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The MODIS/Terra Snow Cover 8-Day L3 Global 0.05Deg CMG (MOD10C2) data set contains snow cover and Quality Assessment (QA) data, latitudes and longitudes in compressed Hierarchical Data Format-Earth Observing System (HDF-EOS) format, and corresponding metadata. This data set consists of 7200 columns by 3600 rows of global arrays of snow cover in a 0.05 degree Climate Modeling Grid (CMG). MODIS snow cover data are based on a snow mapping algorithm that employs a Normalized Difference Snow Index (NDSI) and other criteria tests.
As a condition of using this data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.
The following example shows how to cite the use of this data set in a publication. List the principal investigators, year of data set release, data set title and version number, dates of the data you used (for example, December 2003 to March 2004), publisher: NSIDC, and digital media.
Hall, Dorothy K., George A. Riggs, and Vincent V. Salomonson. 2006, updated daily. MODIS/Terra Snow Cover 8-Day L3 Global 0.05deg CMG V005, [list the dates of the data used]. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.
|Data format||HDF-EOS version 2.9. GeoTIFF available through Reverb | ECHO, NASA's Next Generation Earth Science Discovery tool.|
|Spatial coverage and resolution||Coverage is global, but only tiles over land are produced. Grid resolution is 0.05 degrees.|
|Temporal coverage and resolution||MODIS data extends from 24 February 2000 to present. Temporal resolution is eight days.|
|Tools for accessing and analyzing data||Land Processes Distributive Active Archive Center: MODIS Swath Reprojection Tool Distribution Page
HEG HDF-EOS to GeoTIFF Conversion Tool Web site
Space Science and Engineering Center (SSEC): Terra Orbit Tracks GLOBAL Web site
NSIDC Hierarchical Data Format - Earth Observing System (HDF-EOS) Web site
MODIS Rapid Response System
NASA Goddard Space Flight Center: MODIS Land Global Browse Images
The MODIS Conversion Toolkit (MCTK)
|Grid type and size||The CMG products contain global snow cover arrays of 7200 columns by 3600 rows. Each cell is 0.05 degrees resolution.|
|File naming convention||Example: MOD10C2.A2000057.005.2006260032944.hdf|
|File size||0.5 - 6.0 MB using HDF compression|
|Parameter(s)||Day CMG Snow Cover
Day CMG Confidence Index
Day CMG Clound Obscured
Snow Spatial QA
|Procedures for obtaining data|
Dorothy K. Hall
National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC)
Mail stop 614.1
Greenbelt, MD 20771
Vincent V. Salomonson
Room 809 WBB
Department of Meteorology
University of Utah
Salt Lake City, UT 84112
George A. Riggs
Science Systems and Applications, Inc.
Mail stop 614.1
Greenbelt, MD 20771
NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
The following sites can help you select appropriate MODIS data for your study:
Data are available via the following methods:
Reverb: NASA search and order tool for subsetting, reprojecting, and reformatting data.
Subscription Service: Subscribe to have new data automatically sent when the data become available.
See the MODIS Data at NSIDC: Data Summaries: Web page for other MODIS snow and sea ice products available from NSIDC.
Algorithms that generate snow cover products are continually being improved as limitations become apparent in early versions of data. As a new algorithm becomes available, a new version of data is released. Users are encouraged to work with the most current version of MODIS data available, which is the highest version number. Version 5 (V005), also known as Collection 5, is the most current version of data available from NSIDC. No major changes were made to MOD10C2 V005 data from the previous version.
Please visit the following sites for more information about the V005 data, known data problems, production schedule, and future plans:
MODIS snow products are archived in compressed HDF-EOS format, which employs point, swath, and grid structures to geolocate the data fields to geographic coordinates. This data compression should be transparent to most users since HDF capable software tools automatically uncompress the data. Various software packages, including several in the public domain, support the HDF-EOS data format. See the Software section for details. Also, see the Hierarchical Data Format - Earth Observing System (HDF-EOS) Web site for more information about the HDF-EOS data format, as well as tutorials in uncompressing the data and converting data to binary format.
Data can also be obtained in GeoTIFF format from Reverb | ECHO, NASA's Next Generation Earth Science Discovery Tool.
