Frequently Asked Questions
1. What are the current snow and sea ice products at NSIDC?
2. When and how can I get MODIS data from NSIDC?
3. Which MODIS data products have accompanying browse images?
4. Since Aqua and Terra have the same data products, which one should I use?
5. What is the data format for MODIS data?
6. What are the differences between the available versions of MODIS data?
7. How can I compare the latest version of data to an earlier version of data I used for a research project?
8. What subsetting and reprojection tools are available for MODIS snow and sea ice products?
9. What are the MODIS sensor’s strongest attributes, and how do they compare to other sensors?
10. What are the limitations of MODIS data?
11. What quality control measures are taken for MODIS products?
12. What other data centers archive and distribute MODIS data products?
13. Is a list of published articles that involved MODIS snow or sea ice data in research available?
14. How do I use the MODIS Reprojection Tool (MRT) with MODIS Level-3 Snow Cover Products?
- What are the current snow and sea ice products at NSIDC?
See the Data Summaries Web page for a list of current products.
The MODIS snow cover and sea ice products from the Terra satellite are available with observations beginning 24 February 2000, and from the Aqua satellite with observations beginning 4 July 2002. The lag time between observations and availability of products is only a few days. The processing and reprocessing schedule for all MODIS data determines the lag time. Please refer to Data Versions Web page for the processing and reprocessing schedule.
- When and how can I get MODIS data from NSIDC?
See the Order Data Web page for order options.
- Which MODIS data products have accompanying browse images?
NSIDC does not produce browse imagery for Version 5 (V005) MODIS data because they are now available through each of the order interface options and the Data Pool. All V005 MODIS snow and sea ice products except MOD10CM and MYD10CM have browse images for each data granule.
- Since Aqua and Terra have the same data products, which one should I use?
See the Choosing Terra or Aqua MODIS Data Web page.
- What is the data format for MODIS data?
The data are in Hierarchical Data Format - Earth Observing System (HDF-EOS) format.
HDF-EOS is the standard data format for all EOS data products. HDF-EOS is a multi-object file format developed by The HDF Group. For more information about the HDF-EOS format, tools for extracting binary and ASCII objects from HDF, and a list of other HDF-EOS resources, see the NSIDC HDF-EOS Web pages.
- What are the differences between the available versions of MODIS data?
A version number represents a collection of data with processing refinements for algorithm, instrument, and calibration stabilization. This ensures a continuous time series of data with consistent science quality. When a new version becomes available, a small portion of the previous version remains available to users.
To learn about the different versions of MODIS snow and sea ice products available from NSIDC, see the Data Versions Web page, or visit the MODIS Snow and Sea Ice Global Mapping Project: Validation Status Web site.
- How can I compare the latest version of data to an earlier version of data I used for a research project?
Disclaimer: Due to differences in Science Quality of the products, users are advised not to combine the V004 and V005 data in project applications.
A portion of data from previous versions is available for comparison. Currently, a sample of Version 3 (V003) data from 29 August 2002 (day 241) through 07 October 2002 (day 280), called the Golden Month, is available for all MODIS/Terra snow and sea ice products. Please contact NSIDC User Services to obtain Golden Month data.
- What subsetting and reprojection tools are available for MODIS snow and sea ice products?
The free, stand-alone HDF-EOS To GeoTIFF Conversion Tool (HEG) allows you to reproject, subset, stitch, and convert to GeoTIFF most of the MODIS Aqua and Terra products, as shown in the table below. If MODIS products are accessed via the Data Pool Web site, the HEG converter is automatically used to perform these operations.
The free MODIS Reprojection Tool (MRT) transforms all MODIS/Terra and MODIS/Aqua Level-3 gridded snow and sea ice products from Integerized Sinusoidal (ISIN) and Sinusoidal (SIN) projections to user-specified projections. It also transforms MODIS/Terra and MODIS/Aqua Level-2 snow and sea ice products from HDF-EOS swath format to a uniformly gridded image that is geographically referenced according to user-specified projection and resampling parameters. Refer to FAQ No.14 for more information.
