The short answer
A coordinate format is how a location is written down. A coordinate system or projection is the framework used to place that location on a map. A datum is the reference model used to define where the Earth sits underneath those coordinates.
Format
Examples: DD, DMS, DDM, UTM, UK Grid, Easting/Northing.
It answers: How is the location written?
System / Projection
Examples: latitude/longitude, UTM zones, British National Grid.
It answers: How is the map organised?
Datum
Examples: WGS84, OSGB36.
It answers: What Earth reference are those coordinates tied to?
Why this matters
How they stack together
Think of coordinate data like a four-layer cake: the datum sits at the bottom and defines which model of the Earth you are using. On top of that sits the coordinate system or projection, which defines how positions are laid out. Then comes the coordinate format, which is just the writing style for those positions. Finally, the system type tells you whether you are working with angles (geographic) or metres (projected).
| Layer | Layer Name | Examples | Answers |
|---|---|---|---|
| Bottom (Earth reference) | Datum | WGS84, OSGB36, NAD83 | Which mathematical model of Earth? |
| Middle-bottom (Organization) | System & Projection | Latitude/Longitude, UTM, British National Grid | How is location measured? |
| Middle-top (Notation) | Format | DD, DMS, DDM, UTM notation, UK Grid notation | How is it written down? |
| Top (Type) | System Type | Geographic (angles), Projected (metres) | What units and structure? |
Layer 1: The Datum
A datum is a mathematical model of the Earth. It defines the shape, size, and position of a reference ellipsoid (the mathematical shape used to approximate the Earth). Two coordinates can use the same format and the same system but sit on different datums, and they will point to slightly different places on the ground.
The most common datums are WGS84 (used by GPS, phones, and web maps) and OSGB36 (used for British National Grid mapping). Converting between them without a datum transformation shifts the result by up to 120 metres.
- WGS84: The global standard. Used by GPS devices, smartphones, Google Maps, and international datasets. Based on the WGS84 ellipsoid.
- OSGB36: The British standard. Used for Ordnance Survey mapping, UK planning, surveying, and GIS. Based on the Airy 1830 ellipsoid.
- NAD83: The North American standard. Used across the USA and Canada.
- ED50: The older European standard, now rarely used but still found in legacy datasets.
Layer 2: The Coordinate System & Projection
A coordinate system is the framework that organizes how positions are measured and expressed. It sits on top of a datum.
There are two broad categories: geographic coordinate systems (which use angles/degrees on a globe) and projected coordinate systems (which use a mathematical projection to flatten the Earth onto a grid measured in metres).
- Geographic (angles on a globe): Latitude and longitude use degrees, minutes, and seconds (or decimal degrees) to measure position on a sphere. Examples: WGS84 geographic, OSGB36 geographic.
- Projected (metres on a flat grid): UTM, British National Grid, and State Plane systems flatten the curved Earth onto a 2D grid and measure distance in metres. This makes distance and area calculations trivial.
Geographic vs Projected - why they look so different
A single location in central London might be expressed as:
- Geographic (WGS84): 51.5074, -0.1278 (angles, global reference)
- Projected (British National Grid): 530029, 180380 (metres, grid-based)
Layer 3: The Coordinate Format
A coordinate format is simply how you write down a coordinate. The same value can be written in multiple formats without changing the underlying position.
The most common formats are:
- DD (Decimal Degrees): 51.488865, -0.014573 - the simplest, most digital-friendly form.
- DMS (Degrees Minutes Seconds): 51°29'19.91"N 0°0'52.46"W - traditional, used on printed maps.
- DDM (Degrees Decimal Minutes): 51°29.3318'N 0°0.8744'W - hybrid, common in marine GPS.
- UTM notation: 30N 707256 5708417 - zone-based grid with Easting/Northing.
- UK Grid notation: TQ 37942 78526 - alphanumeric grid square reference.
Changing formats does not change the underlying location. It is purely a notation change.
Layer 4: System Type (Geographic vs Projected)
The system type is about whether the coordinate system uses angles (geographic) or metres (projected).
| System Type | Units | How it works | Best for | Examples |
|---|---|---|---|---|
| Geographic | Degrees (angles) | Measures angles on a sphere | Global sharing, GPS, web maps | Latitude/Longitude (WGS84) |
| Projected | Metres (distance) | Flattens the Earth and measures distance | Measurement, surveying, GIS | UTM, British National Grid |
Where confusion usually starts
The most common mistakes happen when the four layers get mixed up. Here are the most dangerous ones:
- Assuming format = system: DD is not the same as WGS84. DD is a format (how it is written). WGS84 is a datum + geographic system (what it means). You can have DD in other datums too, which is why the same notation can plot in different places.
