1.8   Opening hierarchical data files

Satellite images often come in hierarchically structured formats that include additional data apart from the images themselves. Many of the datasets available on the Internet are stored in one of these formats. Bilko's ability to read these files gives you access to huge amounts of Earth observation data from a wide range of satelllites.

The example we will use here is global SST image for January 2003, derived from measurements made by the AATSR instrument onboard the European Space Agency's Envisat satellite. The file name is ATS_10arcmin_DSST_0301.hdf

Note: If you have not already downloaded this image, please do so now, using the downoad link on the left sidebar (ATS~DSST~0301). Beneath the download links on the sidebar you can find information about the image, which may be useful for studying the data contained in the file.

A hierarchical structure

Open the file ATS_10arcmin_DSST_0301.hdf as described earlier ( T1.1 ), making sure the Extract box on the Open dialog is checked. This will bring up the File structure window ( figure (5K)), which has two frames.

• The left frame displays the hierarchical structure of the file. Folders that contain more than one dataset (or subfolder) may be expanded by clicking on the icon_expand.gif to the left of the folder. You select folders in this frame by clicking on them once with your mouse. The data sets it contains will then appear in the right frame.
• The right frame displays information about a selected data set - such as grid size, and numerical format. To open a dataset you first select it in the left frame, and then you double-click on it in this frame.

Activity: Expand the Map folder and select the different data sets it contains in turn, using the information you see there, and the information in the different sidebar pop-ups to answer the questions below.

Question 1
a)	How many data fields are contained within the Map folder, what are they called, and what information do they contain?
b)	What is the numerical format of this data, and how does this represent the SST derived from the satellite measurements?
c)	Can you think of any reason why it might be useful to have not just the mean SST information, but also the data contained in Pixel grid and Number grid?
d)	Read the sidebar pop-up about the different file types supported by Bilko HDF is one of the hierarchical data files that Bilko can read. What are the other two?

Select the Data folder in the left frame of the file structure window, and double click on it in the right frame. This will open the Extract dialog. Use your answer to the question below to decide how you want to set the Extract options.

Question 2
a)	What is the size of the data grid used for this image?
b)	How does this compare with the data grid used for the ATSR images from 1997 and 1998?
c)	What sample rate would you use to open the AATSR image into a grid comparable to that of the two ATSR images?

Use your answer to question 2c to set the Sample rate used for opening the AATSR image, and click OK.

The Redisplay dialog

Unlike with the previous two images, setting and accepting the Extract options does not open the image. Instead a new dialog appears - the Redisplay dialog ( figure (6K)).

The reason for this is the numerical format used in the image. A standard computer display uses 8-bit data - with the digital values 0-255. This image, however, contains real temperature values, which means that 32-bit floating point data has to be used. The settings in the Redisplay dialogue determines how these real values are 'mapped' onto the 8-bit values of the display.

We will look at this mapping in more details in section 2 of the tutorial. For the moment you will allow Bilko to choose the default settings that best suit this image. To achieve this, however, there is one change you need to make; you must designate the null value used for this image. The use of null values is explained in the sidebar pop-up called null values and missing data. Read this and use the information from the pop-up and the Redisplay dialog to answer the question below.