11 FAQ

Frequently Asked Questions about MODIS VI Products

Q. What is the difference between NDVI and EVI?
A. The Enhanced Vegetation Index differs from NDVI by attempting to correct for atmospheric and background effects. EVI appears to be superior in discriminating subtle differences in areas of high vegetation density, situations in which NDVI tends to saturate. NDVI has been used for several decades, which is advantageous for studying historical changes. Please refer to our page on vegetation indices for more information.

Q. What spatial resolutions are available?
A. 250 m (MOD13Q1), 500 m (MOD13A1), 1 km (MOD13A2), and 0.05 deg (MOD13C1 and MOD13C2).

Q. What temporal resolutions are available?
A. Base products are 16-day (MOD13Q1, MOD13A1, MOD13A2 and MOD13C1). Monthly products (MOD13A3 and MOD13C2) are generated from their 16-day counterparts.

Q. How often are images acquired?
A. MODIS images are collected daily, however the vegetation products are composites of the best pixels from 16 consecutive days. These composites are produced on 16-day cycles. Monthly products are generated by using a weighted-average scheme on corresponding 16-day products.

Q. Where can I order the data from?
A. There are several sources of data, each providing different ways to access the data pool. Please see section “Related Web Sites” (Sec. 10), bullet “Data Access Tools” for detailed information.

Q. How can I order MODIS data from TBRS?
A. The TBRS lab does not keep complete archives of MODIS data due to space limitations. MODIS data may be ordered through the sites listed before.

Q. How much does MODIS data cost?
A. MODIS data are free.

Q. What is a “tile”?
A. Global data from MODIS are organized as units that are 10deg by 10deg at the Equator, but vary according to the latitude. These units are called tiles. The tile coordinate system starts at (0,0) in the UL corner and proceeds right (horizontal) and downward (vertical). The tile in the bottom right corner is (35,17). See http://modis-land.gsfc.nasa.gov/MODLAND_grid.htm for more inforation.

Q. How can I determine the tile and pixel coordinates for a specific site with known geographic coordinates?
A. You can use the MODIS Tile Calculator (http://landweb.nascom.nasa.gov/cgi-bin/developer/tilemap.cgi).

Q. What is the file format of MODIS data?
A. HDF-EOS. Please see Section “Related Web Sites” for links to further details.

Q. How can I read HDF/HDF-EOS data?
A. Some image processing programs, such as ENVI and PCI Geomatics, can read the format directly. If needed, free MODIS tools for converting the data format are available at https://lpdaac.usgs.gov/lpdaac/tools/modis_reprojection_tool.

Q. My software does not recognize the MODIS map projection. What is the projection and how can I change it?
A. The projection is called Sinusoidal (SIN). Use the MODIS Tools, available at https://lpdaac.usgs.gov/lpdaac/tools/modis_reprojection_tool to reproject your data to a more common projection.