2.2 Dealing with missing data

To create a useful histogram we need to prevent missing data (land and cloud pixels) from being included in the plot. This can be done by designating the value of 255 as a null value. You may remember that you have done this already, when you opened the HDF file in section 1.8.

Designating a null value

A null value is basically the numerical value (or range of values) assigned to data points where valid data do not exist. To set the null value in an image, you first need to open the Redisplay dialog. You may do this in different ways:

From the menu bar:
  - either click on Image, then Redisplay on the drop down menu,
  - or hold down the [ALT] key and type [I], then type [D].
Mouse shortcut:
  Right-click on the image and select Redisplay from the pop-up menu.

The effect of 'Applying Stretches'

When you created the previous two histograms you made sure that the 'Apply Stretches' box on the 'New' dialog was unchecked. To see why this is important, create a third histogram based on the whole image, but this time make sure you check the Apply Stretches box before clicking OK to create the histogram.

Study Histograms 2 and 3 and use your observations to answer Question 2 below.