Geog 458: Map Sources and Error January 18, 2006 Georeferencing

Outlines Placenames Placenames Postal addresses Postal addresses Metric georeferencing Metric georeferencing 1D: Linear referencing systems 1D: Linear referencing systems 3D: Unprojected coordinate system 3D: Unprojected coordinate system Geodetic model Geodetic model 2D: Projected coordinate system 2D: Projected coordinate system Map projection: how it works Map projection: how it works Grid coordinate systems: UTM, SPC Grid coordinate systems: UTM, SPC Metadata content standard: spatial reference Metadata content standard: spatial reference

Placenames Simplest form of georeferencing Simplest form of georeferencing Where do you live? I live in university district Where do you live? I live in university district Limited use in GIS due to imprecise location; meaning of placenames Limited use in GIS due to imprecise location; meaning of placenames can vary between people, and with context can vary between people, and with context can be uniquely identified with proper geographic hierarchy can be uniquely identified with proper geographic hierarchy can change over time can change over time To be used in GIS or other applications, it is necessary to systematically identify precise location of names of geographic features To be used in GIS or other applications, it is necessary to systematically identify precise location of names of geographic features GNIS (Geographic Names Information System) GNIS (Geographic Names Information System) In the website, find out what it is, how it works, and purposes In the website, find out what it is, how it works, and purposes

Postal addresses Widely used to identify the location of dwellings Widely used to identify the location of dwellings Based on several assumptions Based on several assumptions Local areas are identified with proper geographic hierarchy Local areas are identified with proper geographic hierarchy Path is unique within local areas Path is unique within local areas Dwellings are numbered consecutively along the path Dwellings are numbered consecutively along the path How does address-matching work? How does address-matching work? Look at TIGER/Line road data Look at TIGER/Line road data Linear interpolation based on topologic data Linear interpolation based on topologic data Any limitation? Any limitation?

Linear referencing Identify location on a network by measuring distance from a defined point of reference along a defined path in the network Identify location on a network by measuring distance from a defined point of reference along a defined path in the network 1-dimensional: stored in a tabular form 1-dimensional: stored in a tabular form Widely used in highway agency (DOT): vary by State, reference point is split along administrative boundary Widely used in highway agency (DOT): vary by State, reference point is split along administrative boundary Look at NHPN data Look at NHPN data

Geodetic model To determine the location precisely, it is necessary to define the shape of earth To determine the location precisely, it is necessary to define the shape of earth Earth is not quite spherical, but rather ellipsoid Earth is not quite spherical, but rather ellipsoid Ellipsoid (a.k.a. spheroid): geodetic model of the earth Ellipsoid (a.k.a. spheroid): geodetic model of the earth Flattening ratio defines how the model differs from sphere Flattening ratio defines how the model differs from sphere Different ellipsoid has different flattening ratio Different ellipsoid has different flattening ratio Ellipsoid can be either earth-centered (global) or not (local), arising from the need to approximate the shape of the earth suited to varying coverage areas Ellipsoid can be either earth-centered (global) or not (local), arising from the need to approximate the shape of the earth suited to varying coverage areas NAD29 is local datum NAD29 is local datum WGS84 is global datum WGS84 is global datum

Unprojected coordinate system 3 dimensional 3 dimensional Latitude: angle from the equator Latitude: angle from the equator Longitude: offset from prime meridian Longitude: offset from prime meridian Geographic unit of measurement Geographic unit of measurement Decimal degrees = degree + minute /60 + second / 3600 Decimal degrees = degree + minute /60 + second / 3600 Other names Other names Geographic system of coordinates Geographic system of coordinates Plate Carree Plate Carree Since it’s unprojected, it gives a heavily distorted image of the earth (e.g. Antarctica) Since it’s unprojected, it gives a heavily distorted image of the earth (e.g. Antarctica)

Projected coordinate system 2-dimensional; involves map projection (transformation of 3D to 2D); a.k.a. planar coordinate system 2-dimensional; involves map projection (transformation of 3D to 2D); a.k.a. planar coordinate system Projection family (the shape of developable surface) Projection family (the shape of developable surface) Cylindrical, conic, planar Cylindrical, conic, planar Projection properties (which geometric property is constant?) Projection properties (which geometric property is constant?) Equal-area, conformal, equidistance, azimuthal Equal-area, conformal, equidistance, azimuthal Tangency: Tangent, secant Tangency: Tangent, secant Aspect: Normal, transverse, oblique Aspect: Normal, transverse, oblique Standard line or standard parallel Standard line or standard parallel Where earth intersects developable surface Where earth intersects developable surface Line of zero distortion Line of zero distortion

Grid coordinate systems UTM UTM Universal Transverse Mercator Universal Transverse Mercator Projection family? Projection family? Projection property? Projection property? Tangency? Tangency? Aspect? Aspect? 1 zone is ? degree 1 zone is ? degree SPC SPC State by state State by state Use three different map projections depending on the shape of zones in State: what are the three projections? Use three different map projections depending on the shape of zones in State: what are the three projections? Difference between SPC27 and SPC83? Difference between SPC27 and SPC83?

Metadata Content Standard: Section 4: Spatial Reference

Reading metadata content standard

Exercise Georeferencing your house Georeferencing your house Local Local Placenames Placenames Postal address Postal address Linear referencing systems Linear referencing systems Geographic Geographic Planar Planar UTM UTM SPC83 SPC83