Title: GIS
1GIS Spatial Analysis in MCH
- Ravi K. Sharma, PhD
- Department of Behavioral Community Health
Sciences, - Graduate School of Public Health,
- University of Pittsburgh,
- Pittsburgh, Pa 15261
2What is Spatial Analysis?
- Various methods of looking geographic patterns in
your data and the relationship between features - The actual methods used can simple (such as
making a map) or complex (such as creating a
spatial data model combining multiple data layers
3Elements of Spatial Analysis
- Formulate your question (s).
- Data requirements based on your question (s)
- Selection of method (s)
- Data processing
- Displaying your results
4Formulate your question (s).
- Start your analysis by asking what you need to
know? Be specific. This could be question such
as How many women of childbearing age live
within two mile radius of a TRI site? Is there a
childhood leukemia cluster in my county? What
neighborhood (s) in my county has significantly
higher death rate from breast cancer? What do
you know about levels of exposure in relation to
distance from the TRI site? - Specificity? analysis ? Methods ?Presentation
5Data requirements based on your question (s)
- Type of data feature ? Methods selected
- Type of feature and attribute data available or
that you can get or create - Data creation refers to calculation of new values
based on existed data or obtaining new layers
6Selection of method (s)
- Decision will be guided by the (1) question for
which you seek answer (2) availability depth of
data (3) processing time effort (4) precision
of results (5) how the results are going to be
applied? - E.G. If you are looking at patterns of mortality
you might decide to simply map the mortality
rates on the other hand if a particular
industrial plant in being charged with causing a
particular disease in a community, you might need
more precise and detailed data.
7Data processing using GIS
- GIS provides the necessary tools for implementing
the selected methods.
8Spatial Public Health/MCH Data
- Geographic data can be either
- Discrete
- Continuous or
- Aggregated by polygon (area)
9Discrete features
- Discrete data are geographic features for which
actual locations can be specified. A feature is
either present or absent at any given spot. A
discrete object has known and definable
boundaries. It is easy to define precisely where
the object begins and ends
10Continuous features
- Continuous data, or a continuous surface,
represents phenomena where each location on the
surface is a measure of the concentration level
or its relationship from a fixed point in space
or from an emitting source. - Continuous data is also referred to as field,
nondiscrete, or surface data. - One type of continuous surface data is derived
from a series of sample points such as ozone
concentrations measurement from air pollution
monitoring stations at fixed locations. - The second type (Progressively varying continuous
data) of continuous surface data includes
phenomena that progressively vary as they move
across a surface from a source. - One type of movement is through diffusion or any
other locomotion where the phenomena moves from
areas with high concentration to areas with less
concentration until the concentration level evens
out, such as a oil spill. - Another type of movement is governed by inherent
characteristics of the moving item or by the mode
of locomotion.
11Spatially aggregated features
- Public health/MCH data is usually available as
summary data for various geographic levels. For
example counts of immunized children, low births
weights babies, childhood leukemia case etc by
census tracts or counties.
12Is your Data Discrete or continuous?
- When representing and modeling many public health
features, the boundaries are not clearly
continuous or discrete. If we conceptualized
consisting of a continuum is created with pure
discrete at one end and pure continuous features
at the other end, most features fall somewhere
between the extremes. The decisive factor for
where a feature falls on the continuous-to-discret
e spectrum is the ease in defining the feature's
boundaries.
13Representation of Geographic Features
- Two basic models for representing geographic
features are - Vector
- Raster
14Representing Spatial Elements
15Representing Spatial Elements
Raster
Stores images as rows and columns of numbers with
a Digital Value/Number (DN) for each cell. Units
are usually represented as square grid cells that
are uniform in size.
Data is classified as continuous (such as in an
image), or thematic (where each cell denotes a
feature type. Numerous data formats (TIFF, GIF,
ERDAS.img etc)
16Representing Spatial Elements
Vector
Allows user to specify specific spatial locations
and assumes that geographic space is continuous,
not broken up into discrete grid squares We store
features as sets of X,Y coordinate pairs.
17Entity Representations
We typically represent objects in space as three
distinct spatial elements
Points - simplest element Lines (arcs) - set of
connected points Polygons - set of connected lines
We use these three spatial elements to represent
real world features and attach locational
information to them.
18Two Major Categories of Maps
- Choropleth maps (Greek choros place, plethos
magnitude) - Classifies areas into categories based values on
one or more variables - Most common method of mapping health data
- Isopleth maps (Greek isos equal)
- Interpolates lines of equal value across the
spatial surface, independent of administrative
boundaries - Examples include weather maps, topographic maps
- Not common in public health . . . But they should
be!
19Map Projections Coordinate System
- Map projections are attempts to represent the
surface of the earth or a portion of the earth on
a flat surface - Since the earth is a spheroid any attempt to
flattened it to a plane any attempt to
represents the earth's surface in two dimensions
causes distortion in the shape, area, distance,
or direction of the data.
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21Properties That Are Distorted
- Projections tend to distort to the following
properties - Conformality Conformal projections preserve
local shape When the scale of a map at any point
on the map is the same in any direction, the
projection is conformal. Meridians (lines of
longitude) and parallels (lines of latitude)
intersect at right angles. - Distance A map is equidistant when it portrays
distances from the center of the projection to
any other place on the map. - Direction A map preserves direction when
azimuths (angles from a point on a line to
another point) are portrayed correctly in all
directions. - Scale Scale is the relationship between a
distance portrayed on a map and the same distance
on the Earth. - Area When a map portrays areas over the entire
map so that all mapped areas have the same
proportional relationship to the areas on the
Earth that they represent, the map is an
equal-area map.
22Geographic Coordinate Systems
- A geographic coordinate system is a reference
system that uses a three-dimensional spherical
surface to determine locations on the earth. - A point is referenced by its longitude and
latitude values. - Longitude and latitude are angles measured from
the earth's center to a point on the earth's
surface. The angles often are measured in degrees
(or in grads)
23A Geographic Coordinate System
This figures shows a geographic coordinate system
where a location is represented by the
coordinates longitude 80 degree East and latitude
55 degree North.