Data Set Documentation

An Introduction to the NOAA/NASA SSM/I Pathfinder Pentad and Monthly Land Products Data Sets

Table of Contents

1.0Introduction

This data set documentation contains information on the structure of the Hierarchical Data Format (HDF) Pentad (5 days) and Monthly Land Products (LP) grid files, instructions on how to get and install HDF on your machine, and a description of two utilities-- extractlg which extracts specified items from the pentad or monthly file, and getfiledesc which pulls out the file description and prints it to the screen. Appendix 1 is a brief discussion of the technical/scientific aspects of the data set, including pertinent references.

The Pathfinder Pentad and Monthly Land Products data sets were created using the SSM/I Pathfinder daily HDF Land Products files. The daily data were grouped into a 1 degree longitude by 1 degree latitude array. The array is 360 x 180 where the first dimension is longitude and the second is latitude. Array location (1,1) contains data between 180 and 179 degrees west longitude, and 89 and 90 degrees north latitude. Array location (2, 2) contains data for the 1 degree square bounded by 179-178 degrees west longitude and 88-89 degrees north latitude. Data for each bin is accumulated for either 5 days (pentad) or one month, as appropriate.

The first pentad begins with January 1 and includes data through January 5. Each succeeding pentad is generated using 5 day increments. During leap year, the pentad beginning on February 26 contains 6 days, including February 29; the other pentads are consistent with non-leap year pentads. The monthly files include all available data in the month. Appendix 2 is a list of the pentad or monthly land products files which were not generated because of instrument or data problems.

Each pentad or monthly file contains a grid of land classification (LCG), the percentage of total pixels in the bin which represent that land classification (LCP), the total number of classes in the bin (LCN), a grid of land surface temperature (LTG), the sum of the squared values of land surface temperature (LTS), the total number of land surface temperature values in the bin (LTN), and a file description. The monthly file is similar to the pentad file. Section 2.0 contains a description of the file structure.

The files were created on a Silicon Graphics VGX class computer with version 3.3, release 4 of the HDF/netCDF library. If you are receiving these files on tape, they were transferred to tape with the UNIX tar facility. All files have been compressed with the IRIX UNIX compress command. The file size for an uncompressed file is 1.6 megabytes.

NOTE: All discussions in this text are for row major applications written in C. If you are using the HDF FORTRAN interface to read the HDF objects, the arrays will be transposed. For example, array A(5,3) in an HDF C interface would become A(3,5) in an HDF FORTRAN interface. This will apply to all HDF data array discussions in this text.

2.0 Pentad and Monthly Pathfinder HDF File Structure

Each pentad or monthly land products file has the following contents:

Table 1.

Objects in the Pathfinder HDF Pentad (Monthly) Land products File

ITEM HDF OBJECT TYPE HDF REF No.
Version Descriptor N/A 1
Pentad (or Monthly) land classification (LCG) Scientific Data Set 2
Percentage of Total Pixels (LCP) Scientific Data Set 3
Number of land classification pixels per bin (LCN) Scientific Data Set 4
Pentad (or Monthly) Land surface temperature (LTG) Scientific Data Set 5
Sum of Squared land surface temperature values (LTS) Scientific Data Set 6
Number of land surface temperature pixels per bin (LTN) Scientific Data Set 7
File Description Annotation 8

Table 2. shows the flag used in the pentad and monthly land products objects. Valid calculated values of surface temperature are scaled before being stored. That is, the pentad or monthly land surface temperature values are multiplied by 10 and then stored as integers. To retrieve a pentad or monthly land surface temperature, you must divide the stored number by 10.

Table 2. Flag For Land Products

FLAG 1
LCG -10
LCP -10
LCN -10
LTG -10
LTS -10
LTN -10

where:

LCG = Pentad or Monthly Land Classification
LCP = Percentage of Total Pixels
LCN = Number of Land Classes per Bin
LTG = Pentad or Monthly Land Surface Temperature in degrees Kelvin
LTS = Sum of the Squared Land Surface Temperatures
LTN = Number of Land Surface Temperature Pixels per Bin
FLAG 1 = No land products data were accumulated in the bin (or missing).

The following sections provide further details on each of the HDF land products:

2.1 Pentad (Monthly) Land Classification (LCG)

This HDF object contains pentad or monthly land classifications derived from the most frequently occurring land classification in a given bin. Each value is stored as a 4-byte integer. There is no scaling factor. Table 3 shows the land classification codes and the associated description.

