Introduction to Coordinate Systems and Projections Rob Juergens, Annette Locke

Introduction We want to give you a basic understanding of Coordinate systems Projecting data Transforming data

Geographic (GCS) Projected (PCS) Coordinate Systems

What is a coordinate system? A way of describing points to answer questions about distance, location, and direction

Distance Q: How far is Redlands? A: 32 million 32 million what? Seems like a lot. 32 million feet from Beijing. What were you expecting? Miles from where I am right now. A coordinate system defines what is expected

Location Q: Where is San Diego? A: (-116.53, 33.82), no wait, it is (-12973635.720, 4006201.638) What do these numbers mean? (-116.53, 33.82) are longitude and latitude in decimal degrees (-12973635.720, 4006201.638) are xy-coordinates in meters How am I supposed to know that? A coordinate system defines what these numbers mean

Direction Q: I am looking at a map and want to place a marker at the North Pole. Where do I put it? A: At the top of the map. Hmm, or at the center. Or … hang on, it’s impossible. The North Pole isn’t on the map! A coordinate system defines where a particular location is on the map or specifies that it isn’t on the map at all

Geographic Coordinate System (GCS) Global – 3D spherical surface Point referenced by longitude and latitude values

Projected Coordinate System (PCS) Flat – 2D surface based on a GCS Point referenced by x, y coordinates on a grid X Y (0,0) X > 0 Y > 0 X < 0 Y < 0

Coordinate System Projected Coordinate System Geographic Coordinate System Projection Linear Unit Datum Prime Meridian Projection Parameters Spheroid Angular Unit

Two ways to specify a coordinate system Well-known ID (WKID) Predefined coordinate systems only Well-known text (WKT) Predefined or custom

Well-Known ID (WKID) Every predefined coordinate system has a WKID For example, GCS_WGS_1984, WKID = 4326 WKID < 32767 is EPSG assigned EPSG Geodetic Parameter Dataset, http://www.epsg-registry.org/ WKID > 32767 is Esri assigned Esri WKID may change Esri  EPSG Old WKID will still work Example, Web Mercator 102100  3857

Geographic Coordinate System Well-Known Text (WKT) GEOGCS[ "GCS_WGS_1984", DATUM[ "D_WGS_1984", SPHEROID[ "WGS_1984", 6378137.0, 298.257223563] ], PRIMEM[ "Greenwich", 0.0], UNIT[ "Degree", 0.0174532925199433] ]

Projected Coordinate System Well-Known Text (WKT) PROJCS[ “World_Mercator", GEOGCS[ "GCS_WGS_1984", DATUM[ "D_WGS_1984", SPHEROID[ "WGS_1984", 6378137.0, 298.257223563] ], PRIMEM[ "Greenwich", 0.0], UNIT[ "Degree", 0.0174532925199433] ], PROJECTION[ "Mercator " ], PARAMETER[ "Central_Meridian", 0.0], PARAMETER[ "Standard_Parallel_1", 0.0], PARAMETER[ "False_Easting", 0.0], PARAMETER[ "False_Northing", 0.0], UNIT[ “Meter", 1.0] ]

SADD = Shape Area Direction Distance Web Mercator distorts area and distance Which is bigger? Greenland South America Antarctica South America is 17 million sq km Antarctica is 14 million sq km Greenland is 2 million sq km

Preserve Shape and Direction Stereographic

Albers Equal Area Conic Preserve Area Albers Equal Area Conic

Preserve Direction Gnomonic

Preserve Direction and Distance Azimuthal Equidistant

What is happening when we project data? Case 1: Both PCSs contain the same GCS PCS A1 PCS A2 (x, y) Projection Projection GCS A (lon, lat) (λ, φ)

What is happening when we project data? Case 2: Each PCS contains a different GCS PCS A1 PCS B1 (x, y) Projection Projection GCS A GCS B (lon, lat) Geographic Transformation Or Datum Transformation (λ, φ)

European Datum 1950 vs. World Geodetic System 1984 Why do we need to transform our data? European Datum 1950 vs. World Geodetic System 1984

Earth-centered datum (WGS 84) Local datum (NAD 27) Changing datums My data Earth’s surface Earth-centered datum (WGS 84) Local datum (NAD 27)

Geographic (Datum) Transformation Converts one GCS to another GCS Defined in a particular direction For example, NAD27 to WGS84 All are reversible Suitable for a particular area May be more than one applicable GT

33 transformations between NAD 27 and WGS 84 Which is best? Depends on the region covered by your data

How do I find transformations? Demo

Now you understand … Coordinate Systems Projecting your data Transforming your data

Where is my data?

Buffers Demo

That’s all folks! Don’t forget to fill out the survey Introduction to Coordinate Systems and Projections

Resources Desktop → Guide Books → Map projections Developer Help http://resources.arcgis.com/en/help Desktop → Guide Books → Map projections Developer Help List of ArcGIS APIs Lining Up Data in ArcGIS, Margaret Maher ESRI Technical paper: Understanding Coordinate Management in the Geodatabase ESRI Technical paper: Understanding Geometric Processing in ArcGIS