SMEX03 Vegetation Data: Oklahoma

Summary

This data set is part of the Soil Moisture Experiment 2003 (SMEX03). The parameters for this data set include Leaf Area Index (LAI), Multispectral Radiometer Reflectance (MSR), plant height, plant density, areal wet biomass, areal dry biomass, and areal water content. The sampling was concentrated in the Little Washita watershed, but several locations from Oklahoma North (ON) and Oklahoma South (OS) were also sampled. The U.S. portion of SMEX03 was conducted during June and 1-18 July 2003. Data are provided in ASCII text files and are available via FTP.

These data were collected as part of a validation study for the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E). AMSR-E is a mission instrument launched aboard NASA's Aqua Satellite on 04 May 2002. AMSR-E validation studies linked to SMEX are designed to evaluate the accuracy of AMSR-E soil moisture data. Specific validation objectives include assessing and refining soil moisture algorithm performance; verifying soil moisture estimation accuracy; investigating the effects of vegetation, surface temperature, topography, and soil texture on soil moisture accuracy; and determining the regions that are useful for AMSR-E soil moisture measurements.

Citing These Data

Jackson, T. J. and L. McKee. 2004. SMEX03 vegetation data: Oklahoma. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Overview Table

Category	Description
Data format	Tab-delimited ASCII text
Spatial coverage and resolution	Southernmost Latitude: 34.8º N Northernmost Latitude: 35.9º N Westernmost Longitude: 98.3º W Easternmost Longitude: 97.6º W
Temporal coverage and resolution	1 July 2003 to 18 July 2003
Files	SMEX03_Raw_MSR_Overpass.txt, SMEX03_Raw_Vegetation.txt, SMEX03_Sum_MSR_Overpass.txt, SMEX03_Sum_Vegetation.txt
File size	5 KB to 52 KB
Parameter(s)	LAI, MSR, plant height, plant density, areal wet biomass, areal dry biomass, and areal water content
Procedures for obtaining data	Data are available via FTP.

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator(s)

Thomas J. Jackson
USDA ARS Hydrology and Remote Sensing Lab
Beltsville, Maryland
USA

Lynn McKee
USDA ARS Hydrology and Remote Sensing Lab
Beltsville, Maryland
USA

Technical Contact

Acknowledgements

Many graduate students and volunteers worked to collect the field data. We would like to thank the Soil Moisture Experiment 2003 Science Team and the Grazinglands Research Laboratory for their assistance.We would also like to thank NASA for their generous contributions to the study. This work was supported by the NASA Aqua AMSR, Terrestrial Hydrology, and Global Water Cycle Programs.

2. Detailed Data Description

Format

Data are in tab-delimited ASCII text files.

File Descriptions

The following files are contained in this data set.

File Name	Description	File Size
SMEX03_Raw_MSR_Overpass.txt	Raw data file of MSR and LAI.	52 KB
SMEX03_Raw_Vegetation.txt	Raw data file containing all data.	30 KB
SMEX03_Sum_MSR_Overpass.txt	Summary data file of MSR and LAI data only.	5 KB
SMEX03_Sum_Vegetation.txt	Summary data file containing all data.	10 KB

 

