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MS2GT: The MODIS Swath-to-Grid Toolbox

Tutorial 3: Gridding 500 m Snow Cover Data over Colorado Using mod10_l2.pl

• Requirements
• Searching for the Data
• Ordering and Downloading the Data
• Creating the mod10_l2.pl Command File
• Creating the listfile
• Creating the gpd and mpp files
• Creating the latlonlistfile
• Running the mod10_l2.pl Command File
• Examining the Results

Requirements

The MS2GT installation provides most of the files you will need for the tutorials. The tutorial shows the process you need to use to create the files when performing operations on data files. The MS2GT installation does not provide the data files, so you will need to find and order the data through the EDG.

In this tutorial, we will put some MODIS 500 m snow cover swath data covering Colorado into an Azimuthal Equal Area spherical projection centered on 40 N 105 W, with the vertical axis of the grid through the center pointing due north towards the top of the grid. We want the grid resolution to be 500 m and we want to use a spherical earth radius value of 6370.997 km. The upper left corner of the grid is at about 44 N 110 W and the lower right corner of the grid at about 36 N 100 W. We want to grid all available MOD10_L2 "channels." These include channel 1 Snow Cover and channel 2 Snow Cover Pixel QA. We could simply order MOD10_L2 data. As you can see in Supported Data Sets, the latlon data are stored at only 5-km resolution in MOD10_L2 granules. We could have mod10_l2.pl work with only the MOD10_L2 granules (i.e. by setting latlonlistfile to "none"), but we decide to order the corresponding MOD03 granules as well, in order to minimize interpolation error.

NOTE: To run this example, you need a machine with at least 100 MB of memory and at least 300 MB of free disk space.

Searching for the Data

We know that the afternoon of September 25, 2000 was fairly clear over Colorado and there was a significant amount of snow on the ground, so we use the EDG to order two MOD10_L2 granules and the corresponding two MOD03 granules acquired on September 25, 2000 at 1745 and 1750 that appear to cover Colorado and the neighboring region. You can use the following values for performing the search using the EDG:

Data Set
MODIS/TERRA SNOW COVER 5-MIN L2 SWATH 500M
MODIS/TERRA GEOLOCATION FIELDS 5-MIN L1A SWATH 1KM

Search Area
Type in Lat/Lon Range:
Northern latitude: 44.0000
Southern latitude: 36.0000
Western longitude: -110.0000
Eastern longitude: -100.0000

Start Date: 2000-09-25  Time (UTC): 17:00:00
End Date:   2000-09-25  Time (UTC): 18:00:00

The search should find four granules having the following names:

MOD03.A2000269.1745.002.2000282120258.hdf
MOD03.A2000269.1750.002.2000282120547.hdf
MOD10_L2.A2000269.1745.002.2000290231013.hdf
MOD10_L2.A2000269.1750.002.2000290231230.hdf

Note that September 25, 2000 is day-of-year 269.

Ordering and Downloading the Data

Order and download the above files to the ms2gt/tutorial_3 directory, where you should have at least 300 MB of free disk space. (You can also download the *.met files that accompany the *.hdf files, but the MS2GT software doesn't use them.)

Creating the mod10_l2.pl Command File

In the ms2gt/tutorial_3 directory, create a text file called colo_2000269_1745.csh, and add the following line:

mod10_l2.pl . colo_2000269_1745 listfile.txt Colorado500.gpd 12 latlonlist.txt

This command specifies the following information (see mod10_l2.pl):

Make colo_2000269_1745.csh executable by typing:

chmod +x colo_2000269_1745.csh

Creating the listfile

In the ms2gt/tutorial_3 directory, create a text file called listfile.txt, and add the following two lines:

MOD10_L2.A2000269.1745.002.2000290231013.hdf
MOD10_L2.A2000269.1750.002.2000290231230.hdf

Note that we list the MOD10_L2 files to be gridded.

