Spatial Queries & Analysis in GIS Dr Nigel Trodd Coventry University.

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 modelsPredictions that support decision-makers Simulations, hypothesis testing

Objectives

In this lecture we willIdentify spatial queriesidentify 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?

SelectExtractInteractive query

examples

What Qs are spatial Qs?

What is where? 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 informationA method of analysis is spatial if the results depend on the locations of the entities being analyzed move the feature(s) and the results

changeOr the analysis modifies the geometry or

creates new geometry

Spatial operations

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

y = f(x)y = f(x)

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

Reclassify

Overlay analysis

Point-in-polygonLine-in-polygonPoint-on-lineLine-on-linePolygon-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

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.

A B

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 entitye.g., to determine areas impacted by a proposed

highway

Point

Line

Area

Contiguity analysis

Which states share a border with Missouri?

…a question of connectivity

Neighbourhood analysiscalculate 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 databaseAnalysis transforms data into informationSpatial analysis transforms spatial data4 basic types of spatial operationSeveral operators of each typeAlgorithms for raster and vector data models