Obtaining and Processing MODIS Data

Transcription

1 Obtaining and Processing MODIS Data MODIS is an extensive program using sensors on two satellites that each provide complete daily coverage of the earth. The data have a variety of resolutions; spectral, spatial and temporal. The MODIS web site, is a good place to begin learning about this important program. This site has links to the Atmospheres, Land and Oceans groups of MODIS. Because the MODIS sensor is carried on both the Terra and Aqua satellites, it is generally possible to obtain images in the morning (Terra) and the afternoon (Aqua) for any particular location. Night time data are also available in the thermal range of the spectrum. You should consider time of day when ordering a scene for a specific day. 1. Identify Data of Interest Before placing an order for data you need to identify the type of data you wish to use. MODIS data can be placed in two broad categories; daily scenes and derived products. You can order a daily scene for any specific date and location; and at different times of day and night as mentioned above. Many consolidated products have been developed from MODIS data. These include 8-day and 16-day composite images, a variety of indices, and a range of global products with varying time scales. Products are separated into four science discipline groups. Data for each group may be obtained from specific sites and have unique import techniques described below. You can learn about each group at the following sites: Land product information can be found at: Atmosphere product descriptions can be found at Ocean data can be found at: Cryosphere data and descriptions are at the National Snow and Ice Data Center Next you need to decide where and when you want data coverage. Locations for full daily scenes and products can be entered using latitude and longitude. If you do not know this information, you could use sites such as Google Earth to locate your area of interest and read the coordinates displayed on the screen. Processed MODIS data such as the Surface Reflectance products are normally segmented into tiles with an area of 10º X 10º using a sinusoidal projection. Dates are in the Julian format, i.e. yyyyddd. There is a Julian Date Converter program on the desktops of the YCEO workstations. Time is in Universal Time Coordinated (UTC). The data are provided in the HDF-EOS format. Copyright 2014 The Yale Center for Earth Observation 1

2 Daily MODIS Scenes Level 1 Individual daily MODIS scenes, MODIS Level 1 products, can be obtained for any part of the earth, every day, since February These files are in the Geographic projection. A complete dataset has a spatial resolution of 1 km and there are 36 bands of data. The data are distributed as digital numbers in 16 bit unsigned integer format. These data should be converted to radiance values, surface reflectance values, and/or brightness/temperature values before performing any analysis. Terra file names for a complete file begin with MOD021KM. Aqua file names begin with MYD021KM. The product name on the MODIS ordering site is: MODIS/Terra Calibrated Radiances 5-Min L1B Swath 1KM V005. In addition you can obtain daily scenes at higher spatial, but lower spectral, resolution. Files with 500m resolution contain the 7 bands of data in the Visible, Near-IR and Mid- IR parts of the spectrum. Files with 250m resolution contain two bands of data in the Red and Near-IR parts of the spectrum. The Terra names for these are MOD02HKM and MOD02QKM respectively. MODIS Products - Level 2 As mentioned earlier, many products have been developed from MODIS data. You should read the product descriptions for each product you intend to use. This will provide information about data range, scaling values, fill values, data type and format, etc. Product data are generally provided in the sinusoidal projection. Below are short descriptions of the surface reflectance and vegetation products. Many more products are available. While the information below is accurate at the time of writing, you should always read the online USGS metadata for any product you plan to work with. Surface Reflectance Products The surface reflectance products are generated from the first two, or seven, bands of the corresponding full 36 band scenes. These provide an estimated at surface spectral reflectance. Several algorithms are applied to various MODIS bands to remove the effects of cirrus clouds, water vapor, aerosols and atmospheric gases. Global surface reflectance products can be obtained at either 250m with 2 bands or 500m with 7 bands, as daily or 8-day composite images. The data type is 16 bit signed integer, which has a theoretical range of values from - 32,768 to +32,768. The documented data range is from -100 to with a fill value of -28,672. If you wish to convert these numbers to a valid reflectance data range, cell values should be divided by 10,000. These data must then be stored with a floating point data type. The data are provided in the HDF-EOS format. MODIS data at version 4 and above use the Sinusoidal projection with the WGS84 datum. A very small sample of common products are listed below. Copyright 2014 The Yale Center for Earth Observation 2

