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MODIS/Terra Sea Ice Extent Daily L3 Global 1km EASE-Grid Day, Version 6
The MODIS/Terra Sea Ice Extent Daily L3 Global 1km EASE-Grid Day (MOD29P1D) data set contains tiles of daily 1 km resolution sea ice extent, ice surface temperature, and quality assessment data gridded to the Equal Area Scalable Earth Grid (EASE-Grid).
Sea ice extent and ice surface temperature algorithms and data sets are the same as Version 5. However, revisions and improvements for Version 6 to algorithm inputs have improved sea ice outputs as follows:
- Revised calibration of MOD02 thermal bands 31 and 32 decreased calculated brightness temperatures by about -0.01 K and improved accuracy for very cold scenes;
- The Version 6 land/water mask in the MOD03 geolocation product increased the accuracy of water body and coastline maps;
- Revisions to the MOD35_L2 cloud mask algorithm have increased the accuracy of cloud detection.
|Temporal Resolution:||1 day|
|Data Contributor(s):||Miguel Román, Dorothy Hall, George Riggs|
|Metadata XML:||View Metadata Record|
As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.Hall, D. K. and G. A. Riggs. 2015. MODIS/Terra Sea Ice Extent Daily L3 Global 1km EASE-Grid Day, Version 6. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/MODIS/MOD29P1D.006. [Date Accessed].
Detailed Data Description
Data files are provided in HDF-EOS2 (V2.17). JPEG browse images are also available.
HDF-EOS (Hierarchical Data Format - Earth Observing System) is a self-describing file format based on HDF that was developed specifically for distributing and archiving data collected by NASA EOS satellites. For more information, visit the HDF-EOS Tools and Information Center.
This section explains the file naming convention used for this MODIS data set with an example. Note that MODIS Terra data file names begin with MOD. MODIS Aqua file names being with MYD.
- Example File Name:
Refer to Table 2 for the valid values for the file name variables listed above.
|A||Acquisition date follows|
|DDD||Acquisition day of year|
|hNN, vNN||Horizontal tile number, vertical tile number (see Grid below).|
|VVV||Version (Collection) number|
|ddd||Production day of year|
|hhmmss||Production hour/minute/second in GMT|
|.hdf||HDF-EOS formatted data file|
External Metadata File
Each HDF-EOS data file (.hdf) has a corresponding Extensible Markup Language external metadata file (.xml) which contains some of the same internal metadata as the HDF-EOS file plus additional information regarding user support, archiving, and granule-specific post-production Quality Assessment (QA). Note that post-production QA metadata will only be present if the granule was evaluated for QA.
Data files are typically between 0.5 - 1.5 MB using HDF compression. XML metadata files are between 5 - 10 KB.
Coverage is global, however only ocean pixels are evaluated for sea ice.
Latitude Crossing Times
Terra's sun-synchronous, near-polar circular orbit is timed to cross the equator from north to south (descending node) at approximately 10:30 A.M. local time.
The gridded resolution is approximately 1 km.
Projection and Grid Description
MOD29P1D/MYD29P1D data sets utilize polar tile grids based on the Lambert Azimuthal Equal-Area projection. Meridians are straight lines that intersect at the poles while lines of latitude are circles with their centers at either pole. The following table lists some of the key parameters for this projection:
|Earth radius||6371228.0 meters|
|Projection origin||North: 90° lat, 0° lon
South: -90° lat, 0° lon
|Orientation||North: 0° lon, oriented vertically at bottom
South: 0° lon, oriented vertically at top
|Upper left corner (m)||-9058902.1845(x)
|Lower right corner (m)|| 9058902.1845(x)
MOD29P1D/MYD29P1D data files are provided as tiles of data gridded in the original EASE-Grid Lambert Azimuthal Equal Area map projection. Tiles contain 951 x 951 cells. The global tile grid is partitioned into separate Northern and Southern Hemisphere polar grids, with half of the tiles (313) in the north and half in the south. The coordinate system, designated by (horizontal, vertical) ordered pairs, starts with (h00,v00) in the upper left corner of the northern grid and proceeds rightward (horizontal) and downward (vertical) to tile (h18, v18) in the bottom right. The southern grid begins where the northern grid ends, with tile (h00,v20) in the upper left and tile (v18,h38) in the lower right.