MOD10C2 V005 consists of 7200 columns by 3600 rows of global arrays of snow cover. The MOD10C2 product was created by merging all of the MOD10A2 tiles for an eight-day period and binning the 500 m data to the 0.05 degree spatial resolution of the CMG cells (Riggs, Hall, and Salomonson 2006). Each data granule contains the following HDF-EOS local attribute fields, which are stored with their associated Scientific Data Set (SDS):
The Eight Day CMG Snow Cover field is a global map of snow cover extent expressed as a percentage of land in each CMG cell for an eight-day period. Snow percentage in each cell of the Day CMG Snow Cover field is calculated using 500 m totals of the number of snow observations and count of land observations in that cell for the day. The percentage of snow-covered land is based on the clear-sky view of land in the CMG cell. So the amount of snow observed in a CMG cell is based on the cloud-free observations mapped into the CMG grid cell for all land in that cell.
percent snow = 100 * count of snow observations/count of land
In V005 data, snow cover ranges from 0 -100 percent.
The Eight Day CMG Confidence Index field indicates how much of the land surface MODIS observes. The index represents an estimate of confidence in each cell's data value and indicates how confident the algorithm is that snow percentage in a cell is correct based on which data: snow, snow-free land, cloud, or unknown, were binned into the grid cell. Thus, the greater the percentage of land in a cell, the higher the confidence is for snow extent in that cell. Cloud obstruction reduces the confidence index.
The Eight Day CMG Cloud Obscured field indicates how much of the land surface in the cell is persistently obscured during the eight-day period. Cloud percentage in each cell of the Day CMG Cloud Obscured field is calculated in the same way as the percentage of snow, except that the count of cloud observations is used. Data from the Day CMG Snow Cover and Day CMG Cloud Obscured fields can be used together to better understand the observed snow. For example, if MODIS views a snow-covered region and no clouds obstruct the view on that day, then percentage of snow cover is 100 percent. If there is 30 percent cloud cover for that day, the percentage of snow cover is 70 percent.
percent cloud obscured = 100 * count of cloud observations/count of land
The Snow Spatial QA field provides additional information on algorithm results for each pixel within a spatial context, and it is used as a measure of usefulness for snow-cover data. The QA information tells if algorithm results were nominal, abnormal, or if other defined conditions were encountered for a pixel (Riggs, Hall, and Salomonson 2006).
A separate ASCII text file containing metadata with a .xml file extension accompanies the HDF-EOS file. The metadata file contains some of the same metadata as in the product file, but also includes other information regarding archiving, user support, and post-production QA relative to the granule ordered. The post-production QA metadata may or may not be present depending on whether or not the data granule was investigated for quality assessment. The metadata file should be examined to determine if post-production QA was applied to the granule (Riggs, Hall, and Salomonson 2006).
The file naming convention common to all MODIS products is MOD10C2.A2000057.005.2006260032944.hdf
Refer to Table 1 for an explanation of the variables used in the MODIS file naming convention.
|Type of product|
|Year of data acquisition|
|Day of year of data acquisition (day 57)|
|Year of production (2006)|
|Day of year of production (day 260)|
|Hour/minute/second of production in GMT (09:11:04)|
|HDF-EOS data format|
Data files are typically between 5 - 6 MB using HDF compression.
Note: New in V005, MOD10C2 data files now use HDF data compression. The extent to which compression reduces the file size varies from image to image, but generally it is a factor of 10 or more.
Coverage is global; however, snow cover is calculated for only tiles that include land. A ±55 degree scanning pattern at 705 km altitude achieves a 2330 km swath with global coverage every one to two days. The following resources can help you select and work with MOD10C2 tiles:
The local equatorial crossing time of the Terra satellite is approximately 10:30 A.M. in a descending node with a sun-synchronous, near-polar, circular orbit.
Gridded resolution is 0.05 degress.
MOD10C2 is in a 0.05 degree CMG.
The CMG products contain global snow cover arrays of 7200 columns by 3600 rows. Each cell is 0.05 degree resolution. The following is a sample image derived from MYD10C1, a similar product. Click on the thumbnail to view a larger image.
