GPS Coordinates: Deep Dive into Latitude, Longitude, and Altitude for Surveying and Aviation

GPS coordinates—the trio of latitude, longitude, and altitude—are the universal language of location, enabling everything from aircraft navigation and land surveys to smartphone maps and tectonic research. Their precision and reliability hinge on international standards, robust reference frames, and careful attention to both spatial and temporal factors. This glossary entry explores the technical heart of GPS coordinates, with a focus on their use in surveying and aviation, guided by ICAO Annexes, WGS84 documentation, and geodetic best practices.

What Are GPS Coordinates?

GPS coordinates specify a position on (or above) the Earth’s surface by providing:

• Latitude (φ): Angular distance north or south of the equator, measured in degrees (−90° to +90°).
• Longitude (λ): Angular distance east or west of the prime meridian at Greenwich, UK (−180° to +180°).
• Altitude (h): Vertical distance above a defined reference surface, usually the reference ellipsoid (ellipsoidal height).

These values are always referenced to a datum—a mathematical model of the Earth’s shape, size, and orientation. The most widely used global datum is WGS84 (World Geodetic System 1984), which underpins all GPS and is mandated for aviation by the International Civil Aviation Organization (ICAO).

Key concept:
Coordinates are meaningful only when accompanied by their datum and, for high-precision, their epoch (the date at which they are valid), due to ongoing tectonic motion and periodic datum updates.

Coordinate Systems and Reference Frames

Geographic Coordinate System (GCS)

The GCS expresses positions in latitude, longitude, and altitude. Latitude and longitude are angular units; altitude is linear (meters or feet). They describe a point on the Earth’s curved surface.

Earth-Centered, Earth-Fixed (ECEF) Cartesian System

ECEF is a 3D Cartesian system with its origin at the Earth’s center of mass:

• X-axis: Intersection of equator and prime meridian
• Y-axis: 90° east along equator
• Z-axis: North Pole

GNSS (Global Navigation Satellite Systems) calculations are performed in ECEF, then transformed to latitude, longitude, and altitude for user applications.

Projected Coordinate Systems

For mapping and engineering, the Earth’s curved surface is projected onto a flat plane (e.g., Universal Transverse Mercator (UTM), State Plane). These use linear units (meters, feet) and are essential for construction, cadastral mapping, and large-scale surveying.

Reference Frame

A reference frame realizes a coordinate system in both space and time. It is defined by a network of surveyed points, orientation, and epoch. The global standard is the International Terrestrial Reference Frame (ITRF), with periodic updates (e.g., ITRF2014, ITRF2020). WGS84 is aligned closely to ITRF for GPS.

ICAO and Reference Frames

ICAO mandates that all published aeronautical data be referenced to WGS84. Accuracy requirements (e.g., runway thresholds within 1 meter horizontally, 0.25 meters vertically) are specified in ICAO Annex 15.

Expressing Position: Latitude, Longitude, and Altitude

Latitude is measured from the equator, positive northward, negative southward.
Longitude is measured from the prime meridian, positive eastward, negative westward.
Altitude (ellipsoidal height) is measured above the reference ellipsoid. For practical purposes (aviation, engineering), altitude is often referenced to mean sea level (orthometric height), requiring a geoid model.

Ellipsoidal vs. Orthometric Height

• Ellipsoidal height (h): Height above the ellipsoid (WGS84)
• Geoid undulation (N): Difference between the ellipsoid and mean sea level
• Orthometric height (H): Height above mean sea level
  H = h − N

Example:
GPS at Los Angeles:

• Latitude: 34.05223° N
• Longitude: 118.24368° W
• Altitude (WGS84): 89.3 m
• Geoid undulation: −34.5 m
• Orthometric height: 123.8 m (for charting and aviation safety)

Datums and Reference Systems

What is a Datum?

A datum is a reference model for the Earth’s size, shape, orientation, and position. It is the foundation for all geodetic, surveying, and mapping activities.

• WGS84: Global datum for GPS, defined by a specific ellipsoid and aligned to the Earth’s center of mass.
• ITRF: International reference frame, periodically updated for plate motion and improved measurement.
• NAD83: North American datum, fixed to the North American tectonic plate.
• Local datums: (e.g., ETRS89, GDA2020) offer regional stability by fixing to specific plates.

Datum Errors:
Using the wrong datum can produce errors of several meters—critical in aviation, property surveys, and engineering.

