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GIS

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What is Spatial Analysis? Various methods of looking geographic patterns in your data and the relationship ... Isopleth maps (Greek: isos equal) ... – PowerPoint PPT presentation

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Title: GIS


1
GIS 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

2
What 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

3
Elements of Spatial Analysis
  • Formulate your question (s).
  • Data requirements based on your question (s)
  • Selection of method (s)
  • Data processing
  • Displaying your results

4
Formulate 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

5
Data 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

6
Selection 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.

7
Data processing using GIS
  • GIS provides the necessary tools for implementing
    the selected methods.

8
Spatial Public Health/MCH Data
  • Geographic data can be either
  • Discrete
  • Continuous or
  • Aggregated by polygon (area)

9
Discrete 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

10
Continuous 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.

11
Spatially 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.

12
Is 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.

13
Representation of Geographic Features
  • Two basic models for representing geographic
    features are
  • Vector
  • Raster

14
Representing Spatial Elements
  • RASTER
  • VECTOR
  • Real World

15
Representing 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)
16
Representing 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.
17
Entity 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.
18
Two 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!

19
Map 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.

20
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21
Properties 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.

22
Geographic 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)

23
A 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.
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