MODIS data extends from 24 February 2000 to present.
Temporal resolution is eight days.
For MOD10C2, eight-day periods begin on the first day of the year and extend into the next year. In some cases, there may not be eight days of input. The data file name indicates the first day in the eight-day period.
The snow mapping algorithm for CMG products classifies pixels as snow, snow-free land, cloud, night, masked (Antarctica), or no data. Snow extent is the primary variable of interest in this data set.
Refer to the MOD10C2 and MYD10C2 Local Snow Cover Attributes, Version 5 document for a key to the meaning of the coded integer values in the Eight Day CMG Snow Cover Field, the Eight Day CMG Confidence Index Field, the Eight Day CMG Cloud Obscured Field, and the Snow Spatial QA Field.
For information regarding the theory for snow mapping and fractional snow cover, please see the Theory of Measurements section in the MODIS/Terra Snow Cover 5-Min L2 Swath 500m, Version 5 guide document (MOD10_L2).
The MODIS science team is responsible for algorithm development. The MODAPS is responsible for product generation and transfer of products to NSIDC.
A snow-mapping algorithm generates global daily and eight-day snow cover products from MODIS data. The algorithm identifies the presence of snow by reflectance properties in each 500 m pixel for each orbit. The snow mapping algorithm is based on the Normalized Difference Snow Index (NDSI). The NDSI is a ratio of the difference between the infrared reflectance of snow in visible and infrared wavelengths. The NDSI is adaptable for a number of illumination conditions, it does not depend on reflectance for a specific band, and it is partially normalized for atmospheric effects. The algorithm uses MODIS bands 4 (0.55 µm) and 6 (1.6 µm) from the Level 1B MOD02HKM product to calculate the NDSI (Hall et al. 1998).
NDSI = (Band 4 - Band 6) / (Band 4 + Band 6)
The MODIS/Terra Calibrated Radiances 5-Min L1B Swath 500m (MOD02HKM) guide document provides the source data that was used to calculate reflectance.
In the MOD10C2 product, the snow cover algorithm maps MOD10A2 eight-day snow cover data at 500 m resolution into the corresponding cell of a 0.05 degree CMG and computes snow and cloud percentages, QA, and a confidence index based on the mapping results. The MODIS Instrument Science Team employed the University of Maryland 1 km global land cover mask to compute the percentage of land in each CMG cell and determine an appropriate classification. CMG cells with at least 12 percent land were classified as land. The global land cover mask was also used in the calculation of the confidence index.
Because Antarctica's surface is typically less than one percent snow-free (a value less than the global error rate for MODIS snow mapping), the algorithm classifies Antarctica as completely snow-covered. This also reduces confusion with cloud signatures (Riggs, Hall, and Salomonson 2006).
As with any upper level product, the characteristics of and/or anomalies in input data may carry through to the output data product. The following product is input to the algorithms used to create the MOD10C2 product:
Quality indicators for MODIS snow data can be found in the following places:
These quality indicators are generated during production or in post-production scientific and quality checks of the data product. For more information on local and global attributes, go to one of the following documents:
The AutomaticQualityFlag is automatically set according to conditions for meeting data criteria in the snow mapping algorithm. In most cases, the flag is set to either Passed or Suspect, and in rare instances it may be set to Failed. Suspect means that a significant percentage of the data were anomalous and that further analysis should be done to determine the source of anomalies. The AutomaticQualityFlagExplanation contains a brief message explaining the reason for the setting of the AutomaticQualityFlag. The ScienceQualityFlag and the ScienceQualityFlagExplanation are set after production, either after an automated QA program is run or after the data product is inspected by a qualified snow scientist. Content and explanation of this flag are dynamic so it should always be examined if present.
The NASA Goddard Space Flight Center: MODIS Land Quality Assessment Web site provides updated quality information for each product.
The MODIS instrument provides 12-bit radiometric sensitivity in 36 spectral bands ranging in wavelength from 0.4 µm to 14.4 µm. Two bands are imaged at a nominal resolution of 250 m at nadir, five bands at 500 m, and the remaining bands at 1000 m. A ±55 degree scanning pattern at 705 km achieves a 2330 km swath with global coverage every one to two days.
