Coordinate Systems & CRS Explained
Every map, DEM and point cloud lives in a coordinate reference system. Get the CRS right and your data lines up to the centimetre; get it wrong and layers drift apart by metres or land in the wrong hemisphere. Here is what you need to know.
What a CRS defines
A coordinate reference system (CRS) is the set of rules that turns numbers into positions on the earth. It says what the coordinates mean, what shape of earth they assume, and where the origin is.
Data with a known CRS can be placed correctly and combined with other data; data without one is just numbers.
Geographic vs projected
A geographic CRS uses latitude and longitude on a curved earth, measured in degrees. It is great for global data but awkward for measuring distances and areas locally.
A projected CRS flattens a region onto a plane and uses metres or feet, which is what most survey and engineering work needs. Projections trade a little accuracy for the convenience of straightforward measurement.
Datums matter
A datum defines the reference surface the coordinates hang on. Two systems can share a projection but use different datums, which shifts positions by anywhere from centimetres to hundreds of metres.
Mixing datums without a transformation is a common and costly source of misalignment.
Why matching is everything
When you overlay a DEM, an orthophoto and CAD vectors, they only line up if they share — or are correctly transformed to — the same CRS. Reading and respecting each layer's CRS is the difference between a clean project and hours chasing a mysterious offset.
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