Does spatial data layer have to have both projected coordinate system and geographic coordinate system?

Question

Since maps are flat, I feel like the PCS is needed for the spatial data layer to know how to draw it out, but I think the layer also needs the GCS for the coordinates to show where the location is on Earth.

Does it need both or just one?

Answer 1

• A GCS (Geographic Coordinate System) defines where the data is located on the earth’s surface.
• A PCS (Projected Coordinate System) tells the data how to draw on a flat surface, like on a paper map or a computer screen. (Reference)

There are two points here:

1. How to draw the spatial data on a plane (paper or screen).

   • If it is PCS, no problem. A PCS is flat, and it's obvious how to draw.
   • If it is GCS, that's a problem. You cannot draw the round Earth on a flat surface. You have to convert it to PCS to draw. But the spatial data doesn't contain two separate coordinate pairs for the features in it. Therefore, GIS applications use a default PCS to draw on a screen (Reference).

2. Where a point is located on Earth.

   • If it is in GCS, no problem. The coordinates of the point tell you where its location is within the related GCS. But you need to know in which GCS it is, before you know where it is on Earth (Reference). Because GPS devices use WGS84 GCS by default. Therefore, you can find the location on Earth after translating to WGS84 in case it is not in WGS84.

   • If it is in PCS, then you need to translate to GCS. Fortunately, all PCS have a GCS on which they are based.

Briefly, it is sufficient for the spatial data to have one coordinate system.

Answer 2

I suspect you're overthinking this problem.

In the abstract, every projected coordinate system (PCS) is based on a geographic coordinate system (GCS), which is based on a datum, which permits conversion between both any GCS and any PCS (except if the PCS is one-way).

But any GCS can be drawn with a plate carrée transcription, treating angular units as Cartesian.

In fact, even data without a coordinate system can be drawn on paper or device with plate carrée. It just can't be translated into any other coordinate system.