3 MOD09GQ - MODIS Surface Reflectance Daily L2G Global 250m This file has a spatial resolution of 250 m and contains two bands of spectral data centered at 645 nm and 858 nm. There are also three bands of additional information on band quality, orbit and coverage, and number of observations. MOD09GA - MODIS Surface Reflectance Daily L2G Global 500m This file has a spatial resolution of 500 m and contains seven bands of spectral data plus three bands of additional information on band quality, orbit and coverage, and number of observations. The spectral range for each band can be found in Appendix A, bands 1 through 7. MOD09Q1 - MODIS Surface Reflectance 8-Day L3 Global 250m This file is a composite using eight consecutive daily 250 m images. The best observation during each eight day period, for every cell in the image, is retained. This helps reduce or eliminate clouds from a scene. The file contains the same spectral information as the daily file listed above, centered at 645 nm and 858 nm. There is one additional band of data for quality control. MOD09A1 - MODIS Surface Reflectance 8-Day L3 Global 500m This file is a composite using eight consecutive daily 500 m images. The best observation during each eight day period, for every cell in the image, is retained. This helps reduce or eliminate clouds from a scene. The file contains the same seven spectral bands of data as the daily file listed above. It also has an additional 6 bands of information concerning quality control, solar zenith, view zenith, relative azimuth, surface reflectance 500 m state flags, and surface reflectance day of year. Vegetation Index Products There are several composite MODIS vegetation products. Sixteen-day composites are available at 250 m, 500 m, 1 km, and 0.05 degree resolutions. There are also monthly composites with 1 km, and 0.05 degree resolutions. Each file contains bands of data for both the traditional Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI). There are also data quality bands in each file. The data type is 16 bit signed integer, which has a theoretical range of values from - 32,768 to +32,768. The documented data range is from to with a fill value of If you wish to convert these numbers to the traditional data range, cell values should be divided by 10,000. These data must then be stored with a float data type of IEEE 4 byte real. The Vegetation Index products have a label prefix of MOD13 for the Terra sensor and MYD13 for the Aqua sensor. On the ordering web page the product name indicates the sensor, composite period, spatial resolution, and data version number. For example, MODIS/TERRA Vegetation Indices 16-day L3 Global 250m Sin Grid V005 will get you the data from Terra for a 16-day composite at 250 m resolution in the sinusoidal projection using the version 5 data. Copyright 2014 The Yale Center for Earth Observation 3

4 2. Obtaining Data MODIS data are freely available over the Internet. Before ordering data, you should review the product descriptions and metadata. This will help you identify the most appropriate data, and more accurately import these data. Data import instructions are contained in Section 3 Importing and Image Processing. Note: While it is tempting to immediately obtain a large amount of data, you should start with just one or two data sets. Only after you have had some experience importing and actually working with these initial data should you consider ordering additional data sets. 2.1 Daily Scenes Level 1 Individual daily MODIS scenes can be obtained at the Level 1 and Atmosphere Archive and Distribution System (LAADS) at: First specify the appropriate satellite(s); Terra, Aqua, or both. Under Group select Level 1 Products for the daily scenes. Then select the Level 1B Calibrated Radiances dataset at the spatial resolution you wish. Next specify Temporal, Collection, and Spatial Selections. The temporal selection will be the starting and ending date of your research period. The Collection will be a processing version level, i.e. 4, 5 or 5. For Spatial selection you can select a MODIS tile or, perhaps more usefully, Latitude/Longitude. You can enter bounding box coordinates or drag a box in the map. Normally you will only enter the Day Coverage Selection. Once this information has been entered, click on the Search button on the bottom of the page. View the browse images of the datasets that meet your search criteria. As of this writing, Version 6 data do not have browse images online so you should make your selections using the Version 5 collection. Then select the same Version 6 dataset if you wish to use that Collection. Once you select the datasets you need, if they are Online, simply click on a file to download it. Otherwise add them to your shopping cart. When you are ready to place your order, go to the shopping cart to review the selected datasets. You should accept the default to order data without post processing and click on the Order button. Enter your address and make sure the FTP Pull option is selected. Click the Order button again. You will receive an when the data are available for download. The order notification will contain the ftp location and an order number. You can use an FTP program, or point your browser to the supplied link, then proceed to the subdirectory of your order number. 2.2 Land Products A selection of Land products are available at the LAADS site described above in the Daily scenes section. For a more complete collection of MODIS Land products you should search for data at either the GloVIS or Earth Explorer USGS sites or the NASA Copyright 2014 The Yale Center for Earth Observation 4

