Spatial Analysis of Vector Data by 2b3U0m2d


									     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
             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
    – Cannot drill within ¼ mile of a river
• Buffers may be feature classes unto themselves
    – Road right-of-way
                           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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