24-04-2012, 11:13 AM
Using QuickBird Imagery in ESRI Software Products
UsingQB.pdf (Size: 476.2 KB / Downloads: 55)
Introduction
Purpose
This document provides instructions for using QuickBird Imagery products in software applications developed by Environmental Systems Research Institute (ESRI). These applications include both ArcView 3.x and ArcMap 9.x, a part of the ArcGIS suite.
Scope
QuickBird Imagery products are delivered in either 8-bit or 16-bit format. However, QuickBird collects data using an 11-bit dynamic range. Bit depth refers to the amount of energy that is stored in an image. To understand bit depth, you have to relate what you see in an image to the way that computers store data. Computers work with binary data, meaning that every number has a value of 0 or 1. To get more complex numbers, you have to string binary numbers together. For example, a string of two binary numbers (referred to as 2-bit data) would result in 4 possible values: 00, 01, 10, and 11. Take that concept a step further and think about 8-bit data. That would result in 28 or 256 possible values. Applying this back to imagery, 8-bit data allows you to store 256 possible values in each pixel. If you are viewing an image in gray tones, 0 corresponds to black, 255 corresponds to white, and all the levels in between correspond to different shades of gray.
Stretching an Image
Contrast stretching involves altering the distribution and range of Digital Number (DN) values. The range of DN values in an image is rarely as large as that of the display. Applying a contrast stretch compensates for this by expanding the histogram to the full range of possible DN values. This stretching typically enhances the image allowing the user more interpretive ability in shadows and other areas in the image. Stretching generally adds contrast and brightness making object differentiation less difficult and linear features more noticeable.
Imagery Usage Instructions
What follows is a step-by-step description of the situations an average user will encounter when using ESRI software products with QuickBird imagery, and how best to display the imagery to meet most user needs.