1. Introduction to Coordinate Systems and Projections
Rob Juergens, Annette Locke

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

3. Geographic (GCS)
Projected (PCS)
Coordinate Systems

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

5. 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

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

7. 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

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

9. 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

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

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

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

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

14. Projected Coordinate System Well-Known Text (WKT)
PROJCS[ “World_Mercator", GEOGCS[ "GCS_WGS_1984", DATUM[ "D_WGS_1984", SPHEROID[ "WGS_1984", , ] ], PRIMEM[ "Greenwich", 0.0], UNIT[ "Degree", ] ], 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] ]

15. 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

16. Preserve Shape and Direction
Stereographic

17. Albers Equal Area Conic
Preserve Area
Albers Equal Area Conic

18. Preserve Direction
Gnomonic

19. Preserve Direction and Distance
Azimuthal Equidistant

20. 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)

21. 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)

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

23. 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)

24. 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

25. 33 transformations between NAD 27 and WGS 84
Which is best?
Depends on the region covered by your data

26. How do I find transformations?
Demo

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

28. Where is my data?

29. Buffers
Demo

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

31. Resources Desktop → Guide Books → Map projections Developer 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