Integrated analysis of spatial and attribute data

Retrieval/classification/measurement functions
Overlay operations
Neighbourhood operations
Connectivity functions

These GIS operations are especially applicable to environmental modelling.

Retrieval/classification/measurement functions

Spatial and attribute data are retrieved but only the attribute data are modified or created, no changes are made to the spatial elements.

• Retrieval operations involved selective search, manipulation and output of data.
  
  
• Classification is the procedure of identifying a set of features belonging to a group and assigning a name to that group.
  
  
• Measurement functions is the calculation of distances between points, length of lines, perimeters and areas of polygons and the size of a group of cells with the same class.
  

Overlay operations

This is divided into arithmetical and logical overlay. Arithmetical overlay includes addition, subtraction, division and multiplication of each value in the data layer by a value in the corresponding location in another data layer Logical overlay involves finding locations where a specified set of conditions occur or do not occur together

Overlay of flood zone with cadastral map 20kb

Neighbourhood operations

These evaluate the characteristics of the area surrounding a specified location - include operations such as search, line in polygon, point in polygon, topographic functions, theissen polygons, interpolation, contour generation

• Search assigns a value to each target feature according to some characteristic of its neighbourhood. The search area is commonly a square, rectangle or circular with the size selected by the operator. The search area is often termed a 'window'. There are 2 types of search functions - those that operate on numerical data and those that operate on thematic data
  
  
  • Numeric functions are functions like the total, average, maximum, minimum and measures of diversity (variance or standard deviation)
    
    
  • Thematic functions are functions like the majority, maximum, minimum and diversity (number of different classes)
    
    
• Line-in-polygon and point-in-polygon operations this is the identification of points and lines contained within polygons. In vector GIS this is a search function in raster GIS this is essentially an overlay operation.
  
  
• Topographic functions topography relates to surface characteristics ie relief. Topographic functions are used to calculate values that describe the topography at specific geographic locations or in the vicinity of locations eg aspect at the location or slope of the area around the location.

Connectivity functions

These are operations that use functions that accumulate values over the area being traversed eg contiguity measures, proximity, network, spread, seek, intervisibility, illumination, perspective view.

• Contiguity measures evaluate characteristics of spatial units that are connected. Common measures are the size of the contiguous area and shortest and longest straight-line distances across the area.
  
  
• Proximity is a measure of the distance between features. A common proximity function is buffer zone generation. A buffer zone is an area of specified width drawn around one or more map features.
  
  
• Network functions A network is a set of interconnected linear features that form a pattern or framework. Networks are commonly used for moving resources from one location to another. There are 3 principal types of network analyses that GIS perform
  
  
  • network loading eg predicting flows of water through a stream network
    
    
  • route optimisation eg routing of emergency vehicles
    
    
  • resource allocation.eg division of an area into service zones

• Spread evaluates phenomena that accumulate with distance eg it can be used to evaluate transportation time or cost over a complex surface. Spread functions have characteristics of proximity and network functions. They can generate accumulation surfaces.
  
  
• Seek (or stream function) performs a directed search outward in a step-by-step manner from a start location using a specified decision rule.
  

Intervisibility (viewshed modelling) can be used to map the area visible from certain locations eg from a scenic lookout. A viewshed is an area that can be seen from specified target locations. These functions make use of digital elevation data.

Viewshed 20 kb

• Illumination portrays the effect of shining a light onto a 3-dimensional surface. The output is a shaded relief image. This function is controlled by:
  
  
  • the position of the illumination source,
    
    
  • the topography and reflectance of the surface
    
    
  • the position and direction from which the model is viewed
    
    
• Perspective A perspective view is a surface portrayed from a viewing position other than vertical. These are primarily presentation tools, useful for showing 3-dimensionality of features on a surface.
  
  (Aranoff, 1989)