- Ignoring the datum: Two coordinates that look identical can point to different places if they are on different datums (WGS84 vs OSGB36, for example). This shift can be 50–120 metres, which is significant in surveying or planning.
- Mixing projected and geographic: Pasting UTM values into a tool expecting latitude/longitude will either fail or produce nonsense because the systems are fundamentally different.
- Assuming projection = datum: A projection (like Transverse Mercator) is a mathematical method for flattening the Earth. A datum is the Earth reference underneath. You can use the same projection on different datums.
Quick reference: Format vs System vs Projection vs Datum
Use this table to quickly check which layer your coordinate belongs to and what you need to convert:
| Example | Format | System/Projection | Datum | Common Use |
|---|---|---|---|---|
| 51.488865, -0.014573 | Decimal Degrees | Geographic (lat/long) | WGS84 | Google Maps, GPS, web apps |
| 51°29'19.91"N 0°0'52.46"W | DMS | Geographic (lat/long) | WGS84 (usually) | Printed maps, navigation |
| 30N 699745 5710157 | UTM notation | Projected UTM (Transverse Mercator) | WGS84 | GIS, military, global projects |
| TQ 37942 78526 | UK Grid notation | Projected British National Grid | OSGB36 | UK planning, surveying, OS maps |
| 530029, 180380 | Easting/Northing | Projected British National Grid | OSGB36 | UK engineering, CAD, planning |
Frequently asked questions
Here are answers to the most common questions that come up when working with coordinate data.
- Q: Is WGS84 a format or a system?
A: WGS84 is a datum + geographic coordinate system. It is not a format. You can express WGS84 coordinates in DD, DMS, or DDM format - all three describe the same WGS84 location, just written differently. - Q: Is UTM a format or a system?
A: UTM is a projected coordinate system (with its own datum, WGS84). The way you write a UTM coordinate (e.g. "30N 699745 5710157") is the format. The system is the framework that defines what those numbers mean. - Q: Can latitude and longitude change if I use a different datum?
A: Yes. The angle values change when you switch datums because the reference point shifts. WGS84 coordinates and OSGB36 coordinates for the same location will have different angle values. - Q: Why do my UK coordinates shift by 100 metres when I convert them?
A: You are probably converting between WGS84 and OSGB36 without applying a datum transformation. This is a common mistake in UK workflows. CoordinateMapper handles this automatically. - Q: Is a projection the same as a datum?
A: No. A projection is a mathematical method (like Transverse Mercator). A datum is an Earth reference. You can apply the same projection to different datums, or use the same datum with different projections. - Q: Can the same numbers mean different places?
A: Yes, absolutely. "51.5, -0.1" in WGS84 is not the same as "51.5, -0.1" in OSGB36. Without knowing the datum, you cannot know which location the numbers refer to.
Real-world scenarios
Here is how format, system, projection, and datum interact in practice:
- Scenario 1: Plotting coordinates on Google Maps
Google Maps expects latitude/longitude in WGS84 datum, written in DD format. If you have UK Grid references, you must convert both the system (BNG → WGS84) and apply the datum transformation (OSGB36 → WGS84). - Scenario 2: A surveyor shares UTM coordinates
You receive "30N 707256 5708417" (UTM notation) on WGS84 datum. If you paste this into a mapping tool expecting lat/long, it will fail. You need to convert the system (UTM → geographic) and optionally change the format (UTM notation → DD). - Scenario 3: UK planning data in a spreadsheet
You have Easting/Northing values (projected, BNG, OSGB36 datum). To share them in an email or upload to a web map, convert to latitude/longitude WGS84. That requires both a projection reversal and a datum transformation. - Scenario 4: Merging datasets from different sources
One source provides WGS84 DD, another provides OSGB36 UK Grid. Before merging, convert both to the same datum and system (usually WGS84 geographic). Never merge without checking the datum first - silent 100-metre shifts are worse than obvious errors.
Takeaway
Coordinate formats, systems, projections, and datums are four separate layers. Confusing them is the root cause of most coordinate mistakes. Before converting, always ask: What format is it written in? What system is it using? What datum is it on? Once you have those answers, conversion becomes straightforward.