Geography 38:376 Intro GIS

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Geography 38376Intro GIS

• Topic 2
• Defining and Measuring Spatial Data
• Chapter 1 Chang
• (Chapter 2 DeMers)

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Spatial Awareness

• We possess an inherent ability to spatially
  organize our environment
• Traditionally, we have used maps to model the
  spatial complexity of the real world
• Ability to think spatially is limited
  particularly in 3-dimensions
• GIS is a new way of representing our 3-D world

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Representing Spatial Objects

• All real or conceptual spatial objects can be
  viewed as a point, line, area or volume
• On a map or GIS these features are represented
  using points, lines, or polygons features
• These are referred to as cartographic entities or
  geographic primitives

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Representing Spatial Objects

• PLP are represented using raster or vector data
  models in a GIS more on this later
• Spatial surrogates represent conceptual features
• Cartographic entity used to represent a real
  world feature is often scale dependent
• E.g. city represented by polygon at large scale,
  point at a small scale

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Representing Spatial Objects
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Point Features

• Points are used to represent real or conceptual
• Point objects
• Surfaces
• Areas too small to be represented by a polygon
• Points can represent either discrete phenomena
• Fire hydrant
• Tree
• Oil well
• Or continuous phenomena sampled at points
• Precipitation
• Snow depth
• Soil pH

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Point Features

• Points represent surfaces by recording an
  elevation or z-value at each x and y location
• Lattice or array of pts.
• Random x,y,z positions
• Vector model records point using x,y,(z) coord
• Raster model records r,c position of one grid cell

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Line Features

• Lines represent real or conceptual features that
  occupy a single dimension have length but no
  width
• Roads
• Rivers
• Pipelines
• Political and administrative boundaries
• Also used to represent real world surfaces
  (e.g.contours)
• In vector model line is arc made up of a series
  of connected points, with beginning and end
• In raster model line is made up of a chain of
  consecutive pixels

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Polygon Features

• Polygons represent real or conceptual 2D objects
  (e.g. regions)
• Countries
• Lakes
• Ecoregions
• In vector model polygon is made up of a series of
  line segments (arcs), each made up of points,
  with the same beginning and ending point
• In raster model polygon is created using a block
  of contiguous pixels

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Measuring Aspatial Attributes

• For each PLP we record spatial and aspatial
  information (characteristics of that feature)
• Attributes are non-spatial or aspatial
  information
• Attribute data measured according to standard
  framework nominal, ordinal, interval, and ratio
• Important b/c determines type of analyses we can
  perform

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Nominal

• Qualitative characteristics
• Common Name
• Land use/cover type
• Occurrence (that it exists)
• Does not indicate magnitude or value
• Represented in a GIS w/ character data type

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Ordinal

• Rank orders features according to est. criteria
• Rank is relative
• Some quantitative analysis, mostly qualitative
• May be represented by
• Character Data Type
• high, medium, low
• Integer Data Type
• numeric scale 1 to 5

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Interval

• Attributes are measured on a scale consisting of
  equally spaced increments
• Has an arbitrary starting point or zero value
• Zero does not mean there is no value
• Not a measure of actual quantity but distance b/w
  values
• Temperature scale is classic example
• Arbitrary datum (ht. above or below some level)
• Can perform quantitative analyses
• May be represented by
• Integer
• Real Numbers (single, double, float)

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Ratio

• Attributes of features measured on a scale
  consisting of equally spaced increments
• Does have a real and meaningful starting point or
  zero value
• Streamflow
• Population
• Yield
• Usually represented by
• Real (single, double, float)

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Measuring Spatial Attributes
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• Next
• Topic 3 Raster and Vector
• Data Models
• Chapters 3, 4, 5 Chang
• (Chapter 4 DeMers)