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

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

3. a fundamental question ...
whywhywhywhywhy
perform
spatial
analysis ?

4. 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

5. 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

6. 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.

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

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

9. 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

10. 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

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

12. y = f(x)

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

14. 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

15. 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

16. 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.

17. Polygon on Polygon overlay for vector dataB 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.

18. Polygon on Polygon overlay for raster data

19. Polygon on Polygon algorithms
Algebraic
Statistical
Boolean

20. 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

21. Point
Area
Line

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

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

25. 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