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Lecture 3:
Vector Data
Analysis
Jianfei Chen
School of Geographical Sciences
GuangZhou University
GuangZhou, 510405 China
Email: [email protected]
Chapter Outline
10.1 Introduction
10.2 Buffering
10.2.1 Applications of Buffering
10.3 Map Overlay
10.3.1 Feature Type and Map Overlay
10.3.2 Map Overlay Methods
Box 10.1 Methods of Map Overlay
10.3.3 Slivers
Box 10.2 Map Overlay Operations using shapefiles
10.3.4 Error Propagation in Map Overlay
10.3.5 Applications of Map Overlay
10.4 Distance Measurement
Box 10.3 Distance Measurement using ArcGIS
10.5 Map Manipulation
Box 10.4 Map Manipulation using ArcGIS
Applications: Vector Data Analysis
Task 1: Perform Buffering and Map Overlay
Task 2: Use a Visual Basic Script to Update Area and Perimeter of a Shapefile
Task 3: Join Data by Location
Basic GIS Data
Models
Raster
Vector
Buffering
Based on the concept of proximity, buffering
separates a map into two areas: one area that is
within a specified distance of selected map
features and the other area that is beyond. The
area that is within the specified distance is
called the buffer zone.
Variations in Buffering
1. The buffer distance or buffer size does not have to be
constant. It can vary according to the values of a given
field. For example, stream buffer sizes may vary
depending on the intensity of adjacent land use.
2. A map feature may have more than one buffer zone.
For example, a nuclear power plant may be buffered
with distances of 5, 10, 15, and 20 miles, thus forming
multiple rings around the plant.
3. Boundaries of buffer zones may remain intact so that
each buffer zone is a separate polygon. Or these
boundaries may be dissolved so that there are no
overlapped areas between buffer zones.
Buffering with different buffer distances
Buffering with four rings
Buffer zones dissolved (top) or
not dissolved (bottom)
Applications of Buffering
A buffer zone is often treated as a protection zone for
planning or regulatory purposes.
•A city ordinance may stipulate that no liquor stores or pornographic shops shall be
within 1000 feet of a school or a church.
•Government regulations may stipulate that logging operations must be at least 2
miles away from any stream to minimize the sedimentation problem and set the 2mile buffer zones of streams as the exclusion zones.
•A national forest may restrict oil and gas well drilling within 500 feet of roads or
highways; within 200 feet of trails; within 500 feet of streams, lakes, ponds, or
reservoirs; or within 400 feet of springs.
•A urban planning agency may set aside land along the edges of streams to reduce
the effects of nutrient, sediment, and pesticide runoff; to maintain shade to prevent
the rise of stream temperature; and to provide shelter for wildlife and aquatic life.
•A resource agency may establish stream buffers or vegetated filter strips to protect
aquatic resources from adjacent agricultural land use practices.
Applications of Buffering
A buffer zone may be treated as a neutral zone and as a tool for
conflict resolution.
In controlling the protesting mass, police may require protesters
to be at least 300 feet from a building. Perhaps the best-known
neutral zone is the demilitarized zone separating North Korea
from South Korea along the 38° N parallel.
Applications of Buffering
Buffer zones may represent the inclusion zones in GIS
applications.
For example, the siting criteria for an industrial park may
stipulate that a potential site must be within 1 mile of a
heavy-duty road. In this case, the 1-mile buffer zones of all
heavy-duty roads become the inclusion zones.
Applications of Buffering
Buffer zones themselves may become the object for analysis.
A forest management plan may define areas that are within
200 feet of streams as riparian zones. Under the plan,
riparian zones are separate from non-riparian areas and are
managed differently. Another example comes from urban
planning in developing countries, where urban expansion
typically occurs near existing urban areas and major roads.
Management of future urban growth should therefore
concentrate on the buffer zones of existing urban areas and
major roads.
Applications of Buffering
Buffering with multiple rings can be useful as a sampling
method.
For example, by buffering stream networks in 10-meter
increments to a distance of 300 meters one can analyze the
composition and pattern of woody vegetation as a function
of distance from the stream network. One can also apply
incremental banding to other studies such as land use change
around urban areas.
Map Overlay
Map overlay combines the geometries and
attributes of two feature maps to create the
output. One of the two maps is called the input
map and the other the overlay map.