Table 1: Column Headings and Definitions SMEX03_Raw_MSR_Overpass.txt

Column Heading	Description
FIELD	Site location identification number, OS is the Oklahoma South region, ON is the Oklahoma North region, LW is Little Washita Watershed, SOY1 is a soybean field located in the Little Washita Watershed
CROP	WW_Stubble is winter wheat that has been harvested.
DATE	Month/day/year
DOY	Day of year
TIME (CDT)	Time of sampling in Central Daylight Time (CDT)
SITE	Number of site within given field
Latitude WGS84	Decimal degrees
Longitude WGS84	Decimal degrees
UTM_Easting WGS84_Zone 14	WGS84, Zone 14, in meters
UTM_Northing WGS84_Zone 14	WGS84, Zone 14, in meters
LAI	Leaf Area Index
CropScan Multispectral Radiometer Reflectance Percent
485 nm AVG	Average of % reflectance at wavelength: 485 nm
485 nm STD	Standard deviation of % reflectance at wavelength: 485 nm
560 nm AVG	Average of % reflectance at wavelength: 560 nm
560 nm STD	Standard deviation of % reflectance at wavelength: 560 nm
650 nm AVG	Average of % reflectance at wavelength: 650 nm
650 nm STD	Standard deviation of % reflectance at wavelength: 650 nm
660 nm AVG	Average of % reflectance at wavelength: 660 nm
660 nm STD	Standard deviation of % reflectance at wavelength: 660 nm
830 nm AVG	Average of % reflectance at wavelength: 830 nm
830 nm STD	Standard deviation of % reflectance at wavelength: 830 nm
850 nm AVG	Average of % reflectance at wavelength: 850 nm
850 nm STD	Standard deviation of % reflectance at wavelength: 850 nm
1240 nm AVG	Average of % reflectance at wavelength: 1240 nm
1240 nm STD	Standard deviation of % reflectance at wavelength: 1240 nm
1640 nm AVG	Average of % reflectance at wavelength: 1640 nm
1640 nm STD	Standard deviation of % reflectance at wavelength: 1640 nm
1650 nm AVG	Average of % reflectance at wavelength: 1650 nm
1650 nm STD	Standard deviation of % reflectance at wavelength: 1650 nm

 

Table 2: Column Headings and Definitions for SMEX03_Raw_Vegetation.txt

Column Heading	Description
FIELD	Site location identification number, OS is the Oklahoma South region, ON is the Oklahoma North region, LW is Little Washita Watershed, SOY1 is a soybean field located in the Little Washita Watershed
CROP	WW_Stubble is winter wheat that has been harvested. Plowed_WW is winter wheat that has been harvest and plowed.
DATE	Month/day/year
DOY	Day of year
TIME (CDT)	Time of sampling in Central Daylight Time (CDT)
SITE	Number of site within field
Latitude WGS84	Decimal degrees, WGS84
Longitude WGS84	Decimal degrees, WGS84
UTM_Easting WGS84_Zone 14	WGS84_Zone 14, in meters
UTM_Northing WGS84_Zone 14	WGS84, Zone 14, in meters
LAI	Leaf Area Index
CropScan Multispectral Radiometer Reflectance Percent
485 nm AVG	Average of % reflectance at wavelength: 485 nm
485 nm STD	Standard deviation of % reflectance at wavelength: 485 nm
560 nm AVG	Average of % reflectance at wavelength: 560 nm
560 nm STD	Standard deviation of % reflectance at wavelength: 560 nm
650 nm AVG	Average of % reflectance at wavelength: 650 nm
650 nm STD	Standard deviation of % reflectance at wavelength: 650 nm
660 nm AVG	Average of % reflectance at wavelength: 660 nm
660 nm STD	Standard deviation of % reflectance at wavelength: 660 nm
830 nm AVG	Average of % reflectance at wavelength: 830 nm
830 nm STD	Standard deviation of % reflectance at wavelength: 830 nm
850 nm AVG	Average of % reflectance at wavelength: 850 nm
850 nm STD	Standard deviation of % reflectance at wavelength: 850 nm
1240 nm AVG	Average of % reflectance at wavelength: 1240 nm
1240 nm STD	Standard deviation of % reflectance at wavelength: 1240 nm
1640 nm AVG	Average of % reflectance at wavelength: 1640 nm
1640 nm STD	Standard deviation of % reflectance at wavelength: 1640 nm
1650 nm AVG	Average of % reflectance at wavelength: 1650 nm
1650 nm STD	Standard deviation of % reflectance at wavelength: 1650 nm
Column Heading	Description
Plant Height	Plant height in cm
Plant Density	Number of plants/m2
Total Wet Wt	Total areal green biomass kg/m2
Leaves Wet Wt	Areal green biomass kg/m2of leaves only
Stalks Wet Wt	Areal green biomass kg/m2 of stalks only
Ears Wet Wt	Areal green biomass kg/m2 of ears only
Total Dry Wt	Total areal dry biomass kg/m2
Leaves Dry Wt	Areal dry biomass kg/m2 of leaves only
Stalks Dry Wt	Areal dry biomass kg/m2 of stalks only
Ears Dry Wt	Areal dry biomass kg/m2 of ears only
Total Water	Total areal water content kg/m2
Leaves Water	Areal water content kg/m2of leaves only
Stalks Water	Areal water content kg/m2 of stalks only
Ears Water	Areal water content kg/m2of ears only
Notes	Sampling notes

