Advanced Geodetic Surveying

Module 2: Advanced Geodetic Surveying

1. Ellipsoids, Geoids, and Datums

Geodetic surveying requires an understanding of Earth’s shape and reference frameworks.

• Ellipsoids:

• Mathematical models approximating Earth’s shape (oblate spheroid).

• Common ellipsoids: WGS84 (global), GRS80 (North America), Clarke 1866 (historical).

• Used in GNSS and mapping for coordinate calculations.

• Geoid:

• The "true" Earth shape based on gravity (mean sea level extended globally).

• Deviates from ellipsoids due to gravitational variations (geoid undulations).

• Critical for orthometric heights (elevations above sea level).

• Datums:

• Reference frames defining coordinate systems (horizontal & vertical).

• Horizontal Datums (e.g., NAD83, ETRS89): Fix ellipsoids to Earth’s surface.

• Vertical Datums (e.g., NAVD88, EGM96): Relate heights to geoid/mean sea level.

2. Coordinate Reference Systems (CRS) and Transformations

Precise positioning requires correct CRS selection and conversions.

• Types of CRS:

• Geographic CRS: Uses latitude/longitude (e.g., WGS84).

• Projected CRS: Flat 2D systems (e.g., UTM, State Plane).

• Local CRS: Site-specific grids for engineering projects.

• Transformations:

• Helmert (7-Parameter): Rotates, scales, and shifts coordinates between datums.

• Grid-Based (NTv2, NADCON): Corrects distortions in regional datums.

• Height Adjustments: Converts ellipsoidal (GNSS) to orthometric heights using geoid models.

3. GNSS (GPS, GLONASS, Galileo) in Engineering Surveys

Global Navigation Satellite Systems (GNSS) revolutionize high-precision surveying.

• Key Systems:

• GPS (USA): Most widely used; L1/L2 frequencies for dual-frequency corrections.

• GLONASS (Russia): Improves coverage in high latitudes.

• Galileo (EU): Offers civilian high-accuracy service (HAS).

• Applications:

• Control network establishment.

• Machine guidance (earthmoving, paving).

• Topographic & as-built surveys.

• Accuracy Factors:

• Satellite geometry (PDOP – Position Dilution of Precision).

• Signal multipath (reflections near obstacles).

• Atmospheric delays (ionosphere/troposphere corrections).

4. Real-Time Kinematic (RTK) and Network RTK (NRTK) Surveying

RTK and NRTK enable centimeter-level real-time positioning.

• RTK Surveying:

• Uses a base station (known coordinates) to transmit corrections to a rover.

• Requires short baselines (<10 km) to minimize errors.

• Typical accuracy: 1–2 cm horizontally, 2–3 cm vertically.

• Network RTK (NRTK):

• Uses multiple base stations (CORS network) for wider coverage.

• Virtual Reference Station (VRS) technology interpolates corrections.

• Advantages: No need for a local base; consistent accuracy over large areas.

• Challenges:

• Radio/Internet dependency for correction data.

• Signal obstructions in urban canyons/forests.

Summary

This module equips surveyors with advanced geodetic knowledge, from Earth’s geometry to cutting-edge GNSS techniques. Mastery of CRS, datums, and RTK/NRTK ensures precision in large-scale engineering projects.