Spatial Queries & Analysis in GIS Dr Nigel Trodd Coventry University

Aim Your aim is to better understand the reasons for and methods of spatial queries and spatial analysis

a fundamental question ... whywhywhywhywhy perform spatial analysis ?

Why? The cost of data input to a GIS is high, and can only be justified by the benefits of analysis / modelling / decision- making performed with the data. 60 polygons per hour = $1 per polygon estimates as high as $40 per polygon 500,000 polygon database costs $500,000 to create using the low estimate $20m using the high estimate

What can justify these costs? Query if it is faster than manual lookup Analyses which are simple in nature but difficult to execute manually map measurement, particularly area Analyses that exploit GIS capabilities for data integration Exploratory spatial data analysis Analyses that develop spatially distributed models Predictions that support decision-makers Simulations, hypothesis testing

Objectives In this lecture we will Identify spatial queries identify a range of spatial operations available to you in a GIS. explain the principles of the algorithms on which these operators are based.

what is data analysis? ...the data information transformation

So, what is a spatial query? Select Extract Interactive query examples

What Qs are spatial Qs? What is where? Where is what? What is found at a location? Where is what? Where can you find a feature? But, spatial queries are NOT spatial analysis

What is spatial analysis? Data analysis transforms data into useful information A method of analysis is spatial if the results depend on the locations of the entities being analyzed move the feature(s) and the results change Or the analysis modifies the geometry or creates new geometry

Spatial operations Reclassifying maps Topological overlay Berry, J.K., 1987, Fundamental operations in computer-assisted map analysis, International Journal of GIS 1 119-36. Reclassifying maps Topological overlay Measuring distance and connectivity Characterizing neighbourhoods ArcGIS has > 1000 functions so it makes sense to group them in some way

y = f(x)

Reclassify Reclassify and then merge adjacent areas with the same attribute i.e. change geometry

Overlay analysis Point-in-polygon Line-in-polygon Point-on-line Line-on-line Polygon-on-polygon But, visually superimposing 2 or more layers is NOT an overlay operation

Point in Polygon overlay Determine whether a point lies inside or outside a polygon used to assign crimes to police beats, voters to enumeration districts, children to school catchments, patients to PCTs

Point in Polygon Algorithm Draw a line from the point to infinity in any direction, and count the number of intersections between this line and each polygon’s boundary. The polygon with an odd number of intersections is the containing polygon: all other polygons have an even number of intersections.

Polygon on Polygon overlay for vector data B A Two polygons (A, B) are intersected to form 9 new polygons. One is formed from both input polygons; four are formed by Polygon A and NOT Polygon B; and four are formed by Polygon B and NOT Polygon A.

Polygon on Polygon overlay for raster data

Polygon on Polygon algorithms Algebraic Statistical Boolean

Proximity analysis: buffering Create a new area within a user-defined distance of an existing entity e.g., to determine areas impacted by a proposed highway

Point Area Line

Contiguity analysis Which states share a border with Missouri? …a question of connectivity

Neighbourhood analysis calculate an output value at a location from the values at nearby locations… as determined by a moving- window

Spatial Queries & Spatial Analysis Queries manipulate the database Analysis transforms data into information Spatial analysis transforms spatial data 4 basic types of spatial operation Several operators of each type Algorithms for raster and vector data models