Coordinate Systems in Ground Mapping

There are several types of coordinate systems used in ground mapping to represent locations and features on the Earth's surface. These systems help geographers, cartographers, and other professionals to accurately depict spatial data. Some of the most used coordinate systems are listed below.

Geographic Coordinate System (GCS)

This system uses latitude and longitude to define the position of a point on Earth's surface. Latitude lines run horizontally and measure the angle north or south of the equator, while longitude lines run vertically and measure the angle east or west of the Prime Meridian.
There are several latitude and longitude reference systems used to describe the location of points on the Earth's surface. Here, we'll discuss some of the most common systems and their characteristics.

Geodetic Latitude and Longitude

This is the most widely used reference system. It is based on an ellipsoidal model of the Earth, which considers the Earth's flattening at the poles and bulging at the equator. Geodetic latitude and longitude are usually expressed in degrees, minutes, and seconds or in decimal degrees.

Suppose you want to find the coordinates of the Eiffel Tower in Paris. Using the geodetic latitude and longitude system, the location would be approximately 48°51'29.6"N (latitude) and 2°17'40.2"E (longitude) in degrees, minutes, and seconds, or 48.858222°N and 2.294500°E in decimal degrees.

Geocentric Latitude and Longitude

This system uses a spherical model of the Earth, where the centre of the Earth is considered as the origin. Geocentric latitude is the angle between a line connecting the Earth's centre and the point of interest and the Earth's equatorial plane. Geocentric longitude is the angle between a reference meridian (usually the Prime Meridian) and the plane containing the Earth's centre, the point of interest, and the Earth's rotational axis. This system is primarily used in satellite and celestial navigation.

Considering the same Eiffel Tower location, the geocentric latitude would be approximately 48.849104°N, and the geocentric longitude would remain 2.294500°E. These values are slightly different from the geodetic coordinates due to the Earth's ellipsoidal shape.

Astronomic Latitude and Longitude

This system is based on the observations of celestial bodies such as stars, the sun, and the moon. Astronomic latitude is the angle between the plane of the observer's horizon and the celestial equator, while astronomic longitude is the angle between the plane of the observer's local meridian and the plane of the celestial meridian. Due to the irregularities in the Earth's mass distribution and the geoid, astronomic and geodetic latitudes and longitudes may differ slightly.

An observer at the Eiffel Tower might use a sextant to measure the angle between the horizon and a known star to determine their astronomic latitude. The observer would also measure the local time of a celestial event (like a star crossing the meridian) to calculate their astronomic longitude. The resulting astronomic latitude and longitude would be close to but not the same as the geodetic coordinates due to local gravitational anomalies.

Grid Latitude and Longitude

This system is a simplified, plane-based representation of the Earth's surface, used primarily for mapping and navigation. Grid systems divide the Earth's surface into a grid of equally spaced lines of latitude and longitude, such as the Universal Transverse Mercator (UTM) system or the Military Grid Reference System (MGRS). Grid coordinates are usually expressed in meters or other linear units, rather than degrees.

Using the Universal Transverse Mercator (UTM) grid system, the Eiffel Tower's coordinates would be approximately 31U 448255mE 5411943mN. This means it is in zone 31U, 448,255 meters easting, and 5,411,943 meters northing.

Geomagnetic Latitude and Longitude

This system is based on the Earth's magnetic field, which is generated by the movement of molten iron in the Earth's outer core. Geomagnetic latitude is the angle between the horizontal plane and the magnetic field lines at a specific location, while geomagnetic longitude is the angle between the plane of a reference magnetic meridian and the plane containing the point of interest and the Earth's magnetic poles. This system is primarily used in studies of the Earth's magnetic field and its effects on navigation and communication systems.

Geomagnetic coordinates depend on the Earth's magnetic field, which varies over time.

As of 2021, the geomagnetic latitude of the Eiffel Tower would be approximately 49.28°N, and the geomagnetic longitude would be around 70.73°E. These values would be used in studies related to the Earth's magnetic field or to analyse magnetic declination for navigation purposes.

Each of these reference systems has its advantages and limitations, and the choice of system depends on the specific application and requirements of the user.

