Coordinate Reference Systems (CRS) Explained
A coordinate reference system (CRS) is the set of rules that turns numbers into geographic positions — it says what the coordinates mean, what shape of earth they assume, and where the origin is. 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.
EPSG codes: how a CRS is named
In practice a CRS is referred to by its EPSG code — a number from a public registry that pins down the complete definition. Software that reads or writes georeferenced data exchanges these codes so that "the same system" means exactly the same system on both sides.
A handful of codes cover most day-to-day encounters:
| EPSG code | What it is | Where you meet it |
|---|---|---|
| EPSG:4326 | WGS 84 geographic — latitude and longitude in degrees | Raw GNSS output, web APIs, KML; not a projection, so distances and areas are not in metres |
| EPSG:32635 | WGS 84 / UTM zone 35N — projected, in metres (one of 60 zones) | Survey and engineering work; the zone number changes with longitude |
| EPSG:3857 | Web Mercator — the projection behind online basemaps | Web maps and tile services; fine for display, wrong for measurement — areas and distances distort away from the equator |
| EPSG:27700 | OSGB36 / British National Grid — a national grid example | Country-specific grids like this keep national mapping consistent; most countries define their own |
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.
Frequently asked questions
What does a coordinate reference system (CRS) actually define?
A 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.
What's the difference between geographic and projected coordinate systems?
A geographic CRS uses latitude and longitude in degrees on a curved earth — great for global data but awkward for local distance and area work. A projected CRS flattens a region onto a plane using metres or feet, trading a little accuracy for the convenience of straightforward measurement, which is what most survey and engineering work needs.
Why does a datum matter if two datasets already share a projection?
A datum defines the reference surface the coordinates hang on, and two systems can share a projection but use different datums, shifting positions anywhere from centimetres to hundreds of metres. Mixing datums without a transformation is a common and costly source of misalignment.
Why is matching coordinate systems important when combining DEMs, orthophotos, and CAD data?
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.
Sources
Work with georeferenced data in STREAM
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