Table 3. Land Classification Codes

Code Number Land Classification
1 dense vegetation
2 composite vegetation & water
3 dense agriculture/range vegetation
4 precipitation over vegetation
6 composite soil & water/wet soil
7 flooded conditions
8 precipitation over soil
9 dry arable soil/medium vegetation
10 desert
13 refrozen snow
14 dry snow
15 semi-arid surface
19 wet snow

The HDF data type used to store the land classification values is DFNT_INT32. It will be necessary to use this type in any software written to access the data. The HDF reference number is 2.

2.2 Percentage of Total Pixels (LCP)

This object contains a grid of the percentage of the total number of pixels which represent the specified land classification in each bin of the grid. A flag value, shown above, is stored in locations where the land classification could not or should not be assigned.

The array size is 360 x 180. The data are stored as 4-byte integers and HDF data type DFNT_INT32. The HDF reference number is 3.

2.3 Number of Land Classes per Bin (LCN)

This object contains the total number of land classifications which have been grouped into each bin in the LCG object; only valid land classifications (i.e., no flag values) are counted. The daily land classification pixels are counted in each bin and a count of the different land classes is kept for the specified number of days and stored in this object. A flag value, shown above, is stored in locations where the land classification could not or should not be assigned.

The array size is 360 x 180. The values are stored as 4-byte integers and HDF data type DFNT_INT32. The HDF reference number is 4.

2.4 Land Surface Temperature (LTG)

This HDF object contains land surface temperatures. The values are in degrees Kelvin. After multiplication by 10, the temperatures are stored as 4-byte integers. Surface temperatures can not be calculated for all land classification types (e.g., snow surfaces). For these situations, a flag value is used.

The array size is 360 x 180. The HDF data type used to store the land surface temperature is DFNT_INT32. The HDF reference number is 5.

2.5 Sum of the Squared Land Surface Temperatures (LTS)

This object contains a grid of the sum of the squared land surface temperature values which are included in each bin of the grid. Daily valid land surface temperatures within a bin for the specified number of days were squared and summed. A flag value, shown above, is stored in locations where the land surface temperatures could not or should not be calculated.

The array size is 360 x 180. The data are stored as 4-byte integers and HDF data type DFNT_INT32. The HDF reference number is 6.

2.6 Number of Land Surface Temperature Pixels per Bin (LTN)

This object contains the number of land surface temperature pixels which have been grouped into each bin in the LTG object. Only valid land surface temperature values ( i.e., no flag values) are counted. The daily land surface temperature pixels are counted in each bin for the specified number of days and stored in this object. A flag value, shown above, is stored in locations where the land surface temperatures could not or should not be calculated.

The array size is 360 x 180. The values are stored as 4-byte integers and HDF data type DFNT_INT32. The HDF reference number is 7.

2.7 File Description

This object is an ASCII description of the pentad or monthly LP grid file. The description can be read with the program "getfiledesc". Section 3.0 contains instructions on how to get the HDF library and section 4.0 contains information on how to compile and create getfiledesc. The HDF object reference number is 8.

An example of a file description is:

SSM/I Pathfinder Land Products
File ID = Land.mon_87213_87243.hdf
This is a LEVEL 3 product.
This product is a monthly composite grid,
including Julian day 87213
through Julian day 87243.
This grid includes 31 days of data.
The grid is a 1-degree by 1-degree
longitude/latitude grid; grid location
(1,1), in the upper left corner, is
located at 90 deg N latitude, 180 deg
longitude. The gridbox covers 1 degree
(90-89 N, 180-179 W) from that location.

SSM/I PATHFINDER Software Version Number 1.0
File Structure Version Number 1.0
HDF Version Number 3.3, Release 3
Tool Set Version Numbers:
extractlg 1.0
getfiledesc 1.0

For more information on the technical content please contact NSIDC User Services.

3.0 How To Get The HDF Library And Tools

HDF Utilities

All land surface data are stored in Hierarchical Data Format (HDF). A thorough discussion of HDF command line utilities is beyond the scope of this manual. However, HDF and HDF utilities are a public domain software developed by The HDF Group. Up-to-date HDF source code, documentation, HDF newsletters, and user support can all be found at The HDF Group Web site.

