G I S Unit 5 Ch 12 Vector

G. I. S Unit 5 Ch 12. Vector Data Analysis Ch 13. Raster Data Analysis

Ch 12. Vector Data Analysis 12. 1 Buffering 12. 2 Overlay 12. 3 Distance Measurement 12. 4 Map Manipulation

Ch 12. Data Exploration 12. 1 Buffering 12. 1. 1 Variations 12. 1. 2 Applications 12. 2 Overlay 1. What it is ? 2. Feature & Type Overlay 3. Overlay Methods 4. Overlay of Shapefiles 5. Slivers 6. Error Propagation 7. Applications of Overlay 12. 3 Distance Measurement 12. 4 Map Manipulation 12. 4. 1 Dissolve 12. 4. 2 Clip 12. 4. 3 Append 12. 4. 4 Select 12. 4. 5 Eliminate 12. 4. 6 Update 12. 4. 7 Erase 12. 4. 8 Split

Introduction • The vector data model uses points and their x-, y-coordinates to construct spatial features of points, lines, and polygons. • Therefore vector data analysis uses the geometric objects of point, line, and polygon. • And the accuracy of analysis results depends on the accuracy of these objects in terms of location and shape. • Because vector data may be topology-based or non topological, topology can also be a factor for some vector data analyses such as buffering and overlay. • A number of analytical tools such as Union and Intersect also appear as editing tools. • Although the terms are the same, they perform different functions. • As overlay tools, Union and Intersect work with both spatial and attribute data. But as editing tools, they only work with spatial data.

Ch 12. Vector. IData Analysis 12. 1 Buffering • Concept of proximity • Buffer Zone = area within the specified distance • Buffering creates two areas – One within specified distance of select features – Other is area beyond specified feature • The positional accuracy of spatial features in a data set determines the accuracy of the buffer zones • Requires only distance measurement • Features for buffering - points, lines, or polygons • Buffering around – points creates circular buffer zones. – lines creates a series of elongated buffer zones. – polygons creates buffer zones that extend outward from the polygon boundaries.

Ch 12. Vector Data Analysis I 12. 1 Buffering

Ch 12. Vector Data Analysis I - Variations 12. 1 Buffering • The buffer distance or buffer size does not have to be constant • It can vary according to the values of a given field (Figure 2). • For example, the width of the riparian buffer can vary depending on its expected function and the intensity of adjacent land use.

Ch 12. Vector Data Analysis I - Variations 12. 1 Buffering • A feature may have more than one buffer zone. • Example, a nuclear power plant may be buffered with distances of 5, 10, 15, and 20 miles, thus forming multiple rings around the plant • although spaced equally from the plant, are not equal in area

Ch 12. Vector Data Analysis I - Variations 12. 1 Buffering • Buffering around line features do not have to be on the same side of line • Buffer zones around polygon can be extended either outward or inward from polygon boundaries. • Boundaries of buffer zones may remain intact or dissolved

Ch 12. Vector Data Analysis 12. 1 Buffering. I - Applications • Buffer zone as protection zone § Prohibition of liquor store from school areas within certain distance § Oil drilling around roads or highways § Stream buffers against runoff § City planning • Buffer zone as neutral zone • Conflict resolution • Multiple ring buffer zone as sampling method • Land-use change around urban areas • Vegetation growth around streams

Ch 12. Vector. IData Analysis 12. 2 Overlay 1. 2. 3. 4. 5. 6. 7. What it is ? Feature & Type Overlay Methods Overlay of Shapefiles Slivers Error Propagation Applications of Overlay

Ch 12. Vector. IData Analysis 12. 2 Overlay 1. What it is ? – Overlay is an operation in GIS – Combines the geometries and attributes of two feature layers to create the output – The geometry of the output represents the geometric intersection of features from the input layers. – Each feature on the output contains a combination of attributes from the input layers, and this combination differs from its neighbors

Ch 12. Vector. IData Analysis 12. 2 Overlay 2. Feature & Type Overlay – For Overlay operations Feature types have to be considered – There are two groups of overlay operations. • The first group uses two polygon layers as inputs. • The second group uses one polygon layer and another layer, which may contain points or lines. – Overlay Operations are classified by Feature type into : • Point-in-polygon • Line-in-polygon • Polygon-in-polygon

Ch 12. Vector. IData Analysis 12. 2 Overlay 2. Feature & Type Overlay (…. cont) • To distinguish the layers, the layer that may be a point, line, or polygon layer is called the input layer, and the layer that is a polygon layer is called the overlay layer. • In a point-in-polygon overlay operation, the same point features in the input layer are included in the output but each point is assigned with attributes of the polygon within which it falls

Ch 12. Vector. IData Analysis 12. 2 Overlay 2. Feature & Type Overlay (…. cont) • For example, a point-in-polygon overlay can find the association between wildlife locations and vegetation types. Figure 5 : Point-in-polygon overlay. The input is a point layer. The output is also a point layer but has attribute data from the polygon layer.

Ch 12. Vector. IData Analysis 12. 2 Overlay 2. Feature & Type Overlay (…. cont) • In a line-in-polygon overlay operation, the output contains the same line features as in the input layer but each line feature is dissected by the polygon boundaries on the overlay layer • Thus the output has more line segments than does the input layer. Each line segment on the output combines attributes from the input layer and the underlying polygon.

Ch 12. Vector. IData Analysis 12. 2 Overlay 2. Feature & Type Overlay (…. cont) • For example, a line-in-polygon overlay can find soil data for a proposed road. The input layer includes the proposed road. The overlay layer contains soil polygons. And the output shows a dissected proposed road, each road segment having a different set of soil data from its adjacent segments. Figure Line-in-polygon overlay. The input is a line layer. The output is also a line layer. 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 polygon layer.

