2. Histograms: An aid to image interpretation

This chapter of the tutorial introduces another Bilko document: Histograms. In Bilko histograms in are distinguished by the file extension .hst. They provide information about the frequency of different data values within an image or image region. This section teaches the basics about Bilko histogram documents, but you will also encounter them in later sections, where they are used as an aid to display and interpret image data and help you reveal features of interest.

Aim and objectives

The chapter demonstrates how to create and interpret histograms of image data. By the end you should be able to

• create a histogram from the whole or part of an image,
• prevent missing data from distorting histogram information,
• derive information from the histogram,
• understand the significance and limitations of Bilko histograms,
• adjust the appearance of histogram documents

Chapter content

1. Creating a histogram of the whole image
2. Dealing with missing data - null values in histograms
3. Obtaining frequency information - using the cursor to read histogram values
4. Changing histogram appearance - labels, fonts, scaling and accumulate
5. Saving the histogram
6. Using histograms to interpret images
7. Histograms and 'Redisplay' - optimizing the loading of 16 or 32-bit images
8. Summary and conclusions - what you should have learnt in this section

Images used in this chapter

The images for this lesson (below) are usually stored in the folder called tutorial_data Throughout the lesson you can use the links on the right sidebar to get information about them.

• atsr199701.gif
  Image of global sea surface temperature (SST) in January 1997, derived from measurements made by ESA's ATSR instrument.

• ATS_10arcmin_DSST_0301.hdf
  Sea surface temperature for January 2003 derived from ESA's AATSR sensor onboard ENVISAT

• ATS_TOA_1COLRA_20040201~.N1
  Level 1 scene of southern Africa recorded by AATSR on Envisat.