Image Processing

Introduction

After data is collected and transmitted to the ground station, it must be processed and converted into a format that is usable by the researcher who will interpret the data. Often satellite-derived data is converted into imagery that provides a visualization of the data collected by the sensor. However, the format of these data in their original form is usually not such that an interpreter can learn much about the target. Often, the data must be processed, enhanced, and manipulated to provide a useful set of information. This technique, which is part science and part art, is called image processing.

Converting the data stream to an image

Satellite image data is sent from the satellite to the ground station in a raw digital format, which is essentially a stream of numerical data. The smallest unit of digital data is a bit. A bit is represented by a binary number, which has only two possible values, 0 or 1. A bit can be used to represent any piece of data that has two states, such as on/off, true/false, or open/closed. With only two potential values, a bit does not offer much flexibility in representing data that is more complex than a binary number. Therefore, data is often stored as a collection of eight bits, resulting in a unit of data called a byte.

A byte is a unit of data that is comprised of 8 bits, thus providing a data element with up to 256 potential values (2^8). Radiometers that measure the intensity of electromagnetic radiation will generally convert the detected energy levels into a value that ranges from 0 to 255 and represent each of these measured energy levels with a single byte. These bytes will be strung together in a pre-determined manner, converted into a signal, and transmitted to the collections facility. Here, the signal will be converted back into a digital stream of bytes where it can be read in and interpreted by processing software. Images generated in this manner are thus referred to as "8-bit digital images."

Characteristics of Images

Though remotely-sensed images are collected from a wide variety of sensors and transmitted to a ground station through many different paths, all image data have certain characteristics in common.

Pixels and Digital Number

When a stream of bytes is received from a satellite sensor, the value of each byte is applied to a single dot, or pixel (short for "picture element"). The numerical value of the pixel, known as its Digital Number (DN), is translated into a shade of gray that ranges somewhere between white and black. These pixels, when arranged together in the correct order, form an image of the target in which the varying shades of gray represent the varying energy levels detected on the target.

The following image illustrates this concept. The Landsat 7 image clip in the upper left of this image is a false color composite image centered over a resevoir in a portion of central Maryland. When a selected portion of the image is magnified several times, it becomes apparent that the image is really just comprised of rows of pixels, each with its own color.

It is important to remember that a satellite image is not just a picture of the target similar to what a simple camera would take. Instead it is a collection of numeric data that is capable of being displayed as an image. The underlying dataset can be manipulated using algorithms (mathematical equations) that correct for errors (like atmospheric interference), re-map the data to a geographical reference point, or extract information that is not readily apparent in the data. The data for two or more images of the same location can even be combined mathematically, creating imagery that is a composite of multiple datasets. These data products, known as derived products, can be generated by performing calculations on the raw numerical (digital numbers) data.

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