WebÞing

Viewing the AVHRR Layers

The HyperDAAC offers browse and retrieval facilities for the Global 1KM AVHRR Dataset:

[AVHRR Scene Selection]

To load an AVHRR scene, go to the Plugin menu and select Browse AVHRR. Select Period, Month, Year and Band from the Form that pops up. The selected scene is loaded in a new Lens.

When viewing a 32Km or 64Km browse image, the scene is loaded from the browse archive on the CDROM. When viewing a scene at resolutions of 1-16Km, the AVHRR data will be retrieved from the EDC server via the Internet.

When you retrieve data over the Internet, you may choose to export it for viewing in other applications. If you select Save As MSB or LSB in the AVHRR Loader, it will write the data to a raw, uncompressed 16-bit file, which is exactly the same as you'd get using a standard Web browser. This is not relevant for the 8-bit data (NDVI,Sat.Zenith,Sol.Zenith,Rel.Azimuth and Data Index), as the original can trivially be extracted from the HyperLens Image files. The choice of LSB/MSB depends on your computer's architecture: Intel and DEC VAX based systems will use LSB, while most others use MSB.

EDC's retrieval facility comp10d relies on a customised compression format that permits fast scene extraction and decompression at different resolutions. However, it cannot extract scenes to exact pixel measurements, but will instead round up the size of an area requested. The HyperDAAC archive uses a different strategy (made possible by the smaller size: just seven full 1Km-resolution products), and can extract exact scenes. HyperDAAC therefore offers two alternative scene extraction facilities:

  1. The Query lens defines an exact scene
  2. The comp10d lens adjusts the scene to dimensions that EDC's facility can match exactly. To avoid errors arising through rounding, the comp10d lens will confirm its computations with the EDC server when extracting a scene. You will need to use the comp10d lens if you are wish to load data from the AVHRR archive into the scene.

These lenses are otherwise identical.

See also