PHC6194 SPATIAL EPIDEMIOLOGY
Spatial Data Types and Spatial Reference System
Hui Hu Ph.D.
Department of Epidemiology
College of Public Health and Health Professions & College of Medicine
January 23, 2019
Spatial Data Types and Spatial Reference System
Lab: PostGIS Part 1
Spatial Data Types and Spatial Reference System
Spatial Data Types
 Geometry:
 the planar type
 the very first model and still the most popular type
 the foundation of the other types
 uses the Cartesian math
 Geography:
 the spheroidal geodetic type
 lines and ploygons drawn on the earth's curved surface
 Raster:
 the multiband cell type
 space as a grid of rectangular cells, each containing a numeric array of values
 Topology:
 the relational model type
 models the world as a network of connected nodes, edges, and faces
 network
 These four types can coexist in the same database:
 e.g. you can have a geometry that defines the boundaries of a plant, and a raster that defines the concentration of toxic waste along each part of the boundary
Geometry Type
We can represent all geographical entities in 2D using 3 building blocks:
Point
Line
Polygon
 Simplified models of reality, and will never perfectly mimic the real thing
 Geometry type treats the world as a flat Cartesian grid
Geography Type
Similar to geometry, but account for the curvature of the earth
Shape of the earth
 Surface: The Earth's real surface
 Ellipsoid: Ideal, smooth surface
 Geoid: Bumpy surface, where gravity is equal for all locations
Shape of the earth (cont'd)
 Gauss determined in the early 19th century that the surface of the earth can be defined using gravitational measurements
 geoid: where gravity is equal for all locations

Geoid is far from spherical
 the core of the earth is not homogenous
 mass is distributed unevenly
 Geoid is the foundation of both planar and
geodetic models
Raster Type
Vector Model
 points, lines, polygons
 geometry and geography type
Raster Model
 exhaustive regular or irregular partitioning of space
Points
Lines
Topology Type
 Network of points, lines, and polygons
 Not concerned with the exact shape and location of geographic features, but with how they're connected to each other
 Useful in many applications:
 parcel (land lot) data, to ensure that the change of one parcel boundary adjusts all other parcels that share that boundary change as well
 road management, water boundaries, etc.
 architecture
Spatial Reference System
 SRS is the production of geodetics and cartography
 geodetics: the science of measuring and modeling the earth
 cartography: the science of representing the earth on flat maps
 Why do we need SRS?
 to bring in data from multiple sources and be able to overlay one atop another
 Many standards of SRS:
 most common one is the European Petroleum Survey Group (EPSG) numbering system
 take any two sources of data with the same EPSG number, and they will overlay perfectly
SRID
 Spatial Reference IDentifier
 It defines all the parameters of our data’s geographic coordinate system and projection.
 An SRID is convenient because it packs all the information about a map projection (which can be quite complex) into a single number.

http://spatialreference.org/ref/epsg/4326/

EPSG is a very recent SRS numbering system
 If you are using data from a few decades ago, you won't find EPSG number