5 Reverb Echo site. Each site has a full collection of MODIS Land products along with different collections of other data. They each have a unique interface so the method of identifying the data you desire is a bit different. In each case you will select a location, a product, and a date of interest. You need to create a free account at each site to obtain data. The USGS GloVis site provides an easy, visual display of each tile to help you select your data. First use the image window in the upper left to navigate to your area of interest and then use the Collections tab to select the MODIS product you want. Once you select the data, click the Add button to place it in your shopping cart. When finished selecting data, place the order and you will receive s confirming the order then providing a link to the data. This is a quick process. Navigate the USGS Earth Explorer site using tabs along the upper left. Under the Search Criteria tab you are offered several methods to locate your site of interest. You can click a point on the map, manually enter coordinates, or upload a KML or shapefile. From this same screen you can filter the data by time. Next select the Data Sets tab and enter MODIS in the Search box or click on the NASA LPDAAC Collections set to locate your specific data. Click on the Results tab to perform the search and view your results. You can add image footprints to the map and view browse images to help refine your search. You can then immediately download the dataset(s) you wish. The NASA Reverb Echo site uses a three step process for getting data. Under Step 1: Select Search Criteria you will draw a box on the map to spatially define the search. Next enter a Search Term such as MODIS or MODIS NDVI, and optionally enter start and end dates. Under Step 2 Select Datasets you will select the specific product(s) you are interested in. This populates Step 3: Discover Granules. Once you fully define the data you are interested in click the Search for Granules button to advance to Step 1: Select Granules. At this point you have several ways to view individual scenes. The Image View shows nice browse images, but List, Map and Timeline Views may also be useful. Place your selected dataset(s) in the shopping cart and when you click Download you will get a text file with a link to the data. Enter the link(s) in your browser to obtain the data. Refer to section 4 below to learn how to import these data into ENVI using the MODIS Conversion ToolKit. 2.3 Atmosphere Products NASA has a good site describing MODIS atmospheric products, data formats, content, etc. Your first step should be to review this site: You can obtain MODIS Atmosphere products from the same LAADS site that MODIS Daily scenes are distributed at: You should follow the same navigation and search procedures as described above, only under Group select Atmosphere Level 2 or Level 3 Products. Copyright 2014 The Yale Center for Earth Observation 5

6 2.4 Ocean Products Ocean products have been derived from MODIS, SeaWIFS, and other sensors and are available at the NASA OceanColor site. You should read the descriptions of the products and processing levels before working with these data. 2.5 Cryosphere Products The National Snow & Ice Data Center (NSIDC) is the site to learn about MODIS snow and ice products. They have definitive data descriptions and links to data sources. As with other types of data, please read the data descriptions before downloading any data. Copyright 2014 The Yale Center for Earth Observation 6

7 3. Image Processing and Import to ENVI Format ENVI can open and process most MODIS HDF data. However, the data formats and specific processing steps differ for daily scenes and the various domain products. Daily scenes have geographic coordinate information embedded in the file. MODIS products are distributed using the sinusoidal projection in 10º tiles. Instructions for processing several types of data are described below. 3.1 MODIS Daily Scene Currently you should use ENVI Classic to open a MOD021KM or MYD021KM full MODIS daily scene. From the main menu select File Open Image File and select your dataset. ENVI automatically converts digital numbers to surface reflectance, radiance, and brightness/ temperature when it opens a full one kilometer daily scene. You will see three separate listings in the Available Bands List window; identified by band name as Reflectance, Radiance, and Emissive respectively (Figure 1). There will also be one file named Latitude and one Longitude that you can ignore. You typically will only be interested in Reflectance and Temperature (Emissivity). There will be no data layer for brightness/temperature if using 250m or 500m files. Initially these files do not display coordinate information, but ENVI can easily generate this. From the main menu select Map Figure 1 Georeference MODIS. Select a file, click on the Spectral Subset button if you wish to use only a few bands of data, and click OK. In the Georeference MODIS Parameters window select the desired output map projection and datum and click OK again. Finally in the Registration Parameters window select a folder and enter a new filename. Note: ENVI may select the UTM projection by default. Typically these datasets cover an area much larger than a single UTM zone so you should select the Geographic projection and click OK. You can spatially subset and reproject the image later if desired. When the georeferencing is complete ENVI places a new file with the prefix Warp in the Available Bands List. Double click the globe icon to display the coordinate information for this scene. You can view this in a display window and confirm the coordinate information using the Cursor Location/Value tool. 3.2 MODIS Land Products i.e. Surface Reflectance, Vegetation Indices, Atmospheric Products As noted above, the MODIS Land product data are provided in HDF format using a sinusoidal projection. You can open these directly in ENVI Standard from File Open Copyright 2014 The Yale Center for Earth Observation 7

