The CSIRO Marine Research Remote Sensing facility automatically receives and archives data from the USA's National Oceanographic and Atmospheric Administration (NOAA) satellites. Up to 18 passes per day are tracked to receive data. The Advanced Very High Resolution Radiometer (AVHRR) data is received on the High Resolution Picture Transmission (HRPT) signal. Within an hour of reception, these data are automatically processed into full resolution sea surface temperature (SST) images. Here, the land pixels and most cloud pixels are flagged with constant values.
An automatic process each day prepares a composite SST image from data collected as above and from similar NOAA HRPT data automatically downloaded from other receiving stations in Australia. These are:
ACRES (Australian Centre for Remote Sensing, Alice Springs, part of GEOSCIENCE Australia)
Australian Institute of Marine Science , Townsville.
Bureau of Meterology , Darwin site.
Western Australian Dept of Land Information and WASTAC (Western Australian Satellite Technology and Applications Consortium) in Perth.
The image you see here includes data from the latest complete six days and is usually less than two days old. Compositing attempts to overcome the problem of cloud coverage. The compositing technique used here takes the median value of a 4 by 4 neighbouhood of 1km resolution pixels over all the data available for that time period. The resolution has then been reduced to approximately 6 km. If there has not been at least one cloud free view of a point on the ground during the composite period, the pixel in the image that corresponds to this point will either be white or show a false low temperature.
The point you select and data retrieved is for some pixel within a 6 by 6 km region.
The GIF image is identified as an active or clickable image, but no image mapping is applied. Rather, the server passes to the "cgi" script the GIF image coordinates of the click point. Knowing the origin of the GIF image, it is a basic calculation to identify the relevant pixel latitude and longitude.
Another script extracts the 8-bit value (the colour) which is converted to SST in Celsius. The script also calculates the nearest longitude and latitude reference for the sample point. The information is sent back to the server via the calling cgi script.
It is important to understand that all the values are approximates and have been rounded to the nearest 1/10th of a degree.
Hint
Once you've selected the sample point closest to your region of interest, bookmark the result HTML URL to extract updated values for the same point later on.
eg.
http://www.marine.csiro.au/cgi-bin/RS_click.cgi?266,277
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