The scan mirror assembly uses a continuously rotating double-sided scan mirror to scan ±55 degrees, driven by a motor encoder built to operate 100 percent of the time throughout the six year instrument design life. The optical system consists of a two-mirror off-axis afocal telescope which directs energy to four refractive objective assemblies: one each for the visible, near-infrared, shortwave-infrared, and longwave-infrared spectral regions (MODIS Web 2003).
|Orbit||705 km, 10:30 a.m. descending node|
|Scan Rate||20.3 rpm, cross track|
|Swath Dimensions||2330 km (cross track) by 10 km (along track at nadir)|
|Telescope||17.78 cm diameter off-axis, afocal (collimated) with intermediate field stop|
|Size||1.0 x 1.6 x 1.0 m|
|Power||162.5 W (single orbit average)|
|Data Rate||10.6 Mbps (peak daytime); 6.1 Mbps (orbital average)|
|Spatial Resolution||250 m (bands 1-2)
500 m (bands 3-7)
1000 m (bands 8-36)
|Design Life||Six years|
For information on the 36 spectral bands provided by the MODIS instrument, see the Spectral Bands Table.
The MODIS scan mirror assembly uses a continuously rotating double-sided scan mirror to scan ±55 degrees with a 20.3 rpm. The viewing swath is 10 km along track at nadir and 2330 km cross track at ±55 degrees.
MODIS instruments were built to NASA specifications by Santa Barbara Remote Sensing, a division of Raytheon Electronics Systems.
MODIS has a series of on-board calibrators that provide radiometric, spectral, and spatial calibration of the MODIS instrument. The blackbody calibrator is the primary calibration source for thermal bands between 3.5 µm and 14.4 µm, while the Solar Diffuser (SD) provides a diffuse, solar-illuminated calibration source for visible, near-infrared, and shortwave infrared bands. The Solar Diffuser Stability Monitor (SDSM) tracks changes in the reflectance of the SD with reference to the sun so that potential instrument changes are not incorrectly attributed to changes in this calibration source. The Spectroradiometric Calibration Assembly (SRCA) provides additional spectral, radiometric, and spatial calibration.
MODIS uses the moon as an additional calibration technique and for tracking degradation of the SD, by referencing the illumination of the moon since the moon's brightness is approximately the same as that of the Earth. Finally, MODIS deep space views provide a photon input signal of zero, which is used as a point of reference for calibration (MODIS Web 2003).
MODIS is a key instrument aboard the Terra satellite, the flagship of NASA's Earth Observing System (EOS). The EOS includes a series of satellites, a data system, and the world-wide community of scientists supporting a coordinated series of polar-orbiting and low inclination satellites for long-term global observations of the land surface, biosphere, solid Earth, atmosphere, and oceans that together enable an improved understanding of the Earth as an integrated system. MODIS is playing a vital role in the development of validated, global, and interactive Earth system models able to predict global change accurately enough to assist policy makers in making sound decisions concerning the protection of our environment. (NASA's MODIS Web Site 2006), (NASA's Terra Web Site 2006), and (NASA's EOS Web Site 2006)
Within this overall context, the objectives of the MODIS snow and ice team are to develop and implement algorithms that map snow and ice on a daily basis, and provide statistics of the extent and persistence of snow and ice over eight-day periods. Data at 500 m resolution enables sub-pixel snow mapping for use in regional and global climate models. A study of sub grid-scale snow-cover variability is expected to improve features of a model that simulates Earth radiation balance and land-surface hydrology (Hall et al. 1998).
The MODIS sensor contains a system whereby visible light from the earth passes through a scan aperture and into a scan cavity to a scan mirror. The double-sided scan mirror reflects incoming light onto an internal telescope, which in turn focuses the light onto four different detector assemblies. Before the light reaches the detector assemblies, it passes through beam splitters and spectral filters that divide the light into four broad wavelength ranges. Each time a photon strikes a detector assembly, an electron is created. Electrons are collected in a capacitor where they are eventually transferred into the preamplifier. Electrons are converted from an analog signal to digital data, and down linked to ground receiving stations (MODIS Web 2003).
