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Interpreting Date Log Files

The date log tables for most composites are available for download at <http://dds.cr.usgs.gov/pub/data/avhrr/globalcomp/dateindexfiles/>.

Interpretation of date log files

The date index band, by itself, has little value without the date log table. The date log table is very simple. It is an ascii file that matches a sceneid to an index value. The following is an example portion of a date log table:

The use of this table is quite simple. The date index band has an index value for each pixel for which there is data. If the index value of the pixel is 10, as in the example, the scene from which the data for that pixel was selected is ao11051193014133. The sceneid can be parsed into its elements:

ao indicating it is an orbital segment

11 NOAA-11 scene

051193 acquired May 11, 1993

014133 acquisition start time (gmt)

With this information you can identify which orbital segment you need if you have an application needing raw data. Or you can establish the length of time the occurred between an observation of a pixel from 10-day composite to the next by parsing the date of observation. This information would be useful in accessing the degree of change overtime. The amount of time between observations of a pixel from consecutive composites can range from 1 day to 21 days, depending on plant conditions, observation angle, and availability of cloud free observations.

Remember, the composites are produced using maximum NDVI. No matter which band of a composite you analyze, the data for that pixel location comes from the same orbital segment as determined by the occurrence of the maximum NDVI value.

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