ASTER HDF Example

This example uses data in 50 thermal infrared channels as taken by the Airborne ASTER instrument. The format of the data is very similar to MODIS Airborne Simulator (MAS), i.e. HDF (not HDF-EOS). Our sample data file was supplied by Simon Hook and is available on the WebWinds data server.

This example shows how to use visual (bounding box) subsetting as well as how to geo-reference the data. If you download the script file for this example, you can create this session semi-automatically using the sample file on the WebWinds server.

Begin with the Desktop Menu (shown above). First select "File" and "New Source". This will produce a File Finder:
 
 
 

Next, if you wish to use a remote data server, enter the IP address or full name of the machine in the textbox labeled "Remote". Then, enter the file name in the "Current File" text box and the path (if it is not already defined in the paths.txt file) in the "Current Disk Path" text box and click on the "Open" button. When the process is complete (the icon is rotating) you should have:

Note that the file type (HDF) is known and that a description of this new source will be saved in "Save Folder" Temp. You can change the "Save Folder" value. Now, press the "Open" button. When the reading process is complete and the icon is again rotating, you should have a window that looks like:
 
 













In this screenshot, we have scrolled down to the area where geometric information is provided for pixels. By selecting (clicking)"PixelLongitude" and then pressing the "Add As Meta" button, we can tell WebWinds that this field will represent the independent variable longitude. We do the same for "Pixel Latitude" and then select (click on) the last file in the "Subfiles" window. This is the one that contains the actual data. At this point, we should have a window that looks like:

Note that "PixelLongitude" and "PixelLatitude" are designated as Axes. If we clicked the "Load" button, the "Status" window would return the message "Insufficient memory, use format button" since the entire data set is too large to fit into memory. Therefore, we need to subset or subsample the data. If we next click on the "Format" button, the result is
 
 













Here, we have clicked on "Subset active". Note that the 50 spectral channels are, by default, represented by "axis 2", whereas the 2 spatial axes of the image are 1 and 3 which are 716 and 4288 pixels wide respectively.

We wish to create a "thumbnail" or subsampled image. To do this, we have subsampled (by setting the numbers in the "step" text pads) all axes by a factor of 10. The spectral axis appears as axis 2 in this example. WebWinds normally displays axes 1 and 2 with axis 3 being used for different slices or wavelength bands. Therefore, we would like to alter the order of the display. This has been done in the above figure in the "Output order" box (132) which represents the displayed order. Once a selection has been made, press the "Apply" button to return to the Data Object window. Note that if the "Return" button is pressed, changes to the format are ignored. Next press the "Load" button to actually read data into memory. Usually, the rotating icon in the Desktop will slow or stop, indicating that WebWinds is hard at work reading this large file. If an "Insufficient memory?" message appears in the "status" text box, go back to the "Format" window and reduce the amount of data to be loaded. Remember to press the "Load" button when returning from the "Format" window and changes have been made.

Note that after loading, there is a new button on the ObjectDisplay window. The "Subset Image" button, when pressed, provides an image of the data we've just created. Here is what that image should then look like:

There are several things to note about this image. First, since the "RGB" button is not pressed, we're looking only at the top slice or spectral band. Second, we've pressed the "Histogram" button which stretches the black and white image. Third, the images "Freeze" button is pressed which means that from now on, no matter what we do to the data object, *this* image will remain unchanged. This happens automatically when the "Subset Image" button is pressed. Finally, there is a bounding box on this image. This box will help us to select an area to look at at full resolution. Move the bounding box so that it covers the desired region. If you press the "format" button in the ObjectDisplay, you can now choose to redraw the selected portion of the image at full spatial and spectral resolution. This is done by selecting "1" in the "1 step" and "3 step" text pads. You can load a portion of the spectral channels by moving the sliders or typing in the "2" text pads for axis 2. Note that the bounding box in the subset image window has automatically converted your selected area into "Start" and "End" values.

Next press the "Apply" button and reload the data. Now you are ready to display the data. Select "Image" from the "Tools" menu on the "Desktop". Now, drag the View Axis object into the image. Use the button.

The image may be subsampled in order to make it fit on the screen (scrolling will come in a later version). This subsampling, above and beyond that specified in the "Format" window, is indicated in the text box near the bottom of the image. The "RGB" button has been pressed in this example. It selects the beginning, middle and end channels (by default) and assigns red, green and blue colors to the intensity values in those channels. After again pressing the "Histogram" button, you should see something like:











You can alter the spectral channels for the red, green and blue components by clicking the "Line Plot" button on the image to get:









By moving the sliders, different spectral channels can be selected.

Finally, if you wish to view the data in standard geographic (latitude/longitude) coordinates, pressing the "GeoRef" button on the ObjectDisplay will produce an Image like:



 
 
WebWinds Home / Oct 5, 2001