Additional information about this grid is available on the following Web pages, including bounding coordinates for each tile, maps that show tile locations for the Northern and Southern Hemisphere grids, and the MODLAND Tile Calculator tool, which can convert between MODIS tile numbers and latitude/longitude:
- MODIS Sea Ice Tile Bounding Coordinates, Northern Hemisphere
- MODIS Sea Ice Tile Bounding Coordinates, Southern Hemisphere
- EASE-Grid Tile Locations for MODIS Sea Ice Products
- MODIS MODLAND Tile Calculator
For descriptions of all the projections and grids used for MODIS data sets, see the MODIS Land team's MODIS Grids Web page. A complete description of EASE-Grid is available at EASE-Grid Data | Overview.
MODIS Terra data extend from 24 February 2000 to present. Complete global coverage occurs every one to two days (more frequently near the poles). To view daily orbit tracks for the Terra satellite, visit the Space Science and Engineering Center | Terra Orbit Tracks Web page.
Over the course of the Terra mission a number of anomalies have resulted in minor data outages. If you cannot locate data for a particular date or time, check the MODIS/Terra Data Outages Web page.
The content of MODIS sea ice data files differs between day and night because visible data are not acquired when Earth's surface is dark. Thermal data are acquired during both day and nighttime. Users should be aware of the following:
- Swath data acquired during daylight, or during a mix of day and night mode, contain variables for both sea ice extent and ice surface temperature;
- Swath data acquired completely in night mode contain only the ice surface temperature variable;
- Daily sea ice data sets are split into separate files for day and night.
The DayNightFlag object, a metadata value stored with the CoreMetadata.0 global attribute, indicates whether the entire swath was acquired during daylight (day), darkness (night), or a mix of day and night (both).
Sea ice extent and ice surface temperature (IST) are the parameters of interest in this data set. Values represent the best observation from all the swath level observations (MOD29/MYD29) mapped into a grid cell for the day. These data are written to data files as Scientific Data Sets (SDSs) according to the HDF Scientific Data Set Data Model.
Data files also contain important metadata, including HDF-EOS global attributes that are assigned to the file and pre-defined and user-defined local attributes assigned to the data fields. For detailed information about HDF-EOS-specific metadata, see An HDF-EOS and Data Formatting Primer for the ECS Project.
The following table lists the SDSs in MOD29P1D/MYD29P1D data files:
|Scientific Data Sets||Description|
|Sea_Ice_by_Reflectance||Sea ice extent map stored as coded integers¹. Pixels are reported as sea ice, ocean, cloud, land, inland water, or other conditions (e.g. missing data). Daylight only.|
|Sea_Ice_by_Reflectance_Spatial_QA||QA data corresponding to the observation selected as the sea ice observation of the day. Stored as coded integers¹. Values are good, other, or a masked class (e.g. land). Daylight only.|
ISTs stored as calibrated data. To convert to kelvins, use scale_factor = 0.01 and add_offset = 0.0 in the following equation²:
The valid range for ISTs is 210 K to 313.20 K.
|Ice_Surface_Temperature_Spatial_QA||QA data corresponding to the observation selected as the IST observation of the day. Stored as coded integers¹. Values are good, other, or a masked class (e.g. land).|
|¹Coded integer keys are stored as Local Attributes with the corresponding SDS.
²Values for scale_factor and add_offset are stored as Local Attributes with the IST SDS.
Software and Tools
The following sites can help you identify the right MODIS data for your study:
- NASA's Earth Observing System Data and Information System | Near Real-Time Data
- NASA Goddard Space Flight Center | MODIS Land Global Browse Images
The following resources are available to help users work with MODIS data:
- The MODIS Reprojection Tool allows users to read data files in HDF-EOS format, specify geographic subsets or science data sets as input to processing, perform geographic transformations to different coordinate systems and cartographic projections, and write output files to formats other than HDF-EOS.
- The HDF-EOS to GeoTIFF Conversion Tool (HEG) can reformat, re-project, and perform stitching/mosaicing and subsetting operations on HDF-EOS objects.
