Lecture 61: Vector GIS Concepts and Operations I

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UST/ENV/PAD/PDD 642/742 Introduction to
Geographic Information Systems
Lecture 6-1 Vector GIS Concepts and Operations I
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Introduction to Topology

• GIS features are represented as Points Nodes,
  Lines Arcs and Polygons Regions
• For generating NEW information, we must know
  more about the linkages that these objects have
  in relation to each other
• Topology is a mathematically explicit rule
  defining linkages or relationships between
  spatial elements/objects
• Topology is that branch of mathematics which
  concerns itself with the concepts of
• DIRECTION
• CONNECTIVITY
• ADJACENCY or CONTIGUITY and
• PROXIMITY

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Terminology and Uses for Topology

• Topological data structures are founded upon two
  types of objects and their generic relationship
• Objects are NODES and ARCS, the basic
  relationship between them being INCIDENCE
• Topological data structures store and manipulate
  logical relationships between map features
• Topological data structures are used to BUILD
  point, line or polygon feature classes, based on
  node and arc-node topology
• They are also be used for quality control of
  spatial data to identify missing or extraneous
  links
• They are also be used in modeling flow and in
  network analyses

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Topology in Action
Direction from and to nodes Line L1 from Node
N1 to Node N2 line L1 therefore has right and
left sides Connectivity connectivity between
one node and another node Adjacency polygon
line-sharing Line L1 common to both
polygons Proximity node X-Y locations
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Topological Data Structures

• Vector Data Structures that incorporate explicit
  spatial
• information about the relative locations of
  objects in the data
• tables
• Required for flow, proximity models and for
  advanced analysis
• within the vector GIS context
• TIGER The Topologically Integrated Geographic
  Encoding
• and Referencing system is a vector data model
  created by the
• US Census.

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TIGER Topological Data
It is essentially a line file within which
points, lines and areas are explicitly addressed,
allowing direct retrieval of census polygon
spatial data. Real-world objects are portrayed in
their true geographic shape within
technology-defined limits.
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Spatial Relationships
NEAR
Polygon A
Polygon B
ADJACENT
Polygon A
Polygon B
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Spatial Relationships
Polygon A
INTERSECTION
Polygon A
CONTAINMENT
Polygon B
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Spatial Selections Examples of Spatial Queries
and Spatial Selections

• How many gas stations are within .5 miles of the
  I-77 highway?
• What interstate highways intersect with the
  Metroparks?
• How many schools are within the City of
  Cleveland?
• Select all parcels that are zoned for
  Residential within the Cuyahoga
• Select all states that are adjacent to Ohio
• Show me all the streets within Shaker Heights
• Select all Counties (or Cities) within the
  Cleveland MSA

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Spatial Relationship Types
1 Are Crossed by the Outline of selects
features that are overlapped by the features of
another layer. E.g. selecting forest polygons
crossed by the outline of roads will select all
forest polygons with one or more road features
inside its boundaries. Forest features will not
be selected if they share a boundary with a road
segment at least part of the road segment must
lie inside the forest polygon 2 Intersects
selects features that are overlapped by the
features of another layer including any features
bordered adjacent by the selecting features.
E.g. selecting forest polygons intersected by
roads will select any forest polygon, with a road
segment running within its boundaries or even
along the forest polygon edge. 3 Are Within a
Distance of selects features within a specified
distance to features in the same layer or in a
different layer. E.g. if you have a layer of
clean and contaminated wells, you can find all
the clean wells within 1500 feet of the
contaminated ones select-by-location in the same
layer. You could also find reservoir and farm
features in other layers that are within 1500
feet of the contaminated wells select-by-location
in different layers. also selects features
adjacent to other features. E.g. you may have
already selected land parcels that are going to
be inspected for ground pollution, and now you
want information about adjacent parcels. In this
case, you would select parcels within zero
distance of the ones youve already selected.
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Spatial Relationship Types (cont.)
4 Have Their center In selects polygon
features in one layer that have their centroids
within the polygon features of another
layer. E.g. you can select all counties in OH by
selecting all county polygons that have their
centers in the State of OH 5 Are Completely
Within selects point, line or polygon features
in one layer that fall completely inside the
polygons of another layer. Edges cannot be
coincident. E.g. you can select lakes completely
within a county. can also select point, line or
polygon features that are a certain distance
within the edges of the polygon they fall inside,
by specifying a buffer distance. E.g. you can
select lakes that are at least 800 feet within a
county boundary. 6 Completely Contain selects
polygon features in one layer that completely
contain the features in other point, line or
polygon layer(s). E.g. select forest polygons
that have lakes completely within them. can also
select polygons that completely contain features
a certain distance within them, by specifying a
buffer distance. E.g. you can select forest
polygons with lakes at least 300 feet within the
outer boundary of the forest polygon.
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Spatial Relationship Types (cont.)
7 Share a Line Segment with selects line and
polygon features that share line segments with
other line and polygon features. You cannot use
this method to select point features. E.g. select
all hydrography line features that share a line
segment with census tract polygons 8 Touch the
Boundary Of can select features using a layer
containing lines this method selects lines and
polygons that share line segments, vertices or
end-points nodes with the lines in the layer.
Lines and polygons will not be selected if they
cross the lines in the layer. E.g. select all
city polygons that touch the boundary of road
line features can also select features using a
layer containing polygons this method selects
lines and polygons that share line segments or
vertices with the polygon boundaries. Lines and
polygons will not be selected if they cross the
polygon boundaries. You cannot use this method to
select point features. E.g. select all highway
lines that touch the boundary of census tract
polygons 9 Are Identical To selects any
feature that has the same geometry as a feature
in another layer. The feature types must
correspond, that is, you would use polygons to
select polygons, lines to select lines, and
points to select points. E.g. select highway line
features that are identical to selected road line
features
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Spatial Relationship Types (cont.)
10 Contain selects polygon features in one
layer that contain the features of another. This
method differs from the Completely Contains
spatial relationship in that the boundaries of
the features can coincide. E.g. with the Contain
spatial relationship, a forest polygon will
contain a lake and thus be selected, even if the
border of the lake and the border of the forest
polygon coincide. The same forest polygon would
not be selected using the Completely Contain
spatial relationship because the borders
coincide. 11 Are Contained By selects point,
line or polygon features in one layer that are
contained by the polygon features of another
layer. E.g. you can select those all counties
that are contained by a state. This method
differs from Are Completely Within in that the
edges of the features can coincide thus, all
counties in OH would be selected by the state of
OH using the Are Contained By method, while not
all counties in OH would be selected using the
Are Completely Within spatial relationship.