Table 3: Column Headings and Definitions for the file: SMEX03_Sum_MSR_Overpass.txt

Column Heading	Description
FIELD	Site location identification number, LW is Little Washita Watershed, SOY1 is a soybean field located in the Little Washita Watershed
CROP	WW_Stubble is winter wheat that has been harvested.
DATE	month/day/year
DOY	Day of year
TIME (CDT)	Time of sampling in Central Daylight Time (CDT)
Latitude WGS84	Decimal degrees, WGS84
Longitude WGS84	Decimal degrees, WGS84
UTM_Easting WGS84_Zone14	WGS84, Zone 14, in meters
UTM_Northing WGS84_Zone14	WGS84, Zone 14, in meters
LAI - AVG	Average of Leaf Area Index
LAI - STD	Standard deviation of Leaf Area Index
Cropscan Multispectral Radiometer Reflectance Percent
485 nm – AVG	Average of % reflectance at wavelength: 485 nm
485 nm – STD	Standard deviation of % reflectance at wavelength: 485 nm
560 nm - AVG	Average of % reflectance at wavelength: 560 nm
560 nm - STD	Standard deviation of % reflectance at wavelength: 560 nm
650 nm - AVG	Average of % reflectance at wavelength: 650 nm
650 nm - STD	Standard deviation of % reflectance at wavelength: 650 nm
660 nm - AVG	Average of % reflectance at wavelength: 660 nm
660 nm - STD	Standard deviation of % reflectance at wavelength: 660 nm
830 nm - AVG	Average of % reflectance at wavelength: 830 nm
830 nm - STD	Standard deviation of % reflectance at wavelength: 830 nm
850 nm - AVG	Average of % reflectance at wavelength: 850 nm
850 nm - STD	Standard deviation of % reflectance at wavelength: 850 nm
1240 nm - AVG	Average of % reflectance at wavelength: 1240 nm
1240 nm - STD	Standard deviation of % reflectance at wavelength: 1240 nm
1640 nm - AVG	Average of % reflectance at wavelength: 1640 nm
1640 nm - STD	Standard deviation of % reflectance at wavelength: 1640 nm
1650 nm - AVG	Average of % reflectance at wavelength: 1650 nm
1650 nm - STD	Standard deviation of % reflectance at wavelength: 1650 nm

 

