AVHRR

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Composite Images

Data Description

This section of the website contains time-series of level 3 composite maps for most of our routinely processed areas. View areas and data availability using the Data Portal. Composite images are derived by merging together cloud-free patches from all individual scenes within a week or month. Clicking on the colour thumbnails will display a higher resolution colour image, and clicking on the date will display a high resolution (non-annotated) grey-scale verson.

You must be a registered user to obtain free access to the thumbnail composite images. Please note that access to the high resolution composite images is restricted to certain authorised users, see Data access.

What is the composite browser directory structure?

What are the composite image filename formats?

Caution on interpretation of composite maps

The patchiness seen in composites is rarely genuine: this artifact is inevitable since each pixel of a composite is the average (usually median) of the corresponding pixels in the source images. The underlying values change during the compositing period, and variable cloud cover cause uncertainty in the measurement of the average and hence patchiness in the resulting map.

There are also measurement errors, for instance for SST data the sea surface warms up during the day, so more accurate composites are derived from nighttime images only, but these composites are likely to have less coverage than if all images were used. Errors may also be due to clouds which are not completely masked by the automated cloud detection algorithms, and hence contaminate the product value.