Transcript Spatial Queries & Analysis in GIS

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 / decisionmaking 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
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?

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
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)
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
A
B
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 movingwindow
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