Table 4: Column Headings and Definitions for SMEX03_Sum_Vegetation.txt

Column Heading	Description
FIELD	Site location identification number, OS is the Oklahoma South region, ON is the Oklahoma North region, LW is Little Washita Watershed, SOY1 is a soybean field located in the Little Washita Watershed
CROP	WW_Stubble is winter wheat that has been harvested. Plowed_WW is winter wheat that has been harvest and plowed.
DATE	Month/day/year
DOY	Day of year
TIME	Time of sampling in CDT
Latitude WGS84	Decimal degrees, WGS84
Longitude WGS84	Decimal degrees, WGS84
UTM_Easting WGS84_Zone 14	WGS84, Zone 14, in meters
UTM_Northing WGS84_Zone 14	WGS84, Zone 14, in meters
LAI – AVG	Average of Leaf Area Index
LAI – STD	Standard deviation of Leaf Area Index
Cropscan Multispectral Radiometer Reflectance Percent
485 nm – AVG	Average of % reflectance at wavelength: 485 nm
485 nm – STD	Standard deviation of % reflectance at wavelength: 485 nm
560 nm – AVG	Average of % reflectance at wavelength: 560 nm
560 nm – STD	Standard deviation of % reflectance at wavelength: 560 nm
650 nm – AVG	Average of % reflectance at wavelength: 650 nm
650 nm – STD	Standard deviation of % reflectance at wavelength: 650 nm
660 nm – AVG	Average of % reflectance at wavelength: 660 nm
660 nm – STD	Standard deviation of % reflectance at wavelength: 660 nm
830 nm – AVG	Average of % reflectance at wavelength: 830 nm
830 nm – STD	Standard deviation of % reflectance at wavelength: 830 nm
850 nm – AVG	Average of % reflectance at wavelength: 850 nm
850 nm – STD	Standard deviation of % reflectance at wavelength: 850 nm
1240 nm – AVG	Average of % reflectance at wavelength: 1240 nm
1240 nm – STD	Standard deviation of % reflectance at wavelength: 1240 nm
1640 nm – AVG	Average of % reflectance at wavelength: 1640 nm
1640 nm – STD	Standard deviation of % reflectance at wavelength: 1640 nm
1650 nm – AVG	Average of % reflectance at wavelength: 1650 nm
1650 nm – STD	Standard deviation of % reflectance at wavelength: 1650 nm
Column Heading	Description
Plant Height - AVG	Average of plant height in cm
Plant Height - STD	Standard deviation of plant height in cm
Plant Density - AVG	Average – Plants/m2
Plant Density - STD	Standard deviation – Plants/m2
Total Wet Wt – AVG	Average of total areal green biomass kg/m2
Total Wet Wt – STD	Standard deviation of total areal green biomass kg/m2
Leaves Wet Wt - AVG	Average – areal green biomass kg/m2 of leaves only
Leaves Wet Wt -STD	Standard deviation – areal green biomass kg/m2of leaves only
Stalks Wet Wt – AVG	Average – areal green biomass kg/m2 of stalks only
Stalks Wet Wt – STD	Standard deviation – areal dry biomass kg/m2 of stalks only
Ears Wet Wt – AVG	Average – areal green biomass kg/m2 of ears only
Ears Wet Wt – STD	Standard deviation – areal green biomass kg/m2 of ears only
Total Dry Wt - AVG	Average of total areal green biomass kg/m2
Total Dry Wt - STD	Standard deviation of total areal green biomass kg/m2
Leaves Dry Wt - AVG	Average – areal dry biomass kg/m2 of leaves only
Leaves Dry Wt – STD	Standard deviation – areal dry biomass kg/m2 of leaves only
Stalks Dry Wt – AVG	Average – areal dry biomass kg/m2 of stalks only
Stalks Dry Wt – STD	Standard deviation – areal dry biomass kg/m2 of stalks only
Ears Dry Wt – AVG	Average – areal dry biomass kg/m2 of ears only
Ears Dry Wt – STD	Standard deviation – areal dry biomass kg/m2 of ears only
Total Water - AVG	Average of total areal water content kg/m2
Total Water – STD	Standard deviation of total areal water content kg/m2
Leaves Water – AVG	Average – areal water content kg/m2 of leaves only
Leaves Water – STD	Standard deviation – areal water content kg/m2 of leaves only
Stalks Water – AVG	Average – areal water content kg/m2 of stalks only
Stalks Water – STD	Standard deviation – areal water content kg/m2 of stalks only
Ears Water - AVG	Average – areal water content kg/m2 of ears only
Ears Water - STD	Standard deviation – areal water content kg/m2 of ears only
Notes	Sampling notes

Spatial Coverage

Southernmost Latitude: 34.8º N
Northernmost Latitude: 35.9º N
Westernmost Longitude: 98.3º W
Easternmost Longitude: 97.6º W

Temporal Coverage

Data were collected daily from 1-18 July 2003

Temporal Resolution

Data were collected 1-3 times on multiple days at multiple sites throughout the project.

Parameter or Variable

Parameter Description

Parameters in this data set are Leaf Area Index (LAI), Multispectral Radiometer Reflectance (MSR), plant height, plant density, areal wet biomass, areal dry biomass, and areal water content. The following table describes the units of measurement and sources of each parameter.

Parameter	Unit of Measurement	Sensor
Leaf Area Index (LAI)	NA	LI-COR LAI-2000
Multispectral Radiometer Reflectance (MSR)	%	CropScan MSR-16R
Plant height	cm	manual data collection
Plant density	plants/m2	manual data collection
Areal wet biomass	kg/m2	manual data collection
Areal dry biomass	kg/m2	manual data collection
Areal water content	kg/m2	manual data collection

 

Sample Data Record

For each sample file of this data set shown below, the first four and last four columns are displayed. The value -99 is a fill value for missing data.

The following sample shows the first five rows from SMEX03_Raw_MSR_Overpass.txt.

