Accuracy
The radius of the semi-major axis of the Earth at the equator is 6,378,160.0 meters resulting in a circumference of 40,075,161.2 meters. The equator is divided into 360 degrees of longitude, so each degree at the equator represents 111,319.9 metres or approximately 111 km. As one moves away from the equator towards a pole, however, one degree of longitude represents a diminishing number of meters, approaching zero at the pole. The number of decimal places required for a particular accuracy at the equator is:
| decimal places |
degrees | distance |
|---|---|---|
| 0 | 1.0 | 111 km |
| 1 | 0.1 | 11.1 km |
| 2 | 0.01 | 1.11 km |
| 3 | 0.001 | 111 m |
| 4 | 0.0001 | 11.1 m |
| 5 | 0.00001 | 1.11 m |
| 6 | 0.000001 | 0.111 m |
| 7 | 0.0000001 | 1.11 cm |
| 8 | 0.00000001 | 1.11 mm |
A value in decimal degrees to an accuracy of 4 decimal places is accurate to 11.1 meters (± 5.55 m) at the equator. A value in decimal degrees to 5 decimal places is accurate to 1.11 meter at the equator. Because the earth is not flat, the accuracy of the longitude part of the coordinates increases the further from the equator you get. The accuracy of the latitude part does not increase so much, more strictly however, a meridian arc length per 1 second depends on latitude at point concerned. The discrepancy of 1 second meridian arc length between equator and pole is about 0.3 metres because the earth is an oblate spheroid.
Read more about this topic: Decimal Degrees
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