The EOS Ground System (EGS) consists of facilities, networks, and systems that archive, process, and distribute EOS and other NASA earth science data to the science and user community. The EOS Data and Operations System (EDOS) processes telemetry from EOS spacecraft and instruments to generate Level-0 products, and maintains a backup archive of Level-0 products (ESDIS 1996). The MODAPS is currently responsible for generation of Level-1A data from Level-0 instrument packet data. These data are then used to generate higher level MODIS data products, including MOD29. MODIS snow and ice products are archived at the NSIDC Distributed Active Archive Center (DAAC) and distributed to EOS investigators and other users via external networks and interfaces (MODIS Web 2003). Data are available to the public through a variety of interfaces.
Diner, D. J., J. V. Martonchik, C. Borel, S. A. W. Gerstl, H. R. Gordon, Y. Knyazikhin, R. Myneni, B. Pinty, and M. M. Verstraete. 1999. MISR Level-2 Surface Retrieval Algorithm Theoretical Basis Document. Pasadena, CA: Jet Propulsion Laboratory.
Hall, Dorothy K., George A. Riggs, and Vincent V. Salomonson. September 2001a. Algorithm Theoretical Basis Document (ATBD) for the MODIS Snow-, Lake Ice- and Sea Ice-Mapping Algorithms. Greenbelt, MD: Goddard Space Flight Center. <http://modis-snow-ice.gsfc.nasa.gov/?c=atbd&t=atbd> .
Hall, Dorothy K., J. L. Foster, D. L. Verbyla, A. G. Klein, and C. S. Benson. 1998. Assessment of Snow Cover Mapping Accuracy in a Variety of Vegetation Cover Densities in Central Alaska. Remote Sensing of the Environment 66:129-137.
Hall, Dorothy K., J. L. Foster, Vincent V. Salomonson, A. G. Klein, and J. Y. L. Chien. 2001b. Development of a Technique to Assess Snow-Cover Mapping Accuracy From Space. IEEE Transactions on Geoscience and Remote Sensing 39(2):232-238.
Klein, A. MODIS Snow Albedo Prototype. 2003. <http://geog.tamu.edu/klein/modis_albedo/> Accessed July 2003.
Klein, A. G., Dorothy K. Hall, and George A. Riggs. 1998. Improving Snow-Cover Mapping in Forests Through the Use of a Canopy Reflectance Model. Hydrologic Processes 12(10-11):1723-1744.
Markham, B. L. and J. L. Barker. 1986. Landsat MSS and TM Post-Calibration Dynamic Ranges, Exoatmospheric Reflectances and At-Satellite Temperatures. EOSAT Technical Notes 1:3-8.
MODIS Science and Instrument Team. MODIS Web. July 2003. <http://modis.gsfc.nasa.gov/> Accessed October 2000.
Riggs, George A., Dorothy K. Hall, and Vincent V. Salomonson. January 2006. MODIS Snow Products User Guide for Collection 4 Data Products. <http://modis-snow-ice.gsfc.nasa.gov/sug_main.html> .
The following acronyms and abbreviations are used in this document:
|ATBD||Algorithm Theoretical Basis Document|
|CMG||Climate Modeling Grid|
|DAAC||Distributed Active Archive Center|
|EDOS||EOS Data and Operations System|
|EGS||EOS Ground System|
|EOS||Earth Observing System|
|EOSDIS||Earth Observing System Data and Information System|
|ESDIS||Earth Science Data and Information System|
|FTP||File Transfer Protocol|
|GMT||Greenwich Mean Time|
|GSFC||Goddard Space Flight Center|
|HDF-EOS||Hierarchical Data Format - Earth Observing System|
|MCST||MODIS Characterization Support Team|
|MODAPS||MODIS Adaptive Processing System|
|MODIS||Moderate Resolution Imaging Spectroradiometer|
|MODLAND||MODIS Land Discipline Group|
|NASA||National Aeronautics and Space Administration|
|NCSA||National Center for Supercomputing Applications|
|NDSI||Normalized Difference Snow Index|
|NSIDC||National Snow and Ice Data Center|
|SDS||Scientific Data Set|
|SDSM||Solar Diffuser Stability Monitor|
|SRCA||Spectroradiometric Calibration Assembly|