1. Projections and coordinate systems

2. Projections and coordinate systems

3. Projections and coordinate systems

4. Projections and coordinate systems
We think of the earth as a sphere
It is actually a spheroid, slightly larger in radius at the equator than at the poles

5. Projections and coordinate systems
Rotate an ellypse around an axis

6. Projections and coordinate systems

7. Projections and coordinate systems
Reference ellipsoid
Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network computations are performed and point coordinates such as latitude, longitude, and elevation are defined.
Historically, different reference ellipsoids have been defined and adopted by different countries.
They are still in use due to technical legacies and juridical issues

8. Projections and coordinate systems
Some reference ellipsoids
Name
Equatorial axis (m) a
Polar axis (m) b
Inverse flattening
(a-b) / a
Airy 1830
Clarke 1866
Bessel 1841
International 1924
297
Krasovsky 1940
GRS 1980
WGS 1984

9. Projections and coordinate systems
...a little problem: gravity

10. Projections and coordinate systems
The Geoid

11. Projections and coordinate systems
Where is the geoid?

12. Projections and coordinate systems
Levelling network
Tide gauge

13. Projections and coordinate systems
What is a Datum

14. Projections and coordinate systems
Example of geodetic datums
Adindan (Ethiopia, Senegal)
Moznet (Mozambique)
Arc 1960 (Kenya, Tanzania)
ED50 (Iraq)
WGS84 is now becoming the International Standard (GPS)

15. Projections and coordinate systems
GPS
Initiated by U.S. Department of Defense
Military planners wanted a technology where a position could be obtained without the use of radio transmissions
Officially named Navigation System with Timing and Ranging (NAVSTAR)
First satellite launched in 1974
The GPS Operational Constellation consists of 24 satellites (21 plus 3 operational spares) that orbit the Earth in very precise orbits twice a day.
GPS satellites emit continuous navigation signals.

16. Projections and coordinate systems
GPS
Each GPS satellite transmits data that indicates its location and the current time.
All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant.
Physically the signal is just a complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.
The same pseudo-random sequence is generated by the satellite and the receiver.
The receiver compares the two sequences to derive the time difference.
The distance from the satellite is the product of this difference by the speed of light.

17. Projections and coordinate systems
GPS

18. Projections and coordinate systems
GPS
Precision
Satellites use atomic clocks, receivers use quartz clocks -> less expensive
Trees, buildings, mountains can prevent or reduce a clear line of sight
Signal refraction in the atmosphere
Signal interference, e.g. reflection from other surfaces
Satellite distribution in the sky (Geometric Diluition of Precision, GDOP)

19. Projections and coordinate systems
GPS
Geometric Diluition of Precision
Good Poor

20. Projections and coordinate systems
GPS
Precision
GPS satellites transmit in
two frequencies: L1
C/A code (coarse acquisition code)
P code (precise code) encrypted
Navigation data L2
Standard Positioning System (SPS)
Only L1 and only C/A code
Precise Positioning System (PPS)
Both L1 and L2 (to minimize atmospheric refraction)
P code (anti-spoofing)

21. Projections and coordinate systems
GPS
Differential GPS
One or more ground based station in a known location record or broadcast the difference between the pseudoranges received from satellites and the actual internally computed pseudoranges.
The difference can be applied by a rover station (individual, car, airplane etc.) to correct its own pseudoranges
Post-processing
A dedicated station
Real time
A network of stations
Signal broadcasted by local radio network
Signal broadcasted by satellite network
(e.g. WAAS, EGNOS)

22. Projections and coordinate systems
GPS

23. Projections and coordinate systems
Other systems
GLONASS – Russia's global navigation system. Fully operational worldwide
Galileo – a global system being developed by the European Union and other partner countries, planned to be operational by 2014 (and fully deployed by 2019)
COMPASS – People's Republic of China's global system, planned to be operational by 2020

24. Projections and coordinate systems
The earth is a spheroid, a map is flat

25. Projections and coordinate systems
Example of projections
Cylindrical
Conical
Azimuthal

26. Projections and coordinate systems
Areas or angles?
Mollweide (equivalent) Mercator (conformal)

27. Projections and coordinate systems
Areas or angles?
At large scales, most of the commonly used projections are conformal.

28. Projections and coordinate systems
UTM

29. Projections and coordinate systems
UTM

30. Projections and coordinate systems

31. Projections and coordinate systems
EPSG Geodetic Parameter Dataset
European Petroleum Survey Group, now absorbed by the International Association of Oil & Gas Producers (OGP)
a collection of definitions of coordinate reference systems and coordinate transformations which may be global, regional, national or local
EPSG Geodetic Parameter Registry
online at:
example: EPSG:3857
3857 is the Spatial Reference System Identifier (SRID)
WGS 84 / Pseudo-Mercator, used by Google maps

32. Projections and coordinate systems
SRIDs are typically associated with a “well known text” (WKT), i.e. a string definition of the coordinate system
“well known text” is a kind of data definition language
Example: EPSG:4326 (correspond to WGS84)
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84", , , AUTHORITY["EPSG","7030"]], AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]], UNIT["degree", , AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]

33. Projections and coordinate systems
There are some websites where you can find complete lists of spatial reference systems and transformations, along with their description and definition in different formats
My favorite ones are

34. Projections and coordinate systems
Coordinate conversion A B A C B