Vicki Drake

Santa MonicaCollege

Earth Sciences Department

COMPUTERS – THE ENVIRONMENT OF GIS

Components of a GIS include:

Hardware – the machinery

  • The host computer (from a stand-alone microcomputer to a large mainframe supporting many users)
  • Devices for handling input and output

Principles modules of Hardware System also include:

  • Processor – CPU
  • Storage – hard disc and/or floppy disc
  • Main Memory – RAM
  • Input and Output – monitor, keyboard, mouse
  • Network Connection - Ethernet

Software

  • Programs that tell the computer what to do
  • The data the program will use

Computer Data

  • Computer data is coded, manipulated and stored by use of an exclusive two-state condition
  • yes/no
  • on/off
  • open/closed
  • This two state condition translates into

“ switch open/switch closed”

  • One of the two exclusive states always exists
  • If one switch provides two different datum…. how much datum from two switches? (4)

Binary Notation

  • This two state condition is represented in binary notation by the use of 1s and 0s
  • Two switches therefore produces four codes: 00, 01, 10, 11
  • Three switches produces 8 codes: 000,001,010,011,100,101,110,111

Processor and Main Memory

  • CPU “series of processors or “chips” (PC uses 8 bit and some high-powered machines may use 16 bits, 32 bits – 64 bits anyone?
  • Tasks performed by the processor execute programs
  • Instructions making up the program are “fetched”, usually one-at-a-time and in sequence, from the main memory
  • A “program” can be quite large
  • Units: bits and bytes

Bits

  • Each binary digit is called a “bit”
  • Current PCs use 8, 16, and 32 bit paths (the number of bits that can be transmitted simultaneously – determined by number of wires running parallel to one another on circuit board)

Bytes - Data Representation

  • Computer storage is made up of bistable elements
  • Usually, as stated, each component represents a “bit” or “binary digit” – either a “0” or a “1”.
  • Every piece of information in any computer is represented, stored, processed and transferred as a sequence of “bits”.
  • A byte – eight bits in a row – is considered to the smallest element of storage
  • A byte can represent 256 different combinations “0” and “1”.
  • These 256 combinations correspond to the decimal integers “0” to “255” inclusive.

Storage Devices

  • A computer contains several different kinds of storage units, that range from fast (but small) units to large (but slow) units
  • Some typical examples include:
  • CPU cache: 265,512,1024 kb
  • Main Memory: Capacity 64 Mb - gigabytes (access time: 10 ns – SD RAM)
  • Hard disc drive: capacity 40 gigabytes or more! (access time 10 milliseconds)
  • Floppy disc drive: capacity 1.44 Mb/disc (access time 100 milliseconds)

Memory Values

  • a kilobyte is 210 *~1000 bytes
  • a megabyte is 220 ~a million bytes
  • a gigabyte is 230 ~a thousand million bytes
  • a millisecond is 10-3 (1/1000) seconds
  • a microsecond is 10-6 (1/1,000,000) seconds
  • a nanosecond is 10-9 (1/1,000,000,000) seconds

ASCII coding system

  • to maximize efficiency, most computers store data n their own internal formats – however – transfer of data requires the use of standard codes which are understood by all systems
  • the most successful standard is ASCII
  • originated before computer communication as a code for Teletypes
  • ASCII assigns the numbers 0 through 127 to 128 characters, including upper and lower case alphabets, numerals 0 through 9 and various special characters
  • 128 different patterns can be generated using 7 bits in different combinations of “on” and “off”
  • any ASCII character can be coded with 7 bits
  • in practice 8 bits (one byte) are used – the extra bit may be used to extend the code to 128 characters
  • For example: in the ASCII code system, code number ‘85’ is an upper case “U” and the system stores a byte with the bit pattern 01010101.
  • In ASCII, characters 0 through 32 often perform special functions
  • I.e., character 7 (00000111) “rings” a bell
  • character 12 (00001100) is the “form feed” or “new page” signal for printers.

Text

  • Every character of text = 1 byte
  • Each character has a particular combination of bits – the “character” code
  • The phrase “one character of text.” uses 22 bytes – one for each letter, one for each space and one for the full stop.
  • Text does not take up much storage space – or require significant time to transmit
  • A CD has a capacity of ~650 MB of ROM – read-only memory – i.e. the text of the Encyclopedia Britannica

Numbers

  • Numbers are usually represented in one of two forms, referred to an “integer” or “real”
  • Integers tend to be represented in computer storage as either 2 or 4 bytes
  • Integer representations cannot deal with numbers such as 123.456, or 0.0000264
  • For these – “real” (or “floating point”) form is used
  • I.e. Some number “n” is given, in floating point format, by the pair of numbers (D, d) such that 10-1<D<1 and “d” in an integer…so:
  • 123.456 is represented as (.123456,3) and
  • 0.0000264 is represented as (0.264, -4)
  • In this way, numbers of almost any useful magnitude can be represented

Images

  • Most commonly, images are represented as an array of “picture elements” or pixels
  • Each pixel represented by a byte, giving either a gray-value (half-tone images) or color value (full-color images)
  • Each byte can hold a numeric value between 0 and 255, each pixel can be described by any of 256 color values
  • Image quality is determined by the fineness of the grid – the “resolution” or “dots per inch (dpi).
  • Compare to text data – images can gobble up vast amounts of computer storage space!

Operating Systems

An operating system (OS) is the software which controls the operation of the computer from the moment it is turned on or “booted”

  • The OS controls all input and output to and from the peripherals as well as the operations of other programs
  • Allows the user to work with and manage files without knowing specifically how the data is stored and retrieved.
  • In multi-user systems – operating systems manger user access to the processor andperipherals and schedule jobs (i.e., printing)

Common Operating Systems include:

IBM PCs and Clones – using MS-DOS

UNIX – for Workstations

The OS is the interface between the user, hardware and application.

Databases

  • Databases are packages designed to create, edit, manipulate and analyze data
  • To be suitable for a database, the data must consist of records which provide information on individual cases, people, places, features, etc.
  • Each record may contain several fields each of which contains one item of information
  • Field contents can be many types: numeric or text, fixed or variable length.
  • There are can several classes of records in a database
  • I.e., an airline reservation database may have the following classes of records: passengers name, phone, flight numbers, aircraft type, registration number, seats, names of pilot, etc.

Functions of a Database

  • Creating and editing records, using customized screens
  • Printing reports (summaries of groups of records) using customized report forms, including subtotals and totals
  • Selecting records based on user-specific rules
  • Updating records based on new information
  • Linking records

Three types of databases

  • Network
  • Hierarchical
  • Relational

All three are used, but the relational model is most successful within GIS –

Most well known: dBase, Oracle, Info

Most databases use the same language (SQL) Standard Query Language for formulating queries.

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© Vicki Drake

Santa MonicaCollege

Fall 2001 Lectures