2023-04-09

Spatial Reference System

What is Spatial Reference System (SRS)

The Spatial Reference System (SRS), also known as Coordinate Reference Systems (CRS), is a framework or set of rules that facilitate the identification of locations in the real world. This concept is a cornerstone in the domain of Geographic Information System (GIS), where defining and interpreting spatial data is crucial.

Consider the scenario where you want to communicate the location of the Empire State Building in New York City to someone. In human interactions, you might say, "350 5th Ave, New York, NY 10118, USA," and it would suffice. However, in the realm of digital information and particularly GIS, a more structured and numerical approach is necessary for machines to process and understand location data efficiently. This is where a system like latitude and longitude, a type of Spatial Reference System, comes into play.

Despite its seemingly straightforward nature, the concept of the Spatial Reference System can pose challenges, primarily due to its inherent variability. Multiple systems are used to represent locations numerically, depending on a multitude of factors such as geographical area, scale, and projection.

Let's illustrate this with a few examples commonly used in Japanese GIS data:

Spatial Reference System	Spatial Reference System Identifier (SRID)
World Geodetic System (JGD2011) - Lat/Long	EPSG: 6668
World Geodetic System (JGD2011) - Plane Rectangular Coordinate System	EPSG: 6669 - 6687
World Geodetic System (JGD2011) - UTM Zone 51 - 55	EPSG: 6688 - 6692
World Geodetic System (JGD2000) - Lat/Long	EPSG: 4612
World Geodetic System (JGD2000) - Plane Rectangular Coordinate System	EPSG: 2443 - 2461
World Geodetic System (JGD2000) - UTM Zone 51 - 55	EPSG: 3097 - 3101
Old Japan Datum (TOKYO Datum) - Lat/Long	EPSG: 4301
Old Japan Datum (TOKYO Datum) - Plane Rectangular Coordinate System	EPSG: 30161 - 30179
Old Japan Datum (TOKYO Datum) - UTM Zone 51 - 55	EPSG: 102151 - 102156
WGS84 - Lat/Long	EPSG: 4326
Web Mercator	EPSG: 3857

Each of these Spatial Reference Systems has an identifier known as the SRID (Spatial Reference System Identifier), commonly referenced using the EPSG (European Petroleum Survey Group) geodetic parameter dataset. It's crucial to understand the differences among these SRS, as using an inappropriate one can lead to significant errors in data interpretation. Despite its complexity, an effective understanding and application of the Spatial Reference System is key to accurate spatial data management in GIS.

Spatial Reference System Identifier (SRID)

A Spatial Reference System Identifier (SRID) is a unique identifier associated with a specific coordinate system, datum, and projection. The SRID is a crucial component of geospatial metadata, used to interpret the geographic coordinates used in a database. Each SRID corresponds to a particular spatial reference system and ensures that spatial data is accurately located on the Earth's surface.

The SRID is critical for the correct interpretation and manipulation of spatial data. It ensures that all spatial operations performed on the data are based on the same spatial reference system, avoiding inconsistencies and errors in data analysis.

Without the correct SRID, operations like calculating distances, areas, or even simple tasks such as displaying the data on a map can lead to inaccurate results, as the interpretation of the coordinate values can vary depending on the spatial reference system. Therefore, it's crucial to know the SRID associated with your data, and if necessary, convert it to the appropriate SRID for your analysis or application.

Common SRIDs and their Uses

Several SRIDs are commonly used in GIS. The most well-known is probably SRID 4326, which represents the World Geodetic System 1984 (WGS84). This system is used globally and is the default for many geospatial databases, GIS software, and GPS devices.

Another example is SRID 3857, representing the WGS84 Web Mercator projection, widely used in online mapping services such as Google Maps, Bing Maps, or OpenStreetMap. Although it introduces distortions in shape, area, and size, especially as one gets further from the equator, its advantage lies in the fact that it preserves angles, making it suitable for web-based maps.

Elements of Spatial Reference Systems

SRS comprises two key elements: the geodetic system and the coordinate system. Both of these elements work together to define the shape of the Earth and give a context to geographic data.

Geodetic Systems

The geodetic system, simply put, is a set of rules that defines the shape of the Earth. Although satellite imagery allows us to view the Earth's shape, some discrepancies occur when calculating the Earth's center and radius. The geodetic system aims to provide a standardized model of the Earth globe to overcome these variations. There are two main types of geodetic systems: Global Geodetic System and Local Geodetic System.

Global Geodetic System

The Global Geodetic System, also known as a Geocentric Geodetic System, uses the Earth's center of gravity as its reference point. Due to the Earth's irregular shape, aligning the Earth and the globe model isn't straightforward, but advancements in satellite technology have made this feasible. In Japan, the most frequently used Global Geodetic Systems are:

Global Geodetic System (JGD2011)
This is the current official geodetic system for Japan, used in all official surveys, including maps produced by the Japan Geospatial Information Authority.
Global Geodetic System (JGD2000)
This was the official geodetic system for Japan from 2002 to 2011. The system was replaced by JGD2011 after considering the crustal movements due to the Great East Japan Earthquake.
Global Geodetic System (WGS84)
Managed by the United States, this geodetic system is utilized for GPS-based navigation.

Local Geodetic System

In contrast to the Global Geodetic System, the Local Geodetic System aligns a specific point on the model globe with a corresponding point on the actual Earth. A well-known Local Geodetic System in Japan is the Japan Geodetic System (Tokyo Datum), which was the official geodetic system from 1918 to 2002. However, it has been mostly phased out due to its incompatibility with modern satellite positioning systems and the significant discrepancies caused by crustal movements.

How to Handle Geodetic Systems

The specific geodetic system should be rigorously adhered to when conducting surveys or other related tasks. However, for less precise applications, the differences among the three types of Global Geodetic Systems may be negligible. Nevertheless, caution must be exercised when distinguishing between the Global Geodetic Systems and the Local Geodetic System (Tokyo Datum), as the same coordinates can point to locations more than 400 meters apart in the real world.

Coordinate Systems

The Coordinate System is another essential element of a Spatial Reference System. There are two main types of coordinate systems: Geographic Coordinate System and Projected Coordinate System.

Geographic Coordinate System

The Geographic Coordinate System defines positions on the Earth's spherical surface using latitude and longitude. It's a straightforward and widely recognized method of identifying locations on the globe.

Projected Coordinate System

Unlike the Geographic Coordinate System, the Projected Coordinate System represents the Earth on a flat plane, making it suitable for map creation. The Earth is divided into several blocks, each of which is flattened or projected onto a plane. There are several types of Projected Coordinate Systems used in Japan: