Georeferencing

Georeferencing

• Used to describe the act of assigning locations to data or information

• Certain requirements include that they are: unique, have shared meaning, and are persistent through time

• Systems of georeferencing– Place names– Postal addresses– Postal codes– Coordinate systems (lat/long, UTM, etc.)

Georeferencing

• Coordinate Systems– Georeference system – Geographic – Projected

• Geodetic Datum– A general survey conducted to determine the position

of points on the Earth’s surface

• Map Projection– The transformation from a spherical Earth to a flat

surface

“Projections” as referred to in ArcGIS

Geographic Coordinate System(measured in Latitude & Longitude)

Projected Coordinate System(measured in feet, meters, int’l feet, etc.)

Geodetic Datum(estimates location of Latitude/Longitude)

Map Projection(uses mathematical equation to project Latitude/Longitude to

flat surface, resulting in a unit of feet, meters, etc.)

Geodetic Datum(estimates location of Latitude/Longitude)

UNITED STATES PUBLIC LAND SURVEY SYSTEM (USPLSS)

Example: Georeferencing System

USPLSS (Township/Range/Section)

• Land parcel system that uses a grid that is subdivided into Township, range, section, & quarter

• Principal Meridians (range) every six miles for E/W

• Baselines (that coincide with Latitude) for townships (N/S)

• Each grid is 36 sq miles and generally referred to as a “township”

• Each township is divided into 1 sq mile “section”

T2N R3E

Township T2N, R3E

NW 1/4, S9, T2N, R3E.

Township S9, T2N, R3E

GEOGRAPHIC COORDINATE SYSTEMS (GCS)

Example: Latitude & Longitude

Longitude Latitude

“a method for describing the position of a geographic location on the Earth's surface using spherical measures of latitude and longitude. These are measures of the angles (in degrees) from the center of the Earth to a point on the Earth's surface” (ESRI)

Latitude / Longitude

• Spherical shape of the Earth is easily divided into 360

• Prime Meridian – separates E & W• Equator - separates N & S• Approximately 70 miles between lines of

Latitude• Distance between lines of longitude vary

depending on latitude

Latitude – 45 45' 33'' or 45.759167

Degrees, Minutes, Second or Decimal Degrees

Longitude –122 43' 59'' or 122.733056

Degrees, Minutes, Second or Decimal Degrees

Ellipsoid or Oblate SphereA sphere slightly flattened at the poles

Geoid – True shape of the Earth caused by variation in the gravitational pull in different Earth densities

GEODETIC DATUMS

“Projections” as referred to in ArcGIS

Geographic Coordinate System(measured in Latitude & Longitude)

Projected Coordinate System(measured in feet, meters, int’l feet, etc.)

Geodetic Datum(estimates location of Latitude/Longitude)

Map Projection(uses mathematical equation to project Latitude/Longitude to

flat surface, resulting in a unit of feet, meters, etc.)

Geodetic Datum(estimates location of Latitude/Longitude)

Geodetic Datums

• Defines the size and shape of the Earth• Provides reference for measuring locations on

the Earth’s surface

• Estimate the location of latitude and longitude• Basis for Geographic & Projected Coordinate

Systems

• A diversity of datums are in use today

NAD 27 and NAD 83

• The North American Datum of 1927 (NAD 27)– Based on the Clarke Ellipsoid of 1866– Computer with a single survey point, Meades Ranch in Kansas as the

datum point– Coordinates are in feet

• The North American Datum of 1983 (NAD 83) – Based on the Geodetic Reference System of 1980 (GRS80) ellipsoid– Computed as a geocentric reference system with no datum point– Coordinates are in meters– Removed significant local distortions from the network– Much more compatible with modern survey techniques– Officially adopted as the legal horizontal datum for the US by the

federal govt and most states

WGS 1984

• World Geodetic Survey 1984• First world geodetic survey was in 1966 to

provide a universal standard for aviation, navigation, space travel, and lack of existing intercontinental survey

• Presently uses 1996 Earth Gravitational Model for Geoid (revised in 2004)

• Error estimated to less than 2cm• Used with Global Positioning Systems (GPS)

48.7438798534299-122.46818353793WGS 1984

48.7438798543649 -122.46818353793NAD 1983

48.7440490722656-122.46690368652NAD 1927

LatitudeLongitudeDatum

Bellingham, WA, coordinates in decimal degrees

PROJECTED COORDINATE SYSTEMS (PCS)

Examples: Universal Transverse Mercator (UTM), State Plane, Oregon Lambert Projection

Projected CS

• ‘defined on a flat, two-dimensional surface…a PCS has constant lengths, angles, and areas across the two dimensions’

• Always based on a map projection that is in turn based on a datum

“Projections” as referred to in ArcGIS

Geographic Coordinate System(measured in Latitude & Longitude)

Projected Coordinate System(measured in feet, meters, int’l feet, etc.)

Geodetic Datum(estimates location of Latitude/Longitude)

Map Projection(uses mathematical equation to project Latitude/Longitude to

flat surface, resulting in a unit of feet, meters, etc.)

Geodetic Datum(estimates location of Latitude/Longitude)

Geographic v. Projected

• Geographic Coordinate Systems– Based on global or spherical coordinate system such as

latitude-longitude

• Projected Coordinate Systems– based on a map projection such as transverse Mercator,

Albers equal area, or Robinson, which provide various mechanisms to project maps of the earth's spherical surface onto a two-dimensional Cartesian coordinate plane

Universal Transverse Mercator (UTM)

• Example of Projected Coordinate System• A series of 60 longitudinal zones, each 6 and

centered over a meridian

• Based on Transverse Mercator projection because of low N/S distortion

• Segments each longitudinal zone into 20 latitudinal zones - 8

UTM

• Northing:– Find the nearest grid line South of a point, and add

the distance North of the line

– South Zones use a “false northing” at the equator that equals 10,000,000N

• Easting: – Find the nearest grid line West of the point, and add

the distance East of the line

– Easting is equal to 0, 500,000m west of the central meridian

The grid value of line A-A is 357,000 meters east. The grid value of line B-B is 4,276,000 meters north. Point P is 800 meters east and 750 meters north of the grid lines; therefore, the grid coordinates of point P are north 4,276,750 and east 357,800.