Universal Transverse Mercator (UTM)

UTM is a widely used planar coordinate system that divides the Earth into 60 zones, each spanning 6 degrees of longitude. It uses a two-dimensional Cartesian coordinate system to represent locations. Each zone has its own central meridian, and distances are measured in meters from this point, with the equator serving as the baseline.

State Plane Coordinate System (SPCS)

The SPCS is used primarily in the United States and divides each state into one or more zones, with each zone using a unique map projection. This system provides highly accurate measurements for areas that are relatively small, such as individual states or regions, making it ideal for engineering and land surveying purposes.

Military Grid Reference System (MGRS)

MGRS is a standardized coordinate system used by NATO countries for military purposes. It is an extension of the UTM system and uses a grid-based alphanumeric system to represent locations. This system provides high precision and is suitable for tactical and strategic military operations.

World Geodetic System (WGS)

The WGS is a standardized coordinate system for Earth, combining both a reference ellipsoid (WGS84) and a geodetic datum. It is used for global positioning, mapping, and navigation purposes, and is the basis for the Global Positioning System (GPS).

The World Geodetic System 1984 (WGS84) is a global reference system for geodesy, mapping, and navigation. It provides a standardized model of Earth's shape, size, and gravitational field, allowing for accurate representation of geographic coordinates across the globe. WGS84 was developed and is maintained by the United States Department of Defence (DoD) and serves as the foundation for the Global Positioning System (GPS).

WGS84 consists of two main components:

Reference Ellipsoid

An ellipsoid is a mathematical model that approximates the shape of the Earth. The WGS84 reference ellipsoid is designed to closely match the Earth's actual shape, which is an oblate spheroid—slightly flattened at the poles and bulging at the equator. The dimensions of the WGS84 ellipsoid are defined by its semi-major axis (equatorial radius) and the inverse flattening value.

Geodetic Datum

A datum is a set of reference points and parameters that defines a coordinate system. The WGS84 geodetic datum provides a global framework for accurately representing latitude, longitude, and altitude coordinates. It includes precise definitions of the Earth's centre of mass, the orientation of the Earth's rotation axis, and the relationship between the reference ellipsoid and the Earth's surface.

Advantages of The WGS84 System

Global Consistency

As a global reference system, WGS84 allows for consistent representation of coordinates across the entire Earth's surface, making it the preferred choice for global positioning, navigation, and mapping applications.

Compatibility with GPS

Since WGS84 serves as the foundation for the Global Positioning System, it enables seamless integration of GPS data with mapping and geospatial applications.

Continual Improvement

WGS84 is periodically updated to account for changes in Earth's shape and gravitational field, ensuring that the system remains accurate and up to date.

Example of WGS84 Coordinates

The Eiffel Tower, a famous landmark in Paris, France, can be represented in WGS84 coordinates as follows:

Latitude: 48.85837° N (48 degrees, 51 minutes, and 30.132 seconds North) Longitude: 2.294481° E (2 degrees, 17 minutes, and 40.1316 seconds East)
In summary, the World Geodetic System 1984 (WGS84) is a crucial tool for modern mapping, geodesy, and navigation, providing a consistent and accurate framework for representing geographic coordinates on a global scale.

Legacy WGS Systems

WGS72

The World Geodetic System 1972 was developed by the United States Department of Defence (DoD) as a global geodetic reference system. It was an improvement over the previous systems, with updated values for Earth's equatorial radius and flattening, as well as adjustments to the geodetic datum. WGS72 was used in various applications, such as satellite geodesy and Doppler satellite surveying, before being replaced by WGS84.

WGS66

The World Geodetic System 1966 was the first global geodetic reference system established by the DoD. It was based on the available Doppler satellite data and ground-based measurements at the time. WGS66 provided a consistent set of parameters for Earth's shape, size, and gravitational field, but it had limitations in terms of accuracy and global coverage.

