PHC7065 CRITICAL SKILLS IN DATA MANIPULATION FOR POPULATION SCIENCE
Spatial Data
Hui Hu Ph.D.
Department of Epidemiology
College of Public Health and Health Professions & College of Medicine
March 18, 2019
Introduction to Spatial Data
Lab: Spatial Data
Introduction to Spatial Data
Introduction to Spatial Data
Data Models
A geographic data model is a structure for organizing geospatial data so that it can be easily stored and retrieved.
Geographic coordinates
Tabular attributes
Spatial Data Models
Vector Model
 points, lines, polygons
Raster Model
 exhaustive regular or irregular partitioning of space
Points
Lines
Common Formats
 Wellknown binary (WKB) and wellknown text (WKT)
 the most common formats for spatial objects
 Keyhole Markup Language (KML)
 an XMLbased format, used by Google
 SRS is always SRID 4326
 Geography Markup Language (GML)
 an XMLbased format used in Web Feature Service
 Geometry JaveScript Object Notation (GeoJSON)
 a format based on JSON
 Scalable Vector Graphics (SVG)
 popular among highend rendering or drawing tools
 Extensible 3D Graphics (X3D)
Shapefiles
.shp  the file that stores the geometry of the feature
.shx  the file that stores the index of the feature geometry
.dbf  the dBASE file that stores the attribute information
.prj  the file that defines the shapefile's projection
.html, .htm, .xml  the files that usually contains metadata
.sbn and .sbx  store additional indices
Coordinate Systems and Projections
3D sphere
Geographic Coordinate System
2D flat
Projected Coordiate System
Geographic Coordinate Systems
 Longitude and latitude
 Units: Degrees (DMS or DD)
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
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
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
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: Spatial Data
git pull
PHC7065Spring2019Lecture7
By Hui Hu
PHC7065Spring2019Lecture7
Slides for Lecture 7, Spring 2019, PHC7065 Critical Skills in Data Manipulation for Population Science
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