					1640 nm		1650 nm
FIELD	CROP	DATE	DOY	...	AVG	STD	AVG	STD
LW03	Pasture	7/01/2003	182	...	32.16	2.93	29.22	3
LW32	Bare_Soil	7/02/2003	183	...	32.72	1.19	39.31	1.71
LW31	Corn	7/02/2003	183	...	25.9	2.74	10.02	0.92
Quarry	Gypsum_Quarry	7/02/2003	183	...	70.2	4.37	35.41	2.48
LAKE	Lake	7/02/2003	183	...	9.92	1.7	5.31	2.19

 

The following sample shows the first five rows from SMEX03_Raw_Vegetation.txt.

					Leaves	Stalks	Ears
		DATE			Water	Water	Water
FIELD	CROP	(m/dd/yyyy)	DOY	...	kg/m2	kg/m2	kg/m2	NOTES:
SOY1	Soybeans	7/03/2003	184	...	-99	-99	-99	Biomass samples taken 7/04/2003
SOY1	Soybeans	7/03/2003	184	...	-99	-99	-99	Biomass samples taken 7/04/2003
SOY1	Soybeans	7/03/2003	184	...	-99	-99	-99	Biomass samples taken 7/04/2003
LW33	Alfalfa	7/04/2003	185	...	-99	-99	-99
LW33	Alfalfa	7/04/2003	185	...	-99	-99	-99

 

The following sample shows the first five rows from SMEX03_Sum_MSR_Overpass.txt.

					1640 nm		1650 nm
FIELD	CROP	DATE	DOY	...	AVG	STD	AVG	STD
LW03	Pasture	7/01/2003	182	...	32.16	2.93	29.22	3
LW32	Bare_Soil	7/02/2003	183	...	32.72	1.19	39.31	1.71
LW31	Corn	7/02/2003	183	...	25.9	2.74	10.02	0.92
Quarry	Gypsum_Quarry	7/02/2003	183	...	70.2	4.37	35.41	2.48
LAKE	Lake	7/02/2003	183	...	9.92	1.7	5.31	2.19

 

The following sample shows the first five rows from SMEX03_Sum_Vegetation.txt.

					Stalks	Ears	Ears
					Water	Water	Water
		DATE			kg/m2	kg/m2	kg/m2
FIELD	CROP	(m/dd/yyyy)	DOY	...	STD	AVG	STD	NOTES:
SOY1	Soybeans	7/03/2003	184	...	-99	-99	-99	Biomass samples taken 7/04/2003
LW33	Alfalfa	7/04/2003	185	...	-99	-99	-99
LW11	Pasture	7/04/2003	185	...	-99	-99	-99
LW12	Pasture	7/05/2003	186	...	-99	-99	-99
LW13	Pasture	7/05/2003	186	...	-99	-99	-99

 

Error Sources

Leaf Area Index

Direct-beam radiation reflected into the sensor from upper leaves in the canopy can be confused with open sky, causing LAI to be underestimated. Samplers were instructed to sample with the sun to their backs, but occasionally direct sunlight may enter the sensor. The data were examined for this and for evidence of variable sky conditions during the measurement sequence.

Multispectral Radiometer

The radiometer performs near simultaneous inputs of incident as well as reflected irradiation. This allows useful measurements of percent reflectance to be obtained during cloudy conditions with incident irradiance levels down to approximately 300 watts/m². Measurements obtained with an incident irradiance level of less than 300 watts/m² had to be discarded. One day it was too cloudy to take any multispectral radiometer measurements.

3. Data Access and Tools

Data Access

Data are available via FTP.

Volume

The total volume is approximately 97 KB.

Related Data Collections

See related information on the NSIDC Soil Moisture Experiment (SMEX) Web site.

4. Data Acquisition and Processing

Theory of Measurements

The goal of vegetation sampling is to generate the vegetation products used to estimate surface soil moisture from passive microwave radiometers. Sampling was designed to coincide with satellite overpasses, such as LandSat Thematic Mapper (TM5) and Terra-MODIS, which can be used to estimate vegetation water content on the regional scale.

Vegetation Moisture

Samples were collected manually. In the laboratory they were weighed, dried at 60º C for 48 to 96 hours, and then weighed again.

