Many beginners run into their first pitfall when overlaying different layers in GIS software: the projection names look identical, yet the data refuses to align. Some even reproject the data repeatedly, only to find the offset remains hundreds of metres. When this happens, it usually means two concepts haven't been clearly distinguished—the datum and the projection.
The datum determines which Earth model is used to calculate coordinates. The projection determines how those calculated positions are written onto a flat surface. Their responsibilities are different, but in software they often appear bundled together, which easily confuses newcomers. Today we’ll sort this out.
What Really Happens When Making a Map
To draw a map, you need to do at least two things:
- Reduce real-world features onto an ellipsoidal surface and assign latitude and longitude. This step relies on the datum.
- Unfold a portion of that ellipsoidal surface onto a plane and write the result as X and Y. This step relies on the projection.
The diagram below illustrates the relationship.
Mathematically, a projection can be inverted, but during the unfolding process at least one property—distance, area, or angle—will be distorted. That is why different projections such as Gauss-Krüger, Lambert, and Mercator exist for different use cases.
Understanding the Datum
A datum describes the shape of the Earth reference surface, as well as its position and orientation relative to the Earth’s centre. It typically consists of two parts:
- Ellipsoid parameters: semi-major axis, semi-minor axis or flattening.
- Reference surface positioning: which surveyed point on the Earth the datum is anchored to, and how its orientation is defined.
For example, the Beijing 1954 coordinate system uses the Krasovsky ellipsoid, with a semi-major axis of about 6,378,245 m and a semi-minor axis of about 6,356,863 m. When mosaicking old and new maps in China, the discrepancies among Beijing 1954, Xi’an 1980, and CGCS2000 are essentially due to different datum and ellipsoid definitions. Furthermore, the same ellipsoid parameters can correspond to multiple datums. WGS 84 and GRS 80 share the same ellipsoid parameters, yet their datum positioning and intended regions differ. In Europe, ETRS89 is a datum fixed to a local tectonic plate, designed to minimise regional drift.
Understanding the Projection
A projection is a set of mathematical transformations that map points from the ellipsoidal surface to planar coordinates. A geographic coordinate system expresses location as latitude and longitude—essentially measuring angles on a given ellipsoid. A projected coordinate system expresses location as planar X and Y, derived from geographic coordinates through projection formulae, and is still tied to the same datum underneath.
- Geographic coordinate system: in the form of latitude and longitude; not yet unfolded onto a plane, but always bound to a specific datum.
- Projected coordinate system: in the form of planar X and Y; obtained by projecting geographic coordinates, likewise bound to a specific datum.
Common Pitfalls
Pitfall 1: Looking only at the projection name while ignoring the datum.
If two layers both claim to be Gauss-Krüger 3-degree zone, but one is tied to Beijing 1954 and the other to CGCS2000, they will definitely be misaligned when overlaid.
Pitfall 2: Treating projection transformation and datum transformation as the same thing.
You can change the projection without changing the datum, and you can change the datum without changing the projection. In China, corresponding solutions such as seven‑parameter transformations and grid‑based transformations are available. Simply choose the one that suits your project specifications.
Summary
In day-to-day work, when coordinates do not match, do not rush to click the reproject button repeatedly. First verify the datum, then verify the projection, and only then consider whether a seven‑parameter or grid‑based transformation is needed. Once these three steps are taken in order, the cause of most overlay offsets can be identified.
What other projection pitfalls have you run into? Feel free to share your experiences in the comments.