Lecture 3: Vector Data Analysis Jianfei Chen School of Geographical Sciences GuangZhou University GuangZhou, 510405 China Email: [email protected].

Lecture 3: Vector Data

Analysis

Jianfei ChenSchool of Geographical SciencesGuangZhou UniversityGuangZhou, 510405 ChinaEmail: [email protected]

Chapter Outline10.1 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 BufferingA 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 2-mile 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

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.

1  2

A B+ =1A 1B

2A 2B

Feature Type and Map Overlay

1. Point in Polygon

2. Line in Polygon

3. Polygon on Polygon

2+

1+

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.

1 

A B 1A

+ =1B

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 B1A 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.

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1

1

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3

Point ID

Polygon ID of joined attribute data

1 2

2 2

3 1

4 3

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.

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3

4

1

1

2

3

4 3

1

1

a 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.

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2

3

2

1

2 2

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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