# Spatial Analysis of Vector Data by 2b3U0m2d

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```									     GIS in Archaeology

Spatial Analysis of Vector Data
Spatial Analysis: levels of sophistication

• Spatial data manipulation: classic GIS capabilities
– Spatial queries & measurement, buffering, map layer overlay
• Spatial data analysis: descriptive and exploratory
– Visualization through data manipulation and mapping
• Spatial statistical analysis: hypothesis testing
– Are data “to be expected” or are they “unexpected” relative to some
statistical model, usually of a random process
• Spatial modeling: prediction
– Constructing models (of processes) to predict spatial outcomes
(patterns)
Vector Analysis Techniques

•   Buffering
•   Overlay
•   Distance Measurement
•   Map Manipulation
Vector Buffering

• Principle of buffering is to create a buffer zone which contains the
area within a given distance of a feature
• Can buffer points, line or polygons
Vector Buffering

• Creating buffer zones
– Most common technique is a set buffer zone around all features
– Can also vary buffer zone distance for features by using a feature
attribute to define the size of the buffer zone
– Can create multiple buffer zone data sets as well
• Note the area size of outer buffers will be larger than the inner buffers
Buffering Applications

• Specify that features must be within a specific distance of a feature
– Must build within ½ mile of an existing road
• Specify that features cannot be within a specific distance of a
feature
– Cannot drill within ¼ mile of a river
• Buffers may be feature classes unto themselves
Vector Overlay

• Vector overlay techniques are used to combine data from multiple
vector layers
• Output features contain a combination of attributes from the input
vector layers
• Vector overlay types
– Point in polygon
– Line in polygon
– Polygon on polygon
Vector Overlay Methods

• Two basic overlay methods
– Union
• Output data set contains all features and data from the input data
– Intersection
• Output contains only those features spatially overlap
Vector Overlay Issues

• Error Propagation
– Positional Errors
• Mapping inaccuracies of input layers can lead to slivers.
– Identification Errors
• Errors in input layer attributes will continue through later overlay analyses
– Accuracy of composite maps tends to decrease as more input layers
are used.
• Accuracy of resultant map can never be better than the errors of the most
inaccurate source layer.
Vector Distance Measurements

• GIS programs contain functions to calculation the distance between
features within the same layer or between different layers
• Distance measurement methods
– Euclidean measurement
• Measures the shortest straight line distance between features
– Non-Euclidean measurement
• Measures distance where euclidean spatial distance is distorted by
other factors
– Driving distance must be calculated along a road network
– Walking distance can take into account the effects of terrain
Map Manipulation Techniques

• Map manipulation can be similar to overlay techniques but it does
not combine attribute data from the input layers
• Dissolve
– Combines map features that share a common attribute
– Combine spatially adjacent features that share a common attribute
• Clip
– Limits spatial extent of a layer to be by another input map layer
• Merge
– Combines spatial discrete input layers to create a map that contains the
maximum extent of all the input layers
– Differs from edge matching in that the boundaries between the input
layers is maintained.
Dissolve              Clip                       Merge

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