- HDFView is a simple, visual interface for opening, inspecting, and editing HDF files. Users can view file hierarchy in a tree structure, modify the contents of a data set, add, delete and modify attributes, and create new files.
- The MODIS Conversion Toolkit (MCTK) plug-in for ENVI can ingest, process, and georeference every known MODIS data set, including products distributed with EASE-Grid projections. The toolkit includes support for swath projection and grid reprojection and comes with an API for large batch processing jobs.
- NSIDC's Hierarchical Data Format | Earth Observing System (HDF-EOS) Web page contains information about HDF-EOS, plus tools to extract binary and ASCII objects, instructions to uncompress and geolocate HDF-EOS data files, and links to obtain additional HDF-EOS resources.
Data Acquisition and Processing
The MODIS science team continually seeks to improve the algorithms used to generate MODIS data sets. Whenever new algorithms become available, the MODIS Adaptive Processing System (MODAPS) reprocesses the entire MODIS collection—atmosphere, land, cryosphere, and ocean data sets—and a new version is released. Version 6 (also known as Collection 6) is the most recent version of MODIS sea ice data available from NSIDC. NSIDC strongly encourages users to work with the most recent version.
Consult the following resources for more information about MODIS Version 6 data, including known problems, production schedules, and future plans:
- MODIS Sea Ice Products User Guide to Collection 6
- The MODIS Snow and Sea Ice Global Mapping Project
- NASA Goddard Space Flight Center | MODIS Land Quality Assessment
- MODIS Land Team Validation | Status for Snow Cover/Sea Ice (MOD10/29)
The MODIS Version 6 (Collection 6) sea ice extent and ice surface temperature algorithms and products are the same as Version 5. However, Version 6 updates to algorithm inputs—in particular, the L1B calibrated radiances, land and water mask, and cloud mask products—have improved the sea ice outputs. Additional details are provided on the MODIS | Data Versions page and in the Quality Assessment section of this user guide.
For more information regarding the theory for sea ice mapping and ice surface temperature retrieval, please see Theory of Measurements in the MODIS/Terra Sea Ice Extent 5-Min L2 Swath 1km, Version 6 (MOD29) documentation.
The MODIS sensor contains a system whereby visible light from 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 generated. 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 downlinked to ground receiving stations. 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 MODIS Science Team develops the algorithms used to detect snow cover and sea ice. The MODIS Data Processing System (MODAPS) generates the MODIS data sets and transfers them to NSIDC. The following sections outline the approach that the algorithm uses to generate the sea ice extent and ice surface temperature maps. Users seeking a fuller description should consult the MODIS Sea Ice Products User Guide to Collection 6.
Derivation Techniques and Algorithms
The MOD29P1D/MYD29P1D data sets comprise single observations drawn from the many MOD29/MYD29 swath level observations acquired during the day at each location. A scoring algorithm selects the most favorable daily observation based on solar elevation, observation coverage in a grid cell, and distance from nadir. The underlying objective aims to select daytime reflectance observations that are near nadir, acquired near noon local time, and cover a large area in a grid cell. Because MODIS collects both visible and thermal data in day mode, the scoring algorithm uses visible data to determine the observation of the day for both reflectance and thermal data. The score for each observation is given by:
- score = (0.5 × solar elevation) + (0.3 × observation coverage) + (0.2 × distance from nadir)
The observation with the highest score for a grid cell is selected as the observation for the day. The corresponding thermal observation is utilized as the IST observation of the day. In situations where the day and night terminator lies within a tile, the IST is mapped on both sides of the terminator; as such, daytime granules may contain regions where IST is mapped without corresponding sea ice by reflectance data.
The sea ice and IST QA values are drawn from the corresponding MOD29/MYD29 sea ice and IST observations selected as the observation of the day. No quality assessment is performed in this product's algorithm. All QA is inherited from MOD29/MYD29.
The input data to MOD29P1D/MYD29P1D are read from intermediate, level-2 gridded products that are produced by mapping all MOD29/MYD29 swaths from a calendar day to grid cells of the Lambert Azimuthal Equal-Area (polar grid) projection. These necessary intermediates (see Table 5) are used as inputs to the daily gridded sea ice products and are neither retained nor archived at NSIDC.
