Geodesy
"Future Geodetic Engineer! Geodesy - the science of Earth's shape, size, and gravity field. Understanding our planet for precise positioning!"
1. Shape of the Earth 🌍
Earth is not a perfect sphere! Understanding its true shape is fundamental to geodesy.
Geoid
Equipotential surface of Earth's gravity field
Mean sea level extended under continents. "True" reference for heights. Irregular surface affected by mass distribution.
Reference Ellipsoid
Mathematical approximation of Earth's shape
Defined by semi-major axis (a) and flattening (f). WGS84: a = 6,378,137m, f = 1/298.257
Key Parameters
- Semi-major axis (a) = Equatorial radius ≈ 6,378 km
- Semi-minor axis (b) = Polar radius ≈ 6,357 km
- Flattening (f) = (a-b)/a ≈ 1/298.257
- Eccentricity (e) = √(a²-b²)/a
2. Coordinate Systems 📍
| System | Coordinates | Application |
|---|---|---|
| Geographic | Latitude (φ), Longitude (λ), Height (h) | Global positioning, maps, GPS |
| Geocentric Cartesian | X, Y, Z (Earth center origin) | Satellite orbits, GPS calculations |
| UTM (Universal Transverse Mercator) | Easting, Northing, Zone | Plane surveying, military maps |
| PRS92 (Phil. Reference System) | φ, λ, h (local datum) | Philippine cadastral surveys |
3. Height Systems 📏
Orthometric Height (H)
Height above the geoid (mean sea level)
Used for: Engineering projects, water flow determination
Ellipsoidal Height (h)
Height above the reference ellipsoid
Directly measured by GPS. Does NOT follow water flow!
Geoid Undulation (N)
h = H + N
Ellipsoidal = Orthometric + Geoid Undulation. In PH, N ranges from -30m to +80m!
4. Geodetic Datums 🌐
Global Datums
- WGS84 - GPS worldwide standard
- ITRF - International Terrestrial Reference Frame
- Geocentric, uses GRS80 ellipsoid
Philippine Datums
- PRS92 - Official local datum
- Origin: Geodetic point at Balanacan, Marinduque
- Clarke 1866 ellipsoid (older maps)
5. GNSS/GPS Fundamentals 📡
Global Navigation Satellite Systems
Positioning Principle:
- Minimum 4 satellites for 3D position + time
- Trilateration using signal travel time
- Distance = Speed of light × Time
Error Sources:
- Ionospheric delay (corrected by dual-freq)
- Tropospheric delay (weather)
- Multipath (signal reflection)
- Satellite clock & ephemeris errors
6. Practice Questions 📚
Common Board Exam Questions
Q1: What is the difference between geoid and ellipsoid?
A: Geoid is the true equipotential surface (irregular, follows gravity), while ellipsoid is a mathematical smooth surface used as reference for coordinates.
Q2: If GPS ellipsoidal height is 125m and geoid undulation is -35m, what is orthometric height?
A: H = h - N = 125 - (-35) = 125 + 35 = 160m
Q3: What is the semi-major axis of WGS84 ellipsoid?
A: 6,378,137 meters (approximately 6,378 km)
🔥 GE Challenge 🔥
Master Earth's shape and positioning! Geoid, ellipsoid, datums, GNSS - the foundation of modern geodesy!
Know the Earth to map the Earth!
Test Your Knowledge! 🧠
Ready ka na ba? Take the practice quiz for Geodesy to reinforce what you just learned.
Start Practice Quiz 📝