Ch 12. Vector. IData Analysis 12. 2 Overlay 2. Feature & Type Overlay (…. cont) • The overlay operation polygon-on-polygon involves two polygon layers. • The output combines the polygon boundaries from the input and overlay layers to create a new set of polygons • Each new polygon carries attributes from both layers, and these attributes differ from those of adjacent polygons. • For example, a polygon-on-polygon overlay can analyze the association between elevation zones and vegetation types

Ch 12. Vector. IData Analysis 12. 2 Overlay 2. Feature & Type Overlay (…. cont) Figure : Polygon-on-polygon overlay. In the illustration, the two layers for overlay have the same area extent. The output combines the geometries and attributes from the two layers into a single polygon layer.

Ch 12. Vector. IData Analysis 12. 2 Overlay 3. Overlay Methods • All overlay methods are based on the Boolean connectors AND, OR, and XOR. – – An overlay operation is called Intersect if it uses the AND connector. An overlay operation is called Union if it uses the OR connector. An overlay operation that uses the XOR connector is called Symmetrical Difference or Difference. And an overlay operation is called Identity or Minus if it uses the following expression: [(input layer) AND (identity layer)] OR (input layer)

Ch 12. Vector. IData Analysis 12. 2 Overlay 3. Overlay Methods • Union – Preserves all features from the inputs – Polygons only

Ch 12. Vector. IData Analysis 12. 2 Overlay 3. Overlay Methods • Intersect – Preserves only those features that fall within the area extent common to the inputs – Point, line, polygon + polygon – Output contains only those in the area you’re studying

Ch 12. Vector. IData Analysis 12. 2 Overlay 3. Overlay Methods • Symmetrical Difference – Preserves features that fall within the area extent that is common to only one of the inputs – Opposite of intersect – Only polygons

Ch 12. Vector. IData Analysis 12. 2 Overlay 3. Overlay Methods • Identity – Preserves only features that fall within the area extent of the layer defined as the input layer – Point, line, polygon + polygon (identity layer)

Ch 12. Vector. IData Analysis 12. 2 Overlay 4. Overlay of Shapefiles • • Unlike the coverage model, both shapefile and geodatabase allow polygons to have multiple components, which may also overlap with one another. This means that overlay operations can actually be applied to a single feature layer: • Union creates a new feature by combining different polygons, and • Intersect creates a new feature from the area where polygons overlap. But when used with a single layer, Union and Intersect are basically editing tools for creating new features. Many shapefile users are aware of a problem with the overlay output: the area and perimeter values are not automatically updated.

Ch 12. Vector. IData Analysis 12. 2 Overlay 5. Slivers • Common digitizing errors • Small polygons along correlated or shared boundary lines of input layers • To remove slivers: – Cluster tolerance – forces points and lines to be snapped together if they fall within the specified distance – Topology rules – force shared boundaries to be coincident before overlaying them – Minimum mapping unit (MMU) – the smallest area unit that will be managed by an organization • Combine slivers with adjacent polygons after overlaying them

Ch 12. Vector Data Analysis I 12. 2 Overlay 6. Error Propagation in Overlay • The generation of errors that are due to the inaccuracies of the input layers • Types: – Positional • Caused by inaccuracies of boundaries that are due to digitizing or interpretation errors – Identification • Caused by the inaccuracies of attribute data such as the incorrect coding of polygon values – Tend to have both • The accuracy of an overlay output decreases as the number of input layers increases • The accuracy decreases if the likelihood of errors occurring at the same locations in the input layers decreases

Ch 12. Vector Data Analysis 12. 2 Overlay 7. Applications of Overlay • For query and modeling • Finding suitable sites • Solving the area interpolation problem – Transferring known data from one set of polygons to another – Estimating population in a school district within multiple census tracts

Ch 12. Vector. IData Analysis 12. 2 Overlay 7. Applications of Slivers

Ch 12. Vector. IData Analysis 12. 3 Distance Measurement • Measuring straight line (Euclidean) distances between features • Measurements can be made from points in a layer to points in another layer, or from each point in a layer to its nearest point or line in another layer. • Measure distance between home and medical providers to evaluate geographic access to health services • Distance measures can be used directly for data analysis

Ch 12. Vector. IData Analysis 12. 4 Map Manipulation 12. 4. 1 Dissolve – Aggregate features that have the same attribute value(s) – Simplify a classified polygon layer – Dissolving the population from different counties to make it the total population of that state

Ch 12. Vector Data Analysis Map Manipulation 12. 4. 12 Clip – Creates a new layer that includes only those features of the input layer that fall within the area extent of the clip layer – To focus only the study area – Clip layer as polygon

Ch 12. Vector Data Analysis Map Manipulation 12. 4. 3 Append – Creates a new layer by piecing together two or more layers – The boundaries separating the features still remain

Ch 12. Vector Data Analysis Map Manipulation 12. 4. 4 Select – Creates a new layer that contains features selected from a user-defined query expression

Ch 12. Vector Data Analysis Map Manipulation 12. 4. 5 Eliminate – Creates a new layer by removing features that meet a user-defined query expression – Removing MMU

Ch 12. Vector Data Analysis Map Manipulation 12. 4. 6 Update – “cut and paste” to replace the input layer by the update layer and its features – Useful when you don’t need to update the whole map

Ch 12. Vector Data Analysis Map Manipulation 12. 4. 7 Erase – Removes unwanted features

Ch 12. Vector Data Analysis Map Manipulation 12. 4. 8 Split – Divides the input layer into two or more layers – Splitting a state up into counties