Creating the gpd and mpp files

See Points, Pixels, Grids, and Cells for a description of the gpd and mpp file formats used by the mapx library in defining a grid and its associated map projection. In the previous two tutorials, we modified existing gpd files and used an existing mpp file (N200correct.mpp) that specified EASE-Grid projections. Here we will create gpd and mpp files from scratch. We'll start with creating the mpp file, Colorado.mpp, in the ms2gt/grids directory (or copy Colorado.mpp from the ms2gt/tutorial_3 directory to the ms2gt/grids directory):

Azimuthal Equal-Area
40.0    -105.0  lat0 lon0
0.0             rotation
100.0           scale (km/map unit)
40.0    -105.0  center lat lon
20.0    60.0    lat min max
-135.0  -75.00  lon min max
 10.00 15.00    grid
0.00    0.00    label lat lon
1 0 0           cil bdy riv
6370.997        Earth equatorial radius (km) -- gctp

• The first line specifies the projection we wish to use: Azimuthal Equal-Area. Since we didn't specify ellipsoid, a spherical projection is assumed.
• The second line specifies the map projection origin: 40 N 105 W.
• The third line specifies the rotation: 0 degrees. This will produce a map with the vertical axis through the center pointing due north towards the top of the map.
• The fourth line specifies an arbitrary scale for the map as opposed to the grid, which will be defined by the gpd file as grid cells per map unit. Here we define a map unit to be 100 km.
• The fifth line specifies the center of the map which is usually (but not necessarily) the map projection origin. Here we set it equal to the map projection origin: 40 N 105 W.
• The next five lines (the sixth through tenth lines) specify parameters that would be useful to programs that produce graphic overlays (see Points, Pixels, Grids, and Cells). They are not used by the MS2GT software, are present in the mpp file as place holders.
• The eleventh line, if present, specifies the equatorial radius to use instead of the default 6371.228 km. Here we specify 6370.997 km, which is the default radius used by the General Cartographic Transformation Package (GCTP).

In preparing the gpd file that defines our grid, we must know following:

• The name of the mpp file that will define our map: Colorado.mpp.
• The number of columns and rows. We don't know these yet, but we do know:
  • There are about 40000 km / 360 degrees = 111 km/deg in longitude at the equator or in latitude anywhere.
  • We want our grid to have 500 m per cell = 0.5 km/cell.
  • We need to span about 10 degrees in longitude (110 W to 100 W) at about 40 N. This works out to about 10 deg * cos(40 deg) * 111 km/deg / (0.5 km/cell) = 1700 cells in longitude = 1700 columns.
  • We need to span about 8 degrees in latitude (44 N to 36 N). This works out to about 8 deg * 111 km/deg / (0.5 km/cell) = 1776 cells in latitude = 1776 rows.
• The number of grid cells per map unit. This is equal to 100 km/map unit / (0.5 km/cell) = 200 cells/map unit
• The grid cell coordinates of the center of the map. Since we want the center of the map to be the center of our grid, and since we have an even number of rows and columns in our grid, the center of our grid will be at the joint corner of the center four cells. But since grid cell coordinates are measured relative to the center of the upper left cell, it turns out that the grid cell coordinates of the center of the map are (1700 columns / 2) - 0.5 = column 849.5 and (1776 rows / 2) - 0.5 = row 887.5.

We now have all the information we need to create the gpd file, called Colorado500.gpd in the ms2gt/grids directory. (Or copy Colorado500.gpd from the ms2gt/tutorial_3 directory to the ms2gt/grids directory):

Colorado.mpp    map projection parameters       # azimuthal equal area
1700 1776       columns rows                    #
200             grid cells per map unit         # 500 meters
849.5   887.5   origin column, row              # center at 40N 150W

Once Colorado500.gpd has been created in the ms2gt/grids directory, use gtest to check that the latitude and longitude values of the upper left and lower right corners are close to what we want and that the center is exactly what we want:

gtest

enter .gpd file name: Colorado500.gpd
> assuming old style fixed format file

gpd: Colorado500.gpd
mpp:Colorado.mpp

forward_grid:
enter lat lon:

inverse_grid:
enter r s: 0 0
lat,lon = 43.875530 -110.300537    status = 1
col,row = -0.000061 0.000366    status = 1
enter r s: 1699 1775
lat,lon = 35.909435 -100.283951    status = 1
col,row = 1699.000000 1774.999634    status = 1
enter r s: 849.5 887.5
lat,lon = 40.000000 -105.000000    status = 1
col,row = 849.500000 887.500000    status = 1
enter r s:

enter .gpd file name:

You can see that the upper left corner values of 43.875530 -110.300537 are close to our target values of 44 N and 110 W ,and that the lower right corner values of 35.909435 -100.283951 are close to our target values of 36 N and 100 W. The center values are exactly equal to our target value of 40 N 105 W.

