What is coordinate precision?
Coordinate precision describes how finely a location is expressed. In Decimal Degrees, it is controlled by the number of decimal places. In grid-based systems like UK Grid references or UTM, it is controlled by the number of digits.
How decimal places affect precision in Decimal Degrees
| Decimal places | Example | Approximate ground distance |
|---|---|---|
| 0 | 52° | ~111 km |
| 1 | 51.5° | ~11.1 km |
| 2 | 51.51° | ~1.1 km |
| 3 | 51.509° | ~111 m |
| 4 | 51.5074° | ~11 m |
| 5 | 51.50740° | ~1.1 m |
| 6 | 51.507400° | ~0.11 m (11 cm) |
How digit count affects precision in grid references
| Digits (per axis) | Example | Describes a square of |
|---|---|---|
| 2 (4 total) | TQ 38 17 | 1 km x 1 km |
| 3 (6 total) | TQ 379 178 | 100 m x 100 m |
| 4 (8 total) | TQ 3794 1785 | 10 m x 10 m |
| 5 (10 total) | TQ 37942 17852 | 1 m x 1 m |
Precision vs accuracy: they are not the same thing
Precision is about how much detail the coordinate carries. Accuracy is about how close the coordinate is to the true position on the ground. A coordinate can be highly precise without being accurate.
How much precision do you actually need?
- Casual location sharing: 4 to 5 decimal places in DD.
- Field survey and GPS data: 5 to 6 decimal places in DD.
- Professional surveying: 6+ decimal places in DD.
- GIS analysis and data storage: 6 decimal places is a safe default.
Takeaway
Coordinate precision is about how finely a location is expressed, not how trustworthy the position is. Matching precision to the task and understanding the difference between precision and accuracy are the most important principles.