8 As EOS MODIS or from ENVI Classic File Open External File EOS MODIS. Select the data layer(s) you wish to use and the new file will have a sinusoidal projection. If you wish to convert the data to geographic currently you must use ENVI Classic to do this. From the main menu select Map Convert Map Projection. We recommend using the MODIS Conversion Tool Kit to import these data. This is described below in Section MODIS Atmosphere Products MODIS Atmosphere products are distributed in HDF format as are the Land products. ENVI Standard can open this file from File Open As EOS MODIS but it only accesses the reflectance and coordinate layers. You are strongly encouraged to open these files using the MODIS Conversion Tool Kit described below in Section MODIS Ocean Products MODIS Ocean products must be opened with a different extension. You should use ENVI Classic to open and reproject these datasets. From ENVI Classic File Open External File Ocean Color then select an appropriate sensor and product type. From the ENVI 5.x Standard interface you can only open the Level L1A dataset. Select Toolbox Extensions convert_oc_data. 3.5 MODIS Snow and Ice Products MODIS Snow and Ice products can be opened as generic HDF files in ENVI Standard from File Open As Generic HDF4 but these files have no projection or coordinate information. We recommend using the MODIS Conversion Tool Kit to import these data and reproject to Geographic if desired. This is described below in Section MODIS Conversion Tool Kit The YCEO Lab uses a plugin module for ENVI that can be used to import and convert the projection of many MODIS products in one step. You access the Toolkit from the ENVI 5.x Standard interface Toolbox Extensions mctk or from the ENVI Classic main menu File Open External File EOS MODIS Conversion Toolkit. From the initial Toolkit dialog box click on the Input HDF button and select your file. This opens the expanded Toolkit dialog box (Figure 2). Under the Processing Options section on the right select the individual bands of data you wish to import. Figure 2 Copyright 2014 The Yale Center for Earth Observation 8

9 Under the Select Output Type section select one of the radio buttons to determine the output coordinate information. The default, Standard, will retain the sinusoidal tile projection. Selecting Projected will result in an expanded dataset in Geographic projection. You normally would NOT select the third option Both. Finally enter a Rootname and Output Path for the new dataset. If you plan to mosaic adjacent tiles you should retain the Standard (sinusoidal) Output Type. These sinusoidal tiles can be perfectly joined, whereas the projected data will have gaps at the edges. Once the tiles are mosaicked you can reproject the single seamless file into the Geographic projection. The MODIS Conversion Tool Kit and Ocean Color plugins for ENVI were developed by Devin White. He has graciously made them publicly available on GitHub at: Copyright 2014 The Yale Center for Earth Observation 9

10 Appendix A Description of the 36 Band in a Full MODIS Scene BAND RANGE nm RANGE um KEY USE reflected emitted Absolute Land Cover Transformation, Vegetation Chlorophyll Cloud Amount, Vegetation Land Cover Transformation Soil/Vegetation Differences Green Vegetation Leaf/Canopy Differences Snow/Cloud Differences Cloud Properties, Land Properties Chlorophyll Chlorophyll Chlorophyll Chlorophyll Sediments 13h Atmosphere, Sediments 13l Atmosphere, Sediments 14h Chlorophyll Fluorescence 14l Chlorophyll Fluorescence Aerosol Properties Aerosol Properties, Atmospheric Properties Atmospheric Properties, Cloud Properties Atmospheric Properties, Cloud Properties Atmospheric Properties, Cloud Properties Sea Surface Temperature Forest Fires & Volcanoes Cloud Temperature, Surface Temperature Cloud Temperature, Surface Temperature Cloud Fraction, Troposphere Temperature Cloud Fraction, Troposphere Temperature Cloud Fraction (Thin Cirrus), Troposphere Temp Mid Troposphere Humidity Upper Troposphere Humidity Surface Temperature Total Ozone Cloud Temperature, Forest Fires & Volcanoes, Surface Temp Cloud Height, Forest Fires & Volcanoes, Surface Temperature Cloud Fraction, Cloud Height Cloud Fraction, Cloud Height Cloud Fraction, Cloud Height Cloud Fraction, Cloud Height This information was obtained from the Land Process Distributed Active Archive Center website on 17 May 2010 at the following URL: Copyright 2014 The Yale Center for Earth Observation 10