User's Guide


The Image Composite Explorer (ICE) was developed for NASA’s Earth Observatory as a tool to let people explore satellite image data. The ICE is a developmental “on ramp” to the world of Earth system science. It is designed to demonstrate how scientists use multi-spectral data to examine our world. Within its intended purpose, the ICE is flexible and can be easily configured teach various lessons using simple HTML parameters, as described in the ICE Programmer’s Guide.

The ICE is coded in Java to run as an applet under common Web browsers. Users on Apple computers, however, will need to use Internet Explorer and the Apple MRJ (Java runtime).

End-user environment

The ICE is run through a Web browser. It was coded to the Java 1.1 API. The ICE has been tested and shown to run on the most common browsers and operating systems. (Here is a complete list.)

ICE runs best with your computer monitor set to 16-bit color on the desktop. The 8-bit (256) color setting generally does not have enough colors, while the 32-bit (true color) setting substantially slows down the program’s interactivity since it forces the creation of images that are 4 bytes per pixel.

The ICE interface provides three thumbnail images in a row across the top, and a larger window beneath that displays one of the thumbnail images currently shown in one of the thumbnails (user’s choice). A label appears above the thumbnails to indicate what wavelength or measurement (sometimes referred to as data product) is currently shown in each. In the large display window, users may probe or interact with the image in various ways.

The ICE Layout
The Image Composite Editor (ICE) simultaneously displays thumbnails of up to three datasets. They can be different types of data over the same area, the same data types over the same area for different time periods, or different wavelengths from a multi-spectral remote sensing image. In addition, the main window displays one large image, which can be a color-coded image, a representation of a mathematical operation applied on the data sets, or a color combination of different wavelengths.

Note the column of buttons to the right of the display windows in the Channel Islands example (the applet will open in a new window). You may find it helpful to keep both windows open as you review the following brief descriptions of the buttons’ functions:


Hitting the Step button (righthand side) will cycle through each of the three thumbnail images in the larger display window.


Press the Probe button and then pass your cursor over the large window. You should see a digital readout of the unit values for each of the three thumbnail scenes as well as the latitude and longitude location of each pixel.

Zoom & Roam

Push the Zoom & Roam radio button if you want to increase the size of the image in the large window. Zooming in is done by a left button click on your mouse; zooming out is a right click. Once you have zoomed in, you may click and hold then drag the image up or down or from side to side to roam around within the scene. (A radio button controls this mode and the “Select Region” mode because both functions require users to “click and drag” with their mouse — both cannot be “on” at the same time.)


The Restore button returns the image in the large window to its original state (e.g., zooms out all the way.)

Plot Transect

Choosing the Plot Transect radio button enables users to draw a line segment across any part of the scene in the large window. A new window will appear graphing the unit values for each of the three parameters shown in the thumbnails along X- and Y-axes. Note that while this window is active, users may continually “click and drag” to redraw the transect line. Moving the transect line without releasing the mouse button provides another interesting way to probe the data (i.e., watch as the unit values shift in real time as you move your line around in the window).

The Transect function simultaneously displays the values of each type of data (chlorophyll, fluorescence, and sea surface temperature, for example) for each pixel along a line chosen by the user.

Select Region and Outline Region

Choosing the Select Region radio button enables users to draw a box around a region of interest by “dragging” the mouse pointer (holding left button while moving the cursor). As Mother Nature rarely works at right angles, the Outline Region button enables users to more precisely trace the contours of some portion of the scene. Once a region is selected by either of these two means, a number of diagrams can be produced in a second window to help users analyze the data. For those diagrams (like a scatter plot) that need two sets of data, users may select which two parameters they wish to compare by clicking on their thumbnails; otherwise, the first two windows will be selected automatically. A yellow frame appears around the thumbnails selected.

Scatter and 3D Scatter

A scatter plot is a graphical comparison of the measures of two or more different things that can change over time and space, called variables. Scatter plots provide a useful way to look for relationships between different variables and can reveal how much one variable, as it changes, affects another. The relationship between two variables as they change is their correlation. We can show the correlation of two changing variables graphically by plotting their measurements along horizontal (X-axis) and vertical (Y-axis) axes. The closer the data points come to making a straight line, the better the correlation. For example, try comparing the number of hours you study for each exam this year to the numerical grade you receive for each exam. If you plot those numbers on a graph, you will probably find a good correlation between them. If the data points appear to cluster along a line that is rising from left to right, then there is a positive correlation between the number of hours you study and the grade you receive. In other words, a positive correlation means when the value of one variable goes up, the other variable goes up too. There can also be negative correlations between variables, as indicated when data points appear to cluster along a line that is moving downward from left to right. If you sell lemonade on a street corner and every day you incrementally raise the price of a cup of your lemonade, over time you will notice that the more money you charge, the fewer cups of lemonade you sell. Plotting these data points would reveal a negative correlation. A negative correlation means that as one variable goes up, the other variable goes down.