Other satellite navigation systems

Other satellite navigation systems, also known as Global Navigation Satellite Systems (GNSS), use their own geodetic reference systems and datums to define coordinates on the Earth's surface. Here are some examples of other GNSS and their respective reference systems:

GLONASS (Russia)

The Russian GLONASS (Globalnaya Navigatsionnaya Sputnikovaya Sistema) uses the Parametry Zemli 1990 (PZ-90) geodetic reference system. The PZ-90 system has undergone several updates since its inception, with the latest version being PZ-90.11.

BeiDou (China)

The Chinese BeiDou Navigation Satellite System employs the China Geodetic Coordinate System 2000 (CGCS2000), also known as the BeiDou Coordinate System. This system is based on a geocentric coordinate frame with a reference ellipsoid close to the WGS84 ellipsoid.

Galileo (European Union)

The European Galileo satellite navigation system uses the Galileo Terrestrial Reference Frame (GTRF), which is designed to be compatible with the International Terrestrial Reference Frame (ITRF). The GTRF is based on a geocentric coordinate frame and is closely aligned with the WGS84 reference system.

NavIC (India)

The Indian Regional Navigation Satellite System (IRNSS), also known as NavIC (Navigation with Indian Constellation), uses the Indian Geodetic Reference System (IGRS). IGRS is based on the GRS80 reference ellipsoid, which is also closely related to the WGS84 ellipsoid.

While each GNSS has its own geodetic reference system, most of these systems are designed to be compatible or closely aligned with the WGS84 system used by GPS. This compatibility enables the integration of data from multiple GNSS into a single global geodetic framework and allows for seamless interoperability in various mapping, navigation, and geospatial applications.

Local Coordinate Systems

Local coordinate systems are specific to regions or projects and may use unique parameters and measurements to define locations. They are often developed for specific purposes, such as engineering projects, land surveying, or environmental monitoring.

Here are a few examples of local coordinate systems:

British National Grid (BNG) - United Kingdom

The BNG is a Cartesian coordinate system used for mapping and surveying purposes across the United Kingdom. It uses the Ordnance Survey National Grid (OSNG) and the Airy 1830 ellipsoid, with the Transverse Mercator projection. BNG coordinates are expressed in eastings and northings, measured in meters.

Swedish Grid (RT90) – Sweden

The RT90 (Rikets Triangelnät 1990) is a local coordinate system used in Sweden for mapping and surveying purposes. It uses the Bessel 1841 ellipsoid and the Gauss-Krüger projection. RT90 coordinates are expressed in meters and include both eastings (X) and northings (Y) values.

NAD83 State Plane Coordinate System (SPCS) - United States

The North American Datum 1983 (NAD83) State Plane Coordinate System is a set of planar coordinate systems designed for regional use within the United States. Each state has one or more SPCS zones, which use either the Lambert Conformal Conic or the Transverse Mercator projection, depending on the state's shape and orientation. NAD83 SPCS coordinates are typically expressed in feet or meters.

Conclusion

When ground mapping with drones, the most commonly used coordinate system is the WGS84 (World Geodetic System 1984). WGS84 serves as the foundation for the Global Positioning System (GPS), which is widely used by drones for accurate positioning, navigation, and geospatial data collection.

Drones typically rely on GPS receivers to obtain their position and altitude, which are expressed in WGS84 coordinates (latitude, longitude, and altitude). When capturing aerial imagery or generating maps, these coordinates are used to georeference the data, ensuring accurate representation of geographic features and precise alignment with other geospatial datasets.

Using WGS84 for ground mapping with drones offers several advantages:

1. Global Consistency: WGS84 provides a consistent geodetic reference system that enables seamless integration of drone-collected data with other geospatial datasets, regardless of their origin or geographic location.

2. GPS Compatibility: Since WGS84 is the basis for GPS, it allows for easy integration of drone positioning data with mapping and geospatial applications, ensuring accurate georeferencing and alignment of aerial imagery.

3. Wide Acceptance: WGS84 is widely used and recognized in the GIS, mapping, and remote sensing communities, making it a practical choice for drone-based ground mapping applications.

It is worth noting that, depending on the specific application, local coordinate systems might be used in combination with WGS84 to provide higher accuracy or meet regional requirements for mapping and surveying. In these cases, data collected using WGS84 coordinates can be transformed or projected into the desired local coordinate system.