Note: The high resolution data order form is only available to registered users. Please read the High Resolution Data documentation if you wish to become a registered HRPT customer.

Which Orderform?

High resolution data, as received from the satellite, is stored in HRPT format. Each scanline combines the image data for all bands or channels from the scanner (5 channels for AVHRR, 8 for SeaWiFS) along with other information such as calibration data, TIP data, spacecraft health, etc. (For full details see the formats documentation). You can order either the HRPT data or simply the extracted image data.

HRPT data
Unless you have software that can cope with HRPT or HDF format files then this option will not be of much interest.
Image data
The image data extracted from the HRPT data. Each channel is available in separate files allowing for easy display and processing.

See below for common features of both HRPT and Image orders.


HRPT Data Orderform

There are several formats for HRPT data; the range differs depending on the satellite scanner.

AVHRR formats include:

SeaWiFS formats include:

The buffered HRPT options are available because some systems can process files faster when buffered to multiples of their I/O block size. The File Formats documentation describes the data layout in more detail and lists programs which can process the data. Ensure that you can process the HRPT format you are ordering before you order it.

The range of lines extracted is determined by the top left and bottom right coordinates (see below for details). The left and right values are not important as each line is the full scan width.

There is an option to receive a calibration file with your AVHRR orders. It will contain a value (percentage albedo or temperature in Kelvin as appropriate) for each channel, for each scan value (0 to 1023). A future option may include scanner angles too, useful for calculating true albedo, sun glint etc.

The option "Full scan width required" applies only to SeaWiFS data in level-1a format and should normally remain selected (ticked). SeaWiFS level-1a data can be ordered as a region rather than the full scan width in order to save space. This option is currently unavailable as NASA software for processing level-1a data cannot cope with reduced-width images.


Image Data Orderform

There are currently the following image formats available:

10-bit data is actually packaged as 10 bits in 16-bit words. In common with the floating point data, it is available in little- or big-endian byte ordering (for example, PC or Sun respectively). The File Formats documentation describes the data layout in more detail. If you select 10-bit data with a grid overlay the appropriate grid pixels will simply have their top (most significant) bit set, allowing you to remove the grid at a later date without affecting the underlying pixel values. If you request calibrated data then the real values are scaled to fit by taking the first 40% of albedo (visible channels) and the range 250 to 320 Kelvin (thermal channels).

Floating point data uses a 4-byte word in IEEE floating point format for each pixel. If you request calibrated data then the values will represent albedo (percentage) or brightness temperature (Kelvin) as appropriate for the channel from the AVHRR scanner, or sensor calibrated radiance values from the SeaWiFS scanner.

The percentage albedo values can be normalised by diving by the cosine of the solar zenith angle; if you wish to do this then please ensure you request the angle files along with your order.

Future image formats may include (depending on demand):

You are able to specify coordinates and dimensions for extracting a particular image area. Coordinates should be entered as the latitude and longitude of the top-left and bottom-right corners (see below for details).

The image data can be extracted in one of the following projections:

For the HRPT projections (HRPT Linearised and HRPT Unlinearised) the output dimensions should (but may not) be specified as zero as the system can determine the natural size for such areas. All other projections require non-zero values for the width and height.

A grid can overlay the image showing coastlines and latitude / longitude lines spaced at 5 degree intervals.

There is an option to receive calibration files with your image orders. They will contain a value (percentage albedo or temperature in Kelvin as appropriate) for each scan value (0 to 1023).

There is an option to receive zenith and azimuth angle files for both satellite and sun with your image orders. The four files will each contain a 32-bit floating point angle for every pixel in the image. The filenames will have extensions satazi.32B, satzen.32B, solazi.32B and solzen.32B.


Common features

Coordinates

Coordinates are entered as latitude and longitude. and should be in the form angle N angle W or angle N angle E, where the angles are in degrees (-90 to +90 for latitude, 0 to 180 for longitude). When specifying latitude and longitude coordinates for reprojected image data (except for the HRPT projections) it is necessary to enter values for output dimensions (width and height) as the pixels will no longer correspond exactly to those in the raw file. Suitable values should be chosen to give an appropriate aspect ratio, since those suggested will be for an HRPT projection. For all projections the coordinates must lie within the pass.

Data transfer

Unless you requested manual data transfer the files will be transferred by ftp to your registered site. They should start to arrive very soon after the order is submitted but depending on the size of the processing queue and the network bandwidth this may take a little while.

If you requested manual data transfer the output files will be left in your private directory. You can then retrieve them using your Web browser using the same account name and password as used for ordering. If you haven't retrieved them within a set period (24 hours?) they will be deleted and you will have to resubmit your order.

Compression

Both HRPT data and Image data files can be compressed to speed up transfers over the network. Compressed files are compatible with GNU Zip and so can be uncompressed with gunzip on unix or a similar utility on other machines (WinZip works on Windows).

Save Profile

To save time and reduce errors when completing the orderform you can tick the option to save a default profile. Values saved include: channels, coordinates, output dimensions, projection and grid overlay. (Note that the output filetype can be set from the preferences editor). The values are entered into the form displayed when no region has been selected.

Current limitations

Currently the following options have some limitations:


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