The contents of HDF are:

README                  describing files and subdirectories in ftp/HDF/. 
FAQ                     frequently asked questions about HDF. 
Documentation/          HDF documentation 
HDF_Current/            Points to most current release of HDF 
HDF3.3r4/               HDF 3.3 release 4 (latest release) 
HDF4.0.alpha/           HDF 4.0 alpha release 
HDF3.3r3/               HDF 3.3 release 3 
prev_releases/          releases previous to HDF 3.3r4: HDF 3.3r3 and HDF 3.2r4 
contrib/                contributions from HDF users outside and inside NCSA 
examples/               examples of HDF programs_good for testing, too 
newsletters/            HDF newsletters 
tarexamples/            compressed tar files of examples 
HDFVset/                README  _ where to get old/new version of HDFVset 
HDF-UCD/                HDF-UCD versions 1.0, 1.1, 1.2, and HDF-UCD documentation 

4.0 How To Compile the Programs extractlg and getfiledesc

Two programs are included with this distribution. They are extractlg.f and getfiledesc.c. Both run on Silicon Graphics, but port easily to other platforms. A makefile, named Make.lp.3, is included with the distribution. It can be used to compile the programs on a UNIX platform. After making changes to the directory paths and flags, use the following UNIX command to run the make-file program:

make -f Make.lp_3 all

This will compile both programs, and two executables will result: "extractlg" and "getfiledesc"

4.1 extractlg

The program, extractlg.f, extracts an HDF object from the pentad or monthly file. It produces a new HDF file with the name of the HDF object selected. The format for using the utility is: extractlg.

For example, the command:

extractlg Land.pen_87241_87245.hdf LCG

will produce a file called LCG_pen.87241. Run the program with no arguments, and it will list all of the options as shown below:

CODE Description Object Type Variable Type
LCG Land Classification Grid SDS INT32
LCP LCG Percent - Total Pixels Grid SDS INT32
LCN LCG Number of Classes in Bin SDS INT32
LTG Land Surface Temperature Grid SDS INT32
LTS LTG Sum Of Squares Grid SDS INT32
LTN LTG Number of Pixels in Bin SDS INT32

(where SDS is Scientific Data Set and INT32 is a 32 bit integer)

Use the code when selecting an object to extract. You may select several codes at once, separated by a space.

4.2 getfiledesc

The getfiledesc program prints the text of the HDF annotation contained in the pentad or monthly file. Its usage is:
getfiledesc <PENTAD OR MONTHLY LP FILENAME>.
The result should look like the example file description found in section 2.7, File Description.

Appendix 1 - Land products

The Land Surface Type Classification Algorithm (Neale et al. 1990) was derived during the calibration/validation of the SSM/I instrument. Valid surface types in the Pathfinder land products data sets were accumulated in 1.0 degree latitude and longitude bins and the most frequent value or mode was selected as the representative land surface type for that bin. Other parameters included in the files were accumulated during the process.

Valid values of the land surface temperatures from the NOAA/NASA SSM/I PATHFINDER level 2 swath data were accumulated in the bins and the bin average value was calculated. The individual valid bin values were also squared and summed.

REFERENCES

Holinger, J. 1991. DMSP Special Sensor Microwave/Imager Calibration/Validation, Final Report, Vol II. Washington, DC, Naval Research Laboratory.

McFarland, M. J., and C. Neale. 1991. Land Parameter Algorithm Validation and Calibration, DMSP Special Sensor Microwave/Imager Calibration/Validation, Final Report, Volume II, Chapter 9. Washington, DC, Naval Research Laboratory.

McFarland, M.J., R. L. Miller, and C. M. U. Neale. 1990. Land Surface Temperature Derived From the SSM/I Passive Microwave Brightness Temperatures, IEEE Transactions on Geoscience and Remote Sensing, 28(5):839-845.

Neale, C.M.U., M.J. McFarland, and K. Chang. 1990. Land Surface-Type Classification Using Microwave Brightness Temperatures From The Special Sensor Microwave/Imager, IEEE Transactions on Geoscience and Remote Sensing. 28(5):829-838.

Appendix 2 - Anomalous Pentad and Monthly Land Products Files

1987 DMSP F8 Satellite

Pentad for the period October 3-7,1987 (87276-87280) has 1 day of data missing.

The December 1987 monthly file was not generated because the SSM/I instrument was turned off on December 3, 1987 due to overheating.

Pentads starting December 2, 1987 (day 336) through December 31, 1987 were not generated for the same reason.

1988 DMSP F8 Satellite

The pentads for the periods January 1- January 5, January 6-January 10, January 11 - January 15, 1988 were not generated because the instrument was not turned on until January 13, 1988.

The January 1988 monthly file was not generated.

The following pentads contained less than 5 days of data: May 6-10,1988 (88127-88131) has 3 days of data missing.

September 23-27, 1988 (88267-88271) has 1 day of data missing.

December 22-26, 1988 (88357-88361) has 2 days of data missing.

December 27-31, 1988 (88362-88366) has 1 day of data missing.

Contact Information:

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Document Information:

Revision Date: 30 January 1997
Review Date: 30 January 1997
Document ID: nsidc-0042
Citation: This field is intentionally left blank.
Document Curators: NSIDC Writers
Document URL: http://nsidc.org/data/docs/daac/nsidc0042_ssmi_land.gd.html