ICAO Standard:
All aeronautical data must specify the datum (WGS84 by default) to avoid ambiguity.

Epoch: The Time Dimension

Why Does Epoch Matter?

Coordinates change over time due to tectonic drift, earthquakes, and land subsidence. The epoch specifies the date when the coordinates are valid.

• High-precision work: Always state the epoch (e.g., WGS84 (G2139, epoch 2021.0)).
• Tectonic drift: Plates move centimeters per year. Over decades, this can shift coordinates by meters.
• Case: A GNSS station in Seoul referenced to epoch 2002.0 will have shifted over 0.5 meters by 2020.

ICAO Application:
Aeronautical publications must include the datum and epoch for all coordinates to ensure universal understanding and safety.

Precision, Accuracy, and Error Sources

• Precision: Repeatability of measurements.
• Accuracy: Closeness to the true value.
• Resolution: Smallest detectable difference.
• Uncertainty: Range within which the true value lies.

Common Error Sources:

• Satellite geometry (DOP): Poor arrangement increases error.
• Atmospheric delays: Ionosphere and troposphere distort GPS signals.
• Multipath: Reflections off surfaces near the receiver.
• Clock errors: Inaccuracies in satellite or receiver clocks.
• Orbital errors: Ephemeris inaccuracies.
• Tectonic/local motion: Physical ground movement.

ICAO Data Quality:
Runway end coordinates must be within 1 meter horizontally and 0.25 meters vertically (Annex 15). All error sources must be documented and, where possible, mitigated.

Surveying Methods Using GPS

• Control Networks: Precisely surveyed, monumented control points form the basis for mapping, engineering, and legal boundaries.
• Traverses: Sequences of measured positions, used to extend control or map boundaries.
• Triangulation/Trilateration: Classical methods (now largely replaced by GPS) for establishing new positions.
• Differential GPS (DGPS): Uses a reference station to send corrections to mobile receivers, boosting accuracy.

Coordinate Change Over Time

Do GPS coordinates change?
Yes, due to tectonic motion and periodic datum updates. Australia’s plate, for example, moves 7 cm/year; over a decade, this means a shift of 70 cm.

• Datum/epoch updates: Reference frames are periodically redefined (e.g., WGS84 updates, new ITRF releases).
• Corrections: Errors or new phenomena can prompt further adjustments.
• Aviation: All changes must be reflected in updated aeronautical data to maintain safety.

RTK, Reference Stations, and High-Precision Positioning

• RTK (Real-Time Kinematic): Uses a fixed, known base station to provide real-time corrections via radio or internet, achieving centimeter-level accuracy.
• Reference Station: Must have precisely known coordinates (correct datum and epoch).
• Network RTK (NRTK): Combines multiple stations to model atmospheric errors, providing corrections over wide areas.
• Datum/Epoch Consistency: Mismatched epochs/datum between base and rover can cause systematic errors of tens of centimeters.

Aviation:
All ground-based augmentation and survey control must reference WGS84 and specify the epoch to ensure data integrity.

Key GPS Data Variables Glossary

Note:
ICAO requires all aeronautical data to specify datum, epoch, quality, and survey method.

Real-World Use Cases

• Land Surveys: Boundary determination, property definition, cadastral mapping with static/RTK GNSS.
• Construction: Engineering layout, machine control, as-built documentation using GNSS.
• Tectonic Monitoring: Permanent GNSS stations track plate movement, supporting scientific and datum updates.
• Aviation: Airfield, runway, waypoint, and navigation aid locations are surveyed in WGS84 and published for global use. Frequent updates ensure data remains current and safe.

Further Resources

• ICAO Annex 15: Aeronautical Information Services
• WGS84 Technical Documentation
• International Terrestrial Reference Frame (ITRF)
• National Geodetic Survey (NGS) - Geodetic Tool Kit
• FIG Guide on the Use of GNSS in Surveying

Summary

GPS coordinates—latitude, longitude, and altitude—are the foundation of modern geospatial practice. Their reliability depends on consistent use of datum, epoch, and robust error mitigation. Precision surveying, international aviation, and scientific research all rely on the accuracy and clarity provided by standardized GPS coordinate systems.

For safety, legal, and engineering integrity, always document:

• The coordinate datum
• The epoch
• The method of survey
• Quality/uncertainty parameters

This ensures that GPS coordinates remain a trustworthy, universal reference for location worldwide.