Within the context of ICE, users can look for correlations among satellite remote sensing measurements of the Earth’s environment. Click on the two thumbnails you wish to compare. Click either Select Region or Outline Region to specify where in the scene you wish to look for a relationship. Then click Scatter; or you may click 3D Scatter to compare all three thumbnails. A new window will appear containing the scatter plot of the area you selected. The more positively correlated the images, the more closely the data points will cluster around a 45-degree angle line moving upward from left to right between the X-axis and Y-axis. High negative correlations will show up as clusters of points along a 45-degree angle line moving downward from left to right. Conversely, the less correlated the images are, the more the data points will appear randomly scattered on the graph. Such images may be only weakly correlated or they may have no corrrelation at all.

You may click and drag on the scatter plot to enlarge it. Clicking “Reset” will restore graph to its original size.

Sample of Scatter Plot
The scatter plot shows the relationship between two datasets over an area. Each point on the plot shows how two things that vary (or “variables,” in this case sea surface temperature and chlorophyll) at one or more pixel locations are related. Pixels with high sea surface temperature values have lower chlorophyll values, and vice-versa. Although there is some scatter among the points in this example, notice that the general trend follows a line that is falling from the upper left to bottom right — typical of a negative correlation.
Sample of 3D Scatter Plot
The 3D scatterplot compares three types of data in a selected area.

Histogram and 3D Histogram

A histogram is a bar graph that shows the frequency of occurrence of specific values for a given measurement. If you flipped a coin 100 times and recorded the number of times it landed on heads and the number of times it landed on tails, you could plot those numbers on a bar graph. That would be a simple histogram plot showing the frequency of heads versus tails. Now suppose you rolled a six-sided die 1,000 times and recorded the number of times each number, 1 through 6, came up. If the die was “normal,” you would expect each number to come up roughly an equal number of times. But if the die was “loaded,” and you rolled a “6” ten times more often than any other number, then your histogram would reflect a disproportionate weighting for the number 6 in the bar chart. (The Shodor Education Foundation offers an excellent tutorial on Histograms.)

Within the context of ICE, a histogram is a count of the pixels within a selected region at each of the image’s 256 shades of gray. Selecting a region and then clicking the Histogram button brings up a window that graphically displays the number of pixels for each shade of gray within the selected region. Histograms are useful for a variety of reasons. For example, a histogram allows users to estimate how large an area is contained within a certain feature of a scene (for example, a cloud, a burn scar, or a phytoplankton bloom). If you know the spatial resolution (per pixel) of the image and the number of pixels at a given shade within the selected region, then you can estimate the physical size of the feature (in two dimensions, of course). The images in the Channel Islands example all have a spatial resolution of 1 kilometer per pixel.

Sample Histogram
The histogram shows the number of pixels that have a particular gray scale value for each of the three data sets (represented by different colors). For example there are approximately 1150 pixels with a chlorophyll value of 64 in this histogram.
Sample of 3D Histogram
The 3D histogram shows the number of pixels with corresponding values in two data sets. It is similar to the scatterplot, but also shows how many pixels have an identical relationship. In this example approximately 3800 pixels have a sea surface temperature of 40 and chlorophyll of 128.

Pick a Color Table

Use the pop-down menu beneath the large display window to assign a color palette to a particular image. A range of palettes is available, depending upon the types of images available. Hit the “Step” button to choose the thumbnail you wish to color.

The Four Modes of ICE

The ICE has four basic modes of operation:

  • Color (red, green, and blue, or RGB) combining
  • Arithmetic combinations
  • Animations
  • Image comparisons

In each of these modes, a common set of tools is provided but there is added functionality too for developing certain types of lessons. We are developing examples on the Earth Observatory for each of these modes. But you can check out the ICE Programmer’s Guide if you wish to preview them now.