Section Sampling

Sampling was performed on sites approximately a quarter section (0.8 km by 0.8 km) in size. The sampling was concentrated in the Little Washita watershed, but several locations from Oklahoma North (ON) and Oklahoma South (OS) were also sampled. Each Little Washita watershed site was sampled twice during the field campaign, Oklahoma North and South sites were only sampled once. Sampling consisted of recording vegetation height and plant density, collecting vegetation biomass samples, and taking reflectance and LAI measurements. Three locations in each of the sites were sampled, and every effort was made to have these three locations coincide in soil moisture sampling points. The sampling was conducted between 09:00 and 15:00 local time.

Transect Sampling

Several Little Washita watershed sites (representing the dominate types of vegetation) were characterized by transect sampling. Reflectance and LAI measurement were collected at each of the soil moisture sampling locations (14 total). A gypsum quarry and a lake were also sampled for calibration purposes. This was done three times, once to coincide with the each Landsat overpass.

Computing Areal Water Content

The following steps were used to compute areal water content.

Row Crops

1. Determine by manual collection the water content for a known number of plants
2. Convert to a per plant basis
3. Calculate the number of plants per square meter using the row and plant spacing
4. Multiply the water content per plant by the number of plants per square meter to get water content per square meter

Non-Row Crops

1. Determine by manual collection the water content for a 0.44 meter by 0.44 meter area
2. Divide the water content by 0.1936 to get water content per square meter

Sensor or Instrument Description

Leaf Area Index Meter

Investigators used LiCor LAI-2000 plant canopy analyzers to measure LAI using an indirect noncontact method based on light transmittance through the canopy. The LAI-2000 calculates LAI from radiation measurements made with a fish-eye optical sensor (148 degree field-of-view). Measurements made above and below the canopy are used to determine canopy light interception at five angles. Measurements are made by positioning the optical sensor and pressing a button, which sends the data to the data logger. Multiple below-canopy readings are taken so that LAI calculations are based on a large sample of the foliage canopy. After collecting above-canopy and below-canopy measurements, the control data logger performs all calculations and the results are available for immediate inspection. For more information, visit www.licor.com/env/Products/AreaMeters/index.jsp .

Multispectral Radiometer

Investigators used MSR-16R multispectral radiometers manufactured by CropScan to measure reflectance. The CropScan MSR is an inexpensive instrument that has up-and-down-looking detectors and the ability to measure sunlight at different wavelengths. The CropScan MSR systems consist of a radiometer, data logger controller (DLC) or A/D converter, terminal, telescoping support pole, connecting cables and operating software. The radiometer uses silicon or germanium photodiodes as light transducers. Matched sets of the transducers with filters to select wavelength bands are oriented in the radiometer housing to measure incident and reflected irradiation. In this experiment the wavelengths measured were 485, 560, 650, 660, 830, 850, 1240, 1640, and 1650 nm. These bands provide data for selected channels of the Landsat Thematic Mapper and MODIS instruments. Channels were chosen to provide Normalized Difference Vegetation Index (NDVI) as well as a variety of vegetation water content indices under consideration. For more information, visit www.cropscan.com/msr.html .

5. References and Related Publications

Please visit the SMEX03 Web site for more information, and see the NSIDC SMEX Web site to access data.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

A/D	Analog to Digital
AMSR-E	Advanced Microwave Scanning Radiometer – Earth Observing System
ASCII	American Standard Code for Information Interchange
AVG	Average
CDT	Central Daylight Time
DLC	Data Logger Controller
DOY	Day of Year
FTP	File Transfer Protocol
LAI	Leaf Area Index
LW	Little Washita Watershed
MSR	Multispectral Radiometer
NASA	National Aeronautics and Space Administration
NDVI	Normalized Difference Vegetation Index
NSIDC	National Snow and Ice Data Center
ON	Oklahoma North
OS	Oklahoma South
SMEX 2003	Soil Moisture Experiment 2003
STD	Standard Deviation
Terra MODIS	Moderate Resolution Imaging Spectroradiometer instrument on Terra satellite
TM5	LandSat Thematic Mapper
UTM	Universal Transverse Mercator
URL	Uniform Resource Locator
WGS84	World Geodetic System 1984

Document Creation Date

December 2006

Document URL

http://nsidc.org/data/docs/daac/nsidc0297_smex03_veg_ok.gd.html