Geographic Information Systems: An Introduction, 3rd Edition

4.1 Introduction

As discussed in Chapter 3, GIS depicts the real world through models involving geometry, attributes, relations, and data quality. In this chapter, the realization of models is described, with the emphasis on geometric spatial information, attributes, and relations. Data quality is discussed further in Chapter 11.

A prerequisite for describing the real world by use of GIS is that the different types of geographical information can be stored in a computer. All the operations in a computer are based on the storage and handling of numbers. This is why the data stored in a computer are known as digital data. In GIS there is a need to store graphical figures, images, numerical values, and plain text. All these forms of data must thus be able to be converted to digital representation.

Digital representation of data in a computer

In principle, only two different numerical symbols or signals can be stored in a computer—0 and 1. The numerical system based on 0 and 1 is known as the binary system. In order to separate different numbers form each other, the stream of 0's and 1's is divided into groups of 8 bits. Each group is known as a byte. The highest number in a binary system with bytes of 8 bits is 255. Numbers stored in this way are known as integers. In practice, a computer can today work with larger units than 8 bits, such as 16, 32, and 64 bits. Using 32 bits (double integer), one can at the most represent a whole number of 4 294 967 295. In order to give negative ...