The constituent pieces that form an SRS:
 ellipsoid
 datum
 projection
Ellipsoid
 Simplifications of the geoid which are generally good enough for most geographic modeling needs
 An ellipsoid is merely a 3D ellipse
 Instead of one ellipsoid to rule us all, people on different continents wanted their own ellipsoids to better reflect the regional curvature of the earth
 Today, the world is settling on the World Geodetic System (WGS 84) and Geodetic Reference System (GRS 80) ellipsoids
 WGS 84 is the standard of choice, and is what all GPS systems are based on
Common ellipsoids and their ellipsoidal parameters
 Lon/lat with different ellipsoid are not the same
 they use different grounding points
 it's important to not just call things lon/lat: you can have NAD27 lon/lat, NAD80 lon/lat, etc. Each will be subtly different
Datum
 Ellipsoid only models the overall shape of the earth
 after picking out an ellipsoid, you need to anchor it to use it for realworld navigation
 even if two reference systems use the same ellipsoid, they can still have different anchors, or datum, on earth
 Defines the position of the spheroid relative to the center of the earth.
 Global datum:
 uses the earth's center of mass as the origin
 Local datum:
 aligns its spheroid to closely fit the earth's surface in a particular area
 a point on the surface of the spheroid is matched to a particular position on the surface of the earth
 the coordinate system origin of a local datum is not at the center of the earth
Coordinate Reference System
 A coordinate reference system is only one necessary ingredient that goes into the making of an SRS and isn't SRS itself
 used to identify a point on your reference ellipsoid
 Most popular coordinate reference system for use is the geographical coordinate system
 also known as geodetic coordinate system or simply lon/lat
 Longitude and latitude
 Units: Degrees (DMS or DD)
Projection
Taking an ellipsoidal earth and squashing it onto a flat surface
 Projection has distortion built in
 because geodetic and 3D globes are ellipsoidal, they by definition do not refer to a flat surface
 Why do we need to have 2D projections?
 the mathematical and visual simplicity that comes with planar (Euclidean) geometry
Distortion
 How exactly you squash an ellipsoidal earth on a flat surface depends on what you are trying to optimize for
 In creating a projection, we try to balance four conflicting features:
 measurement
 shape: how accurately does it represent angles
 direction: is north really north
 range of area supported
 E.g. if you want to span a large area, you have to either give up measurement accuracy or deal with the pain of maintaining multiple SRSs and some mechanisms to shift among them
Projection Types
Cylindrical projections
Conic projections
Azimuthal projections
Orientation of the paper roll around the globe
Main classes of planar coordinate systems
 Lambert Azimuthal Equal Area (LAEA)
 good for measurement and can cover large areas, but not great for shape
 US National Atlas (EPSG:2163)
 Lambert Conformal Conic (LCC)
 preserve shape more than area, good for measurement for the regions they serve, and distort poles
 best used for middle latitudes with eastwest orientation
 Universal Trans Mercator (UTM)
 good for measurement, shape, and direction, but only span sixdegree longitudinal strips, cannot be used for the polar regions
 Mercator
 good for preserve shape and direction, and spanning the globe, but not good for measurement
 common favorites for web map display since we only need to maintain one SRID
 National grid systems
 variant of UTM or LAEA, but are used to define a restricted region, such as a country
 State plane
 US spatial reference systems, usually designed for a specific state
 most are derived from UTM
Universal Transverse Mercator Coordinate System
 World divided into 60 sixdegreewide zones
 From 80S to 84N
 Zones numbered 160 (N&S), W to E, starting at 180W
Differences between projections
What spatial reference system is appropriate?
 Excellent: covers the globe
 Good: covers a large country like the US; the measurements for the area served are usually within a meter for length, area, and distance calculations
 Medium: covers several degrees or a large state; measurements are accurate within meters, but can be as much as 10 meters off
 Bad: measurements don't have useful units
Lab: PostGIS Part 1
git pull
Type Modifiers
geometry(POINT,4326)
data type
subtype type modifier
SRID type modifier
Geometry: Points and Linestrings
POINT
POINTZ
POINTM
POINTZM
LINESTRING
LINESTRINGZ
LINESTRINGM
LINESTRINGZM
 A point in 2D space specified by its X and Y coordinates
 A point in 3D space specified by its X, Y, and Z coordinates
 A point in 2D space with a measured value specified by its spatial X and Y coordinates plus an M value
 A point in 3D space with a measured value specified by its X, Y, and Z coordinates plus an M value
 A linestring in 2D specified by two or more distinct POINTs
 A linestring in 3D specified by two or more distinct POINTZs
 A linestring in 2D specified by two or more distinct POINTMs
 A linestring in 3D specified by two or more distinct POINTZMs
Geometry: Polygons
 Closed linestrings are the building block of polygons
 A polygon contains all the enclosed area, and its boundary is the linestring that forms it
 The enclosed linestring outlining the boundary of the polygon is called the ring of the polygon. More specifically, it's the exterior ring.
POLYGON
Collection of Geometries
 A collection of geometries groups separate geometries that logically belong together:
 multipoints
 multilinestrings
 multipolygons
 geometrycollection
 can contain any kind of geometry as long as all geometries in the set have the same spatial reference system and the same coordinate dimensions
Geography
 In PostGIS, geography starts by assuming that all your data is based on a geodetic coordinate system, specifically the WGS 84 lon/lat SRID of 4326
 Only include support for basic subtypes of points, linestrings, and polygons, and no support for anything above 2D space
 The structure of the geography data mimic those of geometry
 everything you know about geometry applies to geography with no changes except for swapping out the term geometry for geography in both data type and function names
PHC6194Spring2019Lecture3
By Hui Hu
PHC6194Spring2019Lecture3
Slides for Lecture 3, Spring 2019, PHC6194 Spatial Epidemiology
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