For details about the sea ice and ice surface temperature data used as input to this data set, see Derivation Techniques and Algorithms in the MODIS/Terra Sea Ice Extent 5-Min L2 Swath 1km, Version 6 (MOD29) documentation.
As with any upper level product, anomalies in the input data may carry through to the output product. The following products are input to the MODIS daily sea ice algorithm:
MODIS/Terra Sea Ice Extent Daily L2G Global 1km EASE-Grid Day
Sea ice IST and QA
MODIS/Terra Geolocation Angles Daily L2G Global 1km EASE-Grid Day
Solar and sensor geometry
MODIS/Terra Observation Pointers Daily L2G Global 1km Polar Grid Day
Number of observations, coverage observation swath and location
|¹Intermediate product neither retained nor distributed by NSIDC.|
Some error in geolocation may be associated with projecting from latitude and longitude coordinates to this data set's EASE-Grid Lambert Azimuthal equal area projection. Geolocation error may be notable along coast lines, which may appear to shift from day to day between cells of the grid.
Artifacts due to using a coarse-resolution cloud mask to set the ice background flag may appear as clouds having straight edges, and possibly ninety degree corners.
IST is mapped in the region of darkness 85° to 90° solar zenith angle whereas sea ice extent is not. As such, in the terminator region the sea ice extent and IST maps will have different spatial coverages. Furthermore, over the polar regions the number of MODIS acquisitions in daylight varies with the seasons. During the boreal summer, small regions in the Arctic can be imaged many times—in perhaps as many as fourteen swaths—during a 24 hour period. Users should carefully consider which level of sea ice data will best meet their research or application. The objective of the daily sea ice algorithm is to report the putative best observation of a day based primarily on time of overpass and viewing angle, with best defined as nearest local solar noon and closest to nadir. This also maps swath inputs as contiguous spatially and temporally in a tile, with mixed observations occurring along the edges of overlapping swaths. As such, the daily sea ice product may not best suit some users because it only includes one observation out of all the accessible observations in the swath. Sea ice moves, clouds move, the satellite passes overhead rough every 99 minutes: all factors that users should consider when deciding which data to use and how to interpret the daily gridded sea ice data sets.
See the MODIS Data Versions page for the history of MODIS snow and sea ice product versions.
All QA is inherited from the MOD29 sea ice data set. The Sea_Ice_by_Reflectance_Spatial_QA and Ice_Surface_Temperature_Spatial_QA data fields contain the corresponding QA data for the sea ice and ice surface temperature observations selected by the algorithm as the observation of the day. No automated quality assessment is performed within this algorithm. QA data are stored as coded integers and indicate whether algorithm results were nominal, abnormal, or if other defined conditions were encountered.
For more information about the quality indicators utilized by MOD29, see MODIS/Terra Sea Ice Extent 5-Min L2 Swath 1km, Version 6 | Quality Assessment.
In addition, the NASA Goddard Space Flight Center: MODIS Land Quality Assessment Web site provides updated quality information for all MODIS land products.
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 an altitude of 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, and is 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, short- and mid-wavelength infrared, and long wavelength infrared spectral regions.
The MODIS instruments on the Terra and Aqua space vehicles were built to NASA specifications by Santa Barbara Remote Sensing, a division of Raytheon Electronics Systems. Table 4 contains the instruments' technical specifications:
|Orbit||705 km altitude, 10:30 A.M. descending node (Terra), sun-synchronous, near-polar, circular|
|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 m x 1.6 m 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||6 years|
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 short wave infrared bands. The Solar Diffuser Stability Monitor 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 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.
For additional details about the MODIS instruments, see NASA's MODIS | About Web page.
References and Related Publications
Contacts and Acknowledgments
Miguel O. Román
NASA Goddard Space Flight Center
Mail Code: 619
Greenbelt , MD 20771
Dorothy K. Hall
National Aeronautics and Space Administration (NASA)
Goddard Space Flight Center (GSFC)
Mail Code 615
Greenbelt, MD 20771
George A. Riggs
Science Systems and Applications, Inc.
Mail stop 615
Greenbelt, MD 20771
DOCUMENT CREATION DATE
The following table describes the data subsetting, reformatting, and reprojection services that are currently... read more