Input to TRANPLAN
1
+
2
A
B
=
1A 1B
2A 2B
Map overlay combines the geometry and attribute data from
two maps into a single map. Intersect is the map overlay
method in this illustration. The output from intersect include
areas that are common to both input maps. The dashed lines
are not included in the output.
Feature Type and Map Overlay
1. Point in Polygon
2. Line in Polygon
3. Polygon on Polygon
1
+
2
+
+
A
B
2B
+
=
1A
+
Point in polygon overlay. The input is a point map (the
dashed lines are for illustration only and are not part of the
point map). The output is also a point map, which has
attribute data from the overlay polygon map.
1B
+
1
A
B
=
1A
Line in polygon overlay. The input is a line map (the dashed lines
are for illustration only and are not part of the line map). The
output is also a line map. But the output differs from the input in
two aspects: the line is broken into two segments, and the line
segments have attribute data from the overlay polygon map.
1
+
2
A
B
1A
1B
2A
2B
=
Polygon-on-polygon overlay. In the illustration, the two maps
to be overlaid have the same area extent. The output
combines the geometry and attribute data from the two maps
into a single polygon map.
Map Overlay Methods
Union: (input map) OR (overlay map)
Intersect: (input map) AND (overlay map)
Identity: [(input map) AND (overlay map)] OR (input map)
+
=
The union method keeps all areas of the two
input maps in the output.
+
=
The intersect method preserves only the area common to the
two input maps in the output. The dashed lines are for
illustration only; they are not part of the output.
+
Input Map
=
Identity Map
Output Map
The identity method produces an output that has the same
extent as the input map. But the output includes the
geometry and attribute data from the identity map.
Point
ID
Polygon ID of joined attribute
data
4
1
2
3
2
2
3
1
4
3
1
2
2
3
1
The spatial join method involves two maps: the map to assign data
from, and the map to assign data to. Each map can be a point, line,
or polygon map. Data assignment is based on the spatial
relationship of nearest, part of, or intersects. In this illustration,
each point is assigned attribute data of its bounding polygon.
The top boundary in this illustration represents a coastline,
which has a series of slivers. These slivers are formed between
the coastlines from two maps used in an overlay operation. If
the coastlines register perfectly between the two maps, then
slivers will not be present.
Points (and lines) are snapped together if they fall within the specified fuzzy
tolerance. Many slivers along the top boundary (A) are removed by use of a
fuzzy tolerance. The fuzzy tolerance can also snap arcs that are not slivers (B).
Applications of Map Overlay
A map overlay operation computes the geometric intersections of
two feature maps and creates an output with combined features
and attributes from the two input maps. This output map can then
be used for query and modeling purposes.
A more specific application of map overlay is to help solve the
areal interpolation problem, which involves transferring known
data from one set of polygons (source polygons) to another
(target polygons).
Site Analysis
A
B
1
In this illustration, census tracts are shown in thick lines and school districts in thin
lines. Census tract A has a known population of 4000 and B, 2000. Queried from the
result of map overlay, the areal proportion of census tract A in school district 1 is
found to be 1/8 and the areal proportion of census tract B, 1/2. Therefore, the
population in school district 1 can be estimated to be 1500 (4000 x 1/8 + 2000 x 1/2).
Distance Measurements
Distance measurement refers to measuring
straight-line (Euclidean) distances between
points, or between points and their
corresponding nearest points or lines.
1
1
1
3
3
2
4
3
2
4
1
a
1
b
Dissolve removes boundaries of polygons that have the
same attribute value and create a simplified map (b).
Input Map
Clip Map
Output
Clip creates an output that contains only those features of the input
map that fall within the area extent of the clip map. The dashed lines
are for illustration only; they are not part of the clip map.
Merge
=
Merge pieces together two adjacent maps into a single map.
Merge does not remove the shared boundary between the maps.
1
3
2
2
2
2
1
Select creates a new map with selected map features
from the input map.
Eliminate can remove polygons that are smaller than a specified size. Slivers
along the top boundary (A) are therefore eliminated. This illustration uses an
option to preserve the edge at (B) even though polygons making up the edge are
smaller than the specified size.
Input Map
Update Map
Output
Update replaces the input map with the update map
and its map features.
Input Map
Erase Map
Output
Erase removes features from the input map that fall within the
area extent of the erase map. The dashed lines are for illustration
only; they are not part of the erase map.
Input Map
Split Map
Output
Split uses the geometry of the split map to divide
the input map into four separate maps.
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