Basic Geographic Concepts
Real World  Digital Environment
Data in a GIS represent a simplified
view of physical entities or
phenomena
Spatial location
2. Non-spatial properties /attributes
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Basic characteristics of geographic
data

Location

Dimensionality

Continuity (continuous/discrete)

Attributes (level of measurement)
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Basic characteristics of geographic
data – continuity

discrete objects and continuous field
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
“The discrete object view represents
the geographic world as objects with
well-defined boundaries in otherwise
empty space. “

“The continuous field view
represents the real world as a finite
number of variables, each one
defined at every possible position. “
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http://weather.unisys.com/surface/sst.gif
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Continuous vs. discrete

Continuous
– Data values distributed across a surface w/out
interruption
– Key words: What varies and how smooth?
– Examples: elevation, temperature

Discrete
– with well-defined boundaries in otherwise empty
space
– Examples
• Points: Town, power pole
• Lines: Highway, stream
• Areas: U.S. Counties, national parks
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Continuous & discrete?

Some data types may be presented as
either discrete or continuous
– Example
• Population at a point (discrete)
• Population density surface for an area
(continuous)
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Continuous vs. discrete

In computer databases – raster and
vector data model are used for
representing continuous and discrete
objects.
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Raster and vector data model

“Rater representations divide the world into
arrays of cells and assign attributes to the
cells.”

“Vector data uses sets of coordinates and
associated attribute data to define discrete
objects. There are three basic types of vector
objects: points, lines, and polygons. “
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Basic characteristics of geographic data:
Attributes (level of measurement)
 Nominal

Ordinal

Interval

ratio
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Nominal
To identify or distinguish one entity from
another

Simplest/lowest level of attribute

Identification/labeling of data
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Ordinal
Data ranked based on a particular
characteristic
 Gives us insights into logical comparisons
of spatial objects
 Examples:

– Large, small, medium sized cities
– Interstate highway, US highway, State
highway, Country road
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Interval
Numbers assigned to items measured
 Measured on a relative scale rather than
absolute scale

– 0 point in scale is arbitrary
Data can be compared with more precise
estimates of the differences than nominal
or ordinal levels
 Not very common

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Interval
Example: Temperature
 Zero temperature varies according to
the unit of measurement (0 deg. C = 32
deg. F)
 0 deg. C is not the absence of heat 
Absolute zero is identified by 0 Kelvin

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Interval

The difference between values makes sense,
but ratios of interval data do not
 Ex.: A piece of metal at 300 degrees
Fahrenheit is not twice as hot as a piece of
metal at 150 degrees Fahrenheit
– Why? the ratio of these values is different
using Celsius
150 deg. F=66 C
300 deg. F.=149 deg. C
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Ratio
Numbers assigned to items measured
 Measured on an absolute scale (use true 0
point in scaling)

– Measurements of length, volume, density,
etc.

Data can be compared with more precise
estimates of the differences than nominal
or ordinal levels
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Ratio

Examples
– Total precipitation
– Population density
– Volume of stream discharge
– Areas of countries
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Attributes & Mathematical
Comparisons

Nominal
-Not possible

Ordinal
- Compare in terms of greater than, less than,
equal to

Interval/ratio
– Mathematical operations
• Interval: addition, subtraction
• Ratio: add, subtract, multiply, divide
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Bits and Bytes
8 bits = 1 byte
1 bit = 1 binary digit
1 byte = 8 bits
1 0 1 1 1 0 1 0
1024 bytes = 1 K
1000 K = 1 Mb
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The ASCII Coding
If the computer stores everything in 0’s and 1’s, then how the the characters
are represented?
The ASCII (American Standard Code for Information Interchange) coding
assigns the numbers 0 through 127 to 128 characters, including the upper
and lower case alphabets and various special characters, such white space,
the return key etc.
For Example, decimal 85 is assigned to represent upper case U.
In binary base, 01010101 is decimal 85. Thus the computer
represents U as 01010101.
Computer files which contain information coded in ASCII are easily transferred
And processed by different computers and programs. Such files are often called
“ASCII” files or “text” files.
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Basic Geographic Concepts

The difference between discrete and
continuous data

Descriptive Attributes (nominal, ordinal,
interval, and ratio)

Binary counting system
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