recent month, except for a gap from October 1994 to January 1995.
 Spatial gaps permanently at large inland water bodies, and intermittently at other regions due to persistent cloud cover or other issues. 
Update planA new version of these datasets, with processing that includes explicit corrections for atmospheric and angular effects, will begin processing in Q3 2012.
Use limitationCopyright for any data supplied by the Bureau of Meteorology is held in the Commonwealth of Australia and the purchaser shall give acknowledgement of the source in reference to the data. Apart from dealings under the Copyright Act 1968, the purchaser shall not reproduce (electronically or otherwise), modify or supply (by sale or otherwise) these data without written permission from the supplier.
Use contraintsCopyright: Exclusive right to the publication, production, or sale of the rights to a literary, dramatic, musical, or artistic work, or to the use of a commercial print or label, granted by law for a specified period of time to an author, composer, artist, distributor.
ContactIan Grant
Bureau of Meteorology
GPO Box 1289, Melbourne VIC 3001, Australia
Telephone: (03) 9669 4080
Email: I.Grant@bom.gov.au

References

Documentation

NDVI_anomalies_BoM_for_BRS_procedure (Word, 44kb)

Other sources of this dataset

Bureau of Meteorology Climate Maps – Vegetation Index (NDVI)

Tutorials

NASA Earth Observatory – Article: Measuring Vegetation (NDVI & EVI)

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Created by Ian Grant on 2012/03/06 09:17

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