The coordinate system used for a map projection is known as a Projected Coordinate System (PCS).
Understanding Projected Coordinate Systems
A projected coordinate system is the result of transforming a three-dimensional geographic coordinate system (GCS) onto a two-dimensional, flat surface. This flattening process utilizes a mathematical model called a map projection.
As stated in the reference: "A projected coordinate system (PCS) is a GCS that has been flattened using a map projection."
Think of it like peeling an orange and trying to lay the peel flat without tearing or stretching it perfectly – it's impossible to do without some distortion. Map projections apply mathematical rules to minimize specific types of distortion (like area, shape, distance, or direction) when representing the Earth's curved surface on a flat map.
GCS vs. PCS: Key Differences
Before you can create a projected coordinate system, your data must first have a Geographic Coordinate System (GCS). A GCS defines locations on the Earth's curved surface using angular units (like degrees) based on latitude and longitude coordinates. It's like identifying a point on a globe.
| Feature | Geographic Coordinate System (GCS) | Projected Coordinate System (PCS) |
|---|---|---|
| Surface Model | 3D (Sphere or Spheroid) | 2D (Flat Plane) |
| Units | Angular (Degrees) | Linear (Meters, Feet, Kilometers, etc.) |
| Coordinates | Latitude, Longitude | Easting (X), Northing (Y) |
| Purpose | Locating points on the Earth's curved surface | Mapping, measurement, and spatial analysis on a flat map |
| Requirement | Data must have a GCS before it can have a PCS | Created from a GCS using a map projection |
Why Projections and PCS are Essential
While your spatial data might be stored with just a GCS, maps must use a PCS.
- Data vs. Map: According to the reference, "Your data must have a GCS before it knows where it is on earth. Projecting your data is optional, but projecting your map is not. Maps are flat, so your map must have a PCS in order to know how to draw."
- Flat Representation: Since maps are inherently flat representations of a curved surface, a PCS provides the necessary X and Y coordinates to accurately position features on that flat plane. Without a PCS, a mapping program wouldn't know how to display locations relative to each other in a 2D view.
- Accurate Measurement: PCS uses linear units, which makes measuring distances, areas, and performing other spatial analyses on a map accurate within the specific projection's properties. Measurements on a flat map using a GCS alone would be inaccurate due to the distortion of the curved surface.
In summary, a projected coordinate system is the 2D framework resulting from flattening the Earth's GCS using a map projection, and it is fundamental for creating and working with maps.