Also, see the Geolocating HDF-EOS Data Web page for more details.
Product Standalone HEG MOD10_L2X MOD10A1X MOD10A2X MOD10C1X MOD10C2X MOD10CMX MYD10_L2X MYD10A1X MYD10A2X MYD10C1X MYD10C2X MYD10CMX MOD29X MOD29P1DX MOD29P1NX MOD29E1DX MYD29 MYD29P1DX MYD29E1DX MYD29P1NX
- What are the MODIS sensorís strongest attributes, and how do they compare to other sensors?
MODIS provides global coverage every one to two days in 36 spectral bands. Spatial resolution of the MODIS data varies by band from 250 m to 1 km. By contrast, the Advanced Very High Resolution Radiometer (AVHRR) sensor has five spectral bands with a spatial resolution of 4 km or 1 km. For more information about MODIS, visit the MODIS Web site.
- What are the limitations of MODIS data?
MODIS is an optical sensor, which has inherent limitations in observing sea ice and snow. Unlike a passive-microwave sensor, which can detect microwave energy through clouds, MODIS cannot observe the surface when cloud cover is present. MODIS products, therefore, have cloud masks built into the data arrays to prevent clouds from being interpreted as ice and snow. These cloud masks are continually improved as limitations are documented. Finally, the visible bands of MODIS are only used during the day, when reflectance is the dominant mechanism for detecting ice and snow. The thermal bands are used during both day and night to measure sea ice surface temperature by emittance.
- What quality control measures are taken for MODIS products?
The MODIS Land Quality Assessment Web page describes quality control for all MODIS products.
Also, visit the MODIS Snow and Sea Ice Global Mapping Project: Validation Status Web site for more information.
- What other data centers archive and distribute MODIS data products?
The Level 1 and Atmosphere Archive and Distribution System Web site provides quick and easy access to MODIS Level 1 and atmosphere data, geolocation, and cloud mask products.
The Land Processes Distributed Active Archive Center (LP DAAC) archives and distributes MODIS products pertaining to land and vegetation.
The Science Computing Facility (SCF) archives and distributes MODIS ocean color and sea surface temperature products. Please visit the Ocean Color Web site for more information regarding this topic.
- Is a list of published articles that involved MODIS snow or sea ice data in research available?
Yes, the Published Research Web page lists citations of published MODIS research.
- How do I use the MODIS Reprojection Tool (MRT) with MODIS Level-3 Snow Cover Products?
There are several things to consider when using MRT with MODIS Level-3 Snow Cover Products. The following questions and answers should guide you through using MRT with MODIS Level-3 Snow Cover Products.
- Will the data be resampled if the original (native) projection is changed?
Yes, the data will be resampled in some way. For example, nearest neighbor, bilinear, cubic, etc.
- When inputting into MRT, why are the native data that have a sinusoidal projection and WGS84 datum being forced into NODATUM and thus a spherical projection with radius 6371007.181 m?
MRT does not recognize that the datum spheroid, such ast the WGS84 ellipsoid, is different from the sinusoidal projection spheroid. A sphere of radius 6371007.181 m.
- How can you create a GeoTIFF out of MODIS Level-3 Sinusoidal data using MRT?
Change the projection from sinusoidal to UTM with a WGS84 datum. You will end up with gridded data for which both the geographic coordinate system and the projected coordinate system are defined in terms of the WGS84 ellipsoid. You can then use MRT to create a valid GeoTIFF that all software packages should be able to interpret correctly.
- If the HDF-EOS data files were not reprojected to UTM WGS84 prior to creating the GeoTIFF, is it possible to reproject the GeoTIFF output file from sinusoidal NODATUM format to sinusoidal WGS84 datum?
No. MRT can not reproject data into an ellipsoid sinusoidal projection. Thus, it can not create a sinusoidal GeoTIFF that refers to the WGS84 datum. Any particular GeoTIFF can specify one and only one spheroid to be used for both the geographical coordinate system and the projected coordinate system.