Final UTM coordinate: 10 357800E 4276750N

State Plane Coordinate Systems

State plane systems were developed in the 1930’s to provide local reference systems that were tied to a national datum

Different map projections are chosen for each system to minimize distortion based on the state’s shape

Some states use a single state plane zone, while others are divided into several zones

State plane zone boundaries often follow county boundaries

Oregon Statewide PCS

MAP PROJECTIONS

Map Projections

• A map projection translates the locations on the globe onto the flat surface of a map

• All map projections distort the shapes of the features on a map, including area, distance, and direction

• Distortions are less significant if the map projection covers a small area

Map Projections

• Shape (Conformal)– Shape of features is preserved– Meridians and parallels intersect at right angles

• Equal Area (Equivalency)– Mapped areas have the same proportional relationship to

the areas on the Earth that they represent• Distance (Equidistant)

– Distances from the center of the projection to any other place on the map are the same

• Direction– Azimuths (angles from a point on a line to another point)

are portrayed correctly in all directions

Types of Projections

• Three general classes of map projections:

• Cylindrical projections result from projecting a spherical surface onto a cylinder

• Conic projections result from projecting a spherical surface onto a cone

• Azimuthal or Planar projections result from projecting a spherical surface onto a plane

• Other projections use direct mathematical calculations

Planar or Azimuthal Projections

Cylindrical Projections

Conic Projections

Orientation• Normal

– A cylinder projection is normally tangent (a line or a plane that touches a curve or a surface at a point) at the equator

– A cone is normally tangent along a parallel– A plane is normally tangent at the pole

• Transverse– The projection surface is turned 90 from normal– Cylinder – a meridian; Plane – equator; Cone – point at equator

(uncommon)

• Oblique– Projection surface lies somewhere between normal and transverse

Tangency

• Standard Line– The line of tangency between a projected surface and the

surface of the globe– Along this line, the map has no distortion– Away from the line, distortions occur and the amount of

distortion depends on the type of projection• Secant

– When the projected surface intersects the globe, cutting the Earth along a line or lines.

– Intersects the Earth along two separate lines

Planar or Azimuthal Projections

Cylindrical Projections

Conic Projections

EXAMPLES OF MAP PROJECTIONS

Mercator

• Retains shape (in small areas)• Scale is distorted largely at the poles (above

60 )

• Meridians and parallels intersect at right angles

• Any line on map is of constant direction (parallels or meridians)

• Distance only preserved at the equator

Sinusoidal

• Equal area projection• Distorts shape• Angles where the meridians and parallels

cross becomes increasingly acute towards the periphery

• Often presented as “interrupted” for less shape distortion

• Best for areas that are mainly N/S in extent

Interrupted sinusoidal map, with three full lobes per hemisphere

Azimuthal Equidistant

• Preserves distance – from the center of the projection

• Area and shape are not preserved• Useful for airline distances, seismic, and

radiowaves

Robinson

• Compromise projection – not actually geometrically projected on a surface, but are mathematically derived

• A mix of planar and cylindrical projections• Minimizes visually disturbing distortions• Preserves neither area nor shape precisely,

but strives to approximate both

• National Geographic uses this projection for world maps

EXAMPLES OF PROJECTED COORDINATE SYSTEMS

Lambert Conformal Conic

Albers Equal-Area Conic

Lambert Conformal Conic

Albers Equal-Area Conic

Standard Parallels 30 N & 46 N

Standard Parallels 33 N & 45 N

Oregon Statewide PCS

APPLYING CONCEPTS IN ARCGIS

Projections & GIS

• ‘Projections’ is a general term used in ArcGIS for coordinate systems, datums & spheroids

• GIS data files & data frames (in an .mxd) need to have defined projections• GIS data files have a .prj file that stores projection info

• GIS data MUST be defined in the correct projection, which is the projection used to create the data (GPS, Satellite image, digitizing from a paper map, etc.)• If downloading data, it will either have a .prj file that saves

the projection information OR the projection info is available in the documentation on website

Projections & GIS, con’t

• When the first layer is added to a map, it defines the projection in the .mxd data frame• If the first layer added does not have a defined projection,

neither will the data frame in the .mxd• A data frame with an unknown (undefined) projection, will not

be able to project “on-the-fly”• Can define or change the projection of the data frame, by going

to Data Frame properties>Coordinate System• All other files added to the same data frame will be

projected “on-the-fly”• Does not change the data file – it is a temporary • Data must have defined projected or geographic coordinate

system in order to project “on-the-fly”

Projection Tools

• When the Data file has an Unknownprojection– Use tool in ArcToolbox>Data Management Tools>

Projections and Transformations> Define Projection

• When data already has a projection – Use tool in ArcToolbox>Data Management Tools>

Projections and Transformations> Features> Project

Different map projections result in different spatial relationships between regions

Why Change Projections?

• What is the area & geographic extent you are mapping? • World, National, State or local projected coordinate

system

• Do you need the calculate area, length, or distance?• Equal area, equidistant, conformal

• Are you trying to create a specific visual appearance?

Additional Resources

• http://www.fes.uwaterloo.ca/crs/geog165/mapproj.htm

• http://nationalatlas.gov/articles/mapping/a_projections.html

• http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj.html

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