Creating the latlonlistfile

In the ms2gt/tutorial_3 directory, create a text file called latlonlist.txt, and add the following two lines:

MOD03.A2000269.1745.002.2000282120258.hdf
MOD03.A2000269.1750.002.2000282120547.hdf

Note that we list the MOD03 files from which to extract the 1-km latitude and longitude data to be used in place of the latlon data in the corresponding MOD10_L2 files in listfile.

Running the mod10_l2.pl Command File

Run the shell script containing the mod10_l2.pl command by changing to the ms2gt/tutorial_3 directory, and then typing:

colo_2000269_1745.csh

You'll see messages display while the mod10_l2.pl script runs various IDL and C programs.

In this example, the programs are:

1. extract_latlon - an IDL procedure for extracting latitude and longitude data from a MOD10_L2 or MOD03 file. This program calls another IDL procedure, modis_ancillary_read. In this example, extract_latlon is called twice, once for each of the two MOD03 files. Two binary floating-point files are created per call containing latitude and longitude data, respectively. The mod10_l2.pl script concatenates the two latitude files and the two longitude files to create a single latitude file and a single longitude file, and the pre-concatenated files are deleted.
2. extract_chan - an IDL procedure for extracting channel data from a MOD10_L2 file. This program calls another IDL procedure, modis_snow_read. In this example, extract_chan is called four times: two times for each of the two MOD10_L2 files; on each call, channel 1 or channel 2 is extracted. One binary byte file is created per call containing the channel data. The mod10_l2.pl script concatenates each pair of channel files, creates two concatenated channel files, and then deletes the pre-concatenated channel files.
3. ll2cr - a C program for converting latitude, longitude pairs to column, row pairs for a particular grid. The grid specified in this example is Colorado500.gpd. The concatenated latitude and longitude files are read and two binary floating-point files are created containing column and row numbers, respectively. The mod10_l2.pl script then deletes the concatenated latitude and longitude files.
4. interp_colrow - an IDL procedure for interpolating column, row pairs from a lower-resolution swath format to a higher-resolution swath format, in this case from 1 km to 500 m. The interpolation must be performed on a scan's worth of data at a time because the column and row numbers are discontinuities at scan boundaries. The interp_colrow procedure calls a function called congridx for each scan's worth of column and row arrays. The congridx function is called once for the column array and once for the row array. The congridx function first performs an extrapolation of the given array to a slightly expanded array, which it then interpolates (bicubic interpolation is used here) to a fully expanded array. The final array is extracted from the fully expanded array. The mod10_l2.pl script then deletes the pre-interpolated column and row files.
5. fornav - a C program for performing forward navigation from a swath to a grid. In this example, fornav is called two times, once for each of the two concatenated channel files. On each call, the column and row files are read as well. An elliptical weighted maximum algorithm is applied during forward navigation to minimize holes and aliasing in the gridded data. One binary byte file is created per call containing the gridded data. The mod10_l2.pl script then deletes the concatenated channel files as well as the column and row files.

The final message should contain the string:

MOD10_L2: MESSAGE: done

Examining the Results

Enter the command:

ls -l *.img

You should see something like this:

-rw-r--r--    1 haran    nsidc     3019200 Apr 27 13:04 colo_2000269_1745_rawm_snow_01700_01776.img
-rw-r--r--    1 haran    nsidc     3019200 Apr 27 13:04 colo_2000269_1745_rawm_snqa_01700_01776.img

Each file contains a gridded array of 1700 columns and 1776 rows of binary byte values (1700 * 1776 * 1 = 3019200 bytes).

The file naming convention for gridded MOD10_L2 files is as follows:

<tag>_<conversion><weight_type>_<chan>_<columns>_<rows>.img

• <tag> is the mod10_l2.pl tag parameter
• <conversion> is:
  • raw - raw (1-byte unsigned integers)
• <weight_type> is:
  • m - maximum
• <chan> is the channel name and is one of:
  • snow - channel 1 - Snow Cover - 8-bit unsigned
    • See MODIS/Terra Snow Cover L2 and L3 Daily and 8-Day 500 m for a table of coded values.
  • snqa - channel 2 - Snow Cover PixelQA - 8-bit unsigned
    • See MODIS Snow Cover Bit Processing for a description of coded values.
• <columns> is the number of columns in the grid
• <rows> is the number of rows in the grid

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