Download computers – the environment of gis

Vicki Drake
Santa Monica College
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 highpowered 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-ata-time and in sequence, from the main memory
 A “program” can be quite large
 Units: bits and bytes
© Vicki Drake
Santa Monica College
Fall 2001 Lectures
1
Bits
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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
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a
a
a
a
a
a
kilobyte is 210 *~1000 bytes
megabyte is 220 ~a million bytes
gigabyte is 230 ~a thousand million bytes
millisecond is 10-3 (1/1000) seconds
microsecond is 10-6 (1/1,000,000) seconds
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
© Vicki Drake
Santa Monica College
Fall 2001 Lectures
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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
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I.e., character 7 (00000111) “rings” a bell
 character 12 (00001100) is the “form feed” or “new page”
signal for printers.
Text
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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
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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
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© Vicki Drake
Santa Monica College
Fall 2001 Lectures
Most commonly, images are represented as an array of “picture
elements” or pixels
Each pixel represented by a byte, giving either a gray-value (halftone 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!
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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
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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.
© Vicki Drake
Santa Monica College
Fall 2001 Lectures
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