2.3 Coordinate Systems, Datums, and Projected Coordinate Systems

A fundamental requirement for any GIS professional is understanding spatial references. If coordinate systems are incorrectly defined or projected, layers will not align properly in the map, and spatial analysis results (like calculating area or distance) will be entirely wrong.

Geographic Coordinate Systems (GCS)

A Geographic Coordinate System (GCS) uses a three-dimensional spherical surface to define locations on the earth.

• Units: A GCS measures locations in angular units, most commonly decimal degrees (Latitude and Longitude).
• Components: A GCS includes an angular unit of measure, a prime meridian (usually Greenwich), and a Datum (which includes a spheroid).
• Examples: WGS 1984 (World Geodetic System 1984) and NAD 1983 (North American Datum 1983).

Because a GCS represents a curved 3D surface, you cannot accurately measure distances in linear units (like meters) or areas across a GCS directly. The distance between lines of longitude shrinks as you move from the equator to the poles.

The Role of the Datum

The Earth is not a perfect sphere; it is slightly wider at the equator (an ellipsoid or spheroid). A Datum is a mathematical model that defines the position of the spheroid relative to the center of the earth. It provides a frame of reference for measuring locations on the surface.

• Local Datums: Aligned to closely fit the earth's surface in a particular area. For example, NAD 1983 is optimized for North America. If you use NAD 1983 to map Europe, the coordinates will be significantly inaccurate.
• Global Datums: Designed to fit the entire earth's surface as closely as possible. WGS 1984, the datum used by the GPS system, is a global datum.

Projected Coordinate Systems (PCS)

A Projected Coordinate System (PCS) is defined on a flat, two-dimensional surface. Unlike a GCS, a PCS has constant lengths, angles, and areas across the two dimensions. A PCS is always based on an underlying GCS.

• Units: A PCS uses linear units, typically meters or feet (US Survey Feet or International Feet).
• Components: A PCS contains a geographic coordinate system, a map projection, projection parameters (like standard parallels and central meridian), and linear units of measure.

Map Projections and Distortion

The process of projecting a 3D sphere onto a 2D plane is called a map projection. It is mathematically impossible to flatten a sphere without introducing distortion. Every map projection distorts at least one (and usually more) of these four spatial properties:

1. Shape (Conformal projections preserve shape locally)
2. Area (Equal-area projections preserve area)
3. Distance (Equidistant projections preserve distance from specific lines or points)
4. Direction (Azimuthal projections preserve true direction from a center point)

[!IMPORTANT] Choosing the correct PCS depends entirely on the purpose of your map or analysis. If you are calculating parcel acreages, you must use an Equal-Area projection. If you are doing navigation, you need a Conformal projection.

Common Projected Coordinate Systems

• State Plane Coordinate System (SPCS): Used primarily in the United States. It divides states into multiple zones to minimize distortion (typically under 1 part in 10,000). Zones oriented east-west use the Lambert Conformal Conic projection, while north-south zones use Transverse Mercator. It is ideal for local, highly accurate mapping.
• Universal Transverse Mercator (UTM): Divides the earth into 60 zones, each 6 degrees of longitude wide. It is highly accurate within the specific zone but distorts heavily outside of it.
• Web Mercator (Auxiliary Sphere): The standard projection for web maps (ArcGIS Online, Google Maps). It preserves shape locally (conformal) but drastically distorts area at the poles (making Greenland look as large as Africa). Never use Web Mercator for area or distance calculations.

Spatial Reference Properties in ArcGIS

In ArcGIS Pro, a "Spatial Reference" encompasses more than just the coordinate system. When creating a feature class, the spatial reference includes:

1. Coordinate System: The GCS or PCS.
2. X/Y Tolerance: The minimum distance between coordinates before they are considered identical (snapped together during topological operations).
3. X/Y Resolution: The precision of the coordinates; the smallest allowable separation between values.
4. Z and M Properties: Optional tolerance and resolution for elevation (Z) and linear referencing (M).

Once a feature class is created, its spatial reference cannot be altered. To change the spatial reference of the data permanently, you must project it into a new feature class.