Geog
258: Maps and GIS
February
27, 2006
GIS:
Analytics
Geographic questions
GIS provides tools to answer
geographic questions
Geographic questions range
from very simple (how far is from A to B) to complex one (where should be the suitable
site for a new landfill?)
The process to answer
geographic questions can be thought of the series of input – operation –
output, where operations can be thought of a function that transforms input to
output
Answering geographic
questions with GIS follows these steps below:
1) Frame the question
2) Select the data relevant to answering questions
(input)
3) Choose analysis methods (operation)
4) Process the data (process diagram)
5) Look at the results (output)
For example,
1) Question: Where is nearest
hospital from my place?
2) Data: my place, hospital
3) Analysis method: Distance
(myplace, hospital)
4) Process diagram:
List distance measures
between myplace and hospital;
Pick the minimum among the
list
5) Output: Hospital with the
minimum distance
One of the most valuable
skills in GIS is the ability to take a real problem and convert it into a
series of GIS operations. So what types of operations are available in GIS?
Taxonomy of GIS operations
Attribute operations
Decreasing information contents
Increasing information contents
Spatial operations
Object operations
Unary operations
Binary operations
Topological
Metric
Field operations
Local
Focal
Zonal
Types of attribute operations
Operations performed on
attributes attached to geographic features. Let’s say you perform operations on
attributes of the hospital nearest your place (e.g. the number of beds in the
hospital).
There are two kinds of operation
performed on a single attribute value: reducing information content or
increasing information content.
The operation reduces the
information content if level of measurement is lowered.
e.g. select from the hospital where the number of bed is
more than 100
The operation increases the
information content if level of measurement is raised.
e.g. rank the hospital based on multiple nominal
attributes
Types of spatial operations
Can be divided into two types
of operations depending on data model
Operations performed on
discrete object: object operation
Operations performed on
continuous field: field operation
The difference between object
and field operations comes from the fact that geometric dimensionality of
object is easily identified whereas that of field is not
1. Object operation
1.1 Unary spatial operations
Calculate geometric
properties intrinsic to a single feature
Possible geometric properties
of a single feature are different depending on the dimensionality of a feature
1.1.1 Operations on
zero-dimensional feature
Location
Where is
1.1.2 Operations on
one-dimensional feature
Location, distance,
direction, sinuosity
In which direction is this
road heading? Direction (A) = 180 degree
How long is this runway?
Length (A) = 1000 meter
1.1.3 Operations on
two-dimensional feature
Location, distance
(parameter), direction, area, shape, compactness, and so on
How large is this land
parcel? Area (A) = 10 acre
1.2 Binary spatial operations
Calculate spatial
relationships between features
Spatial relationships between
features can be divided into topological and metric
1.2.1 Topological
Qualitative spatial relation;
invariant properties of geometric figures under continuous deformation (e.g.
adjacency, containment, overlap); it defines spatial relations
1.2.2 Metric
Quantifiable spatial relation
(e.g. distance, direction); it refines spatial relations
In the cell below, write down
all possible spatial relations (both metric and topological)
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Point |
Line |
Polygon |
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Point |
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Line |
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Polygon |
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Not too straightforward
Most of the time, cases are
generalized into one of the followings:
Point-to-point metric
relation
Point-to-polygon topological relation
Polygon-to-polygon
topological relation
e.g. To measure the distance between
For example,
To answer the question “Is
this café in WIFI zone?”
Set café = a AS point
Set WIFI zone = b AS polygon
Choose topological relation
between café and WIFI zone, say within
Within (a, b) = True or False
Q. Dimensionality of features
and type of operations?
1. How far is my place to the
nearest ramp to I5
2. The proximity of my place
to crime hot spots
3. Demographics of areas
likely to be affected by flooding zone
2. Field operation
2.1
Local operation
Value
of the new field at a given location in the spatial framework depends only on
the value of the input field at that location
e.g. Identify mountain ranges with elevation over 2000 feet
2.2
Focal operation
Value
of the resulting field at a given location depends on the values that the input
field assumes in a small neighborhood of the location
e.g. What is the slope gradient at point A?
2.3
Zonal operation
Zonal
operations are naturally associated with aggregate operators or the integration
function.
e.g. What is the average elevation of a set of river basins?
Overlay: a key tool in GIS
1) Vector overlay
Which road segments are
likely be affected by flood?
Think of polygons in Layer A
as flood zones
Think of lines in Layer B as
road
With overlay, it is possible to
identify roads that are likely to be flooded.
Is road within flood zone?
Set road in Layer B as line
Set flood zone in Layer A as
polygon
Choose topological operation
within (B, A)
Identify subset of B where
within (B, A) is True
2) Raster overlay
Estimate the total runoff in
a precipitation event.
The equation for the model
is:
where:
S = the surface slope,
categorized into values of 1 (0 to 3 degrees), 2 (3 to 6 degrees), 3 (6 to 9
degrees), or 4 (greater than 9 degrees),
C = ground cover coefficient,
a value of 10 for dense, broad leaf cover, 20 for grass or mixed coniferous
forest, 30 for sparse canopy forest, and 40 for bare ground,
P = Precipitation in
millimeters, and
R = Runoff volume of water,
in liters per square meter
These are elements of process
diagram: input – operation - output
1) Input
2) Operation
(S * C * P)/160
3) Output
Q. What type of operation
among the following is used in this run-off model application?
a) local b) focal c) zonal