How Many Decimal Places for GPS 座標? Precision 解説
When working with Latitude and Longitude in decimal degrees (e.g., 38.8895 N, -77.0353 W), the number of decimal places dictates the precision of the location. A common mistake is providing either too few decimals or far too many (implying millimeter precision from a smartphone).
精度スケール：各桁の意味
At the equator, one degree of latitude equals about 111 kilometers. Therefore, moving the decimal point one place scales the precision by a factor of 10. (Note: Longitude lines converge at the poles, so the east-west distance gets smaller as you move north/south, but latitude distances remain roughly constant).
| Decimals | Example | Approximate Accuracy (at Equator) | Use Case |
|---|---|---|---|
| 0 | 40 | 111 km (69 miles) | Identifying a country or large state. |
| 1 | 40.1 | 11.1 km (6.9 miles) | Identifying a large city or county. |
| 2 | 40.12 | 1.11 km (0.69 miles) | Town or neighborhood level. |
| 3 | 40.123 | 111 meters (364 feet) | Identifying a specific neighborhood block or large estate. |
| 4 | 40.1234 | 11.1 meters (36.4 feet) | Identifying a specific parcel or street. (Common car GPS output). |
| 5 | 40.12345 | 1.11 meters (3.6 feet) | The Sweet Spot. Identifying a specific tree, door, or person. Matches smartphone GPS accuracy. |
| 6 | 40.123456 | 11.1 centimeters (4.4 inches) | Specialized mapping, differential GPS, laying out rough construction. |
| 7 | 40.1234567 | 1.11 centimeters (0.44 inches) | Survey-grade RTK GPS. Requires professional geodetic equipment. |
| 8+ | 40.12345678 | 1.11 millimeters | Tectonic plate tracking. Essentially meaningless for static mapping. |
見せかけの精度の罠
Software developers frequently export Latitude and Longitude as raw 64-bit floating-point numbers containing 13 or 14 decimal places (e.g., 40.1234567890123). This is false precision.
Unless the coordinate was generated by a geodetic surveyor using an RTK base station, taking a phone GPS reading and displaying 14 decimal places implies the coordinate is accurate to the size of a single human cell. In reality, the phone's hardware error circle was 4 meters wide. All digits after the 5th decimal place are random numerical noise.
DECIMAL(9,5) or DECIMAL(9,6)) before inserting into a database can speed up geospatial queries significantly.
Convert decimal coordinates to DMS or UTM and see how the precision holds up:
—Decimal to DMS 座標変換ツールUTM精度の比較
Unlike Lat/Long, UTM coordinates are measured in actual meters. Therefore:
- 0 decimal places (e.g., 400,123) = 1 meter precision.
- 1 decimal place (e.g., 400,123.4) = 10 centimeter precision.
- 2 decimal places (e.g., 400,123.45) = 1 centimeter precision.
This linear simplicity is why civil engineers prefer UTM or State Plane over calculating decimals of a degree.
FAQ
How many decimal places does Google Maps use?
When you right-click a location in Google Maps, the web interface typically copies coordinates to 5, 6, or 7 decimal places. Google's internal APIs often accept data at any precision but calculate paths based on routing nodes.
Why do longitude distances change by location?
Lines of longitude converge at the North and South Poles. At the equator, 1 degree of longitude is ~111km. At 60 degrees North (e.g., Oslo), 1 degree of longitude is only ~55km wide. Latitude distances, however, stay relatively constant pole to pole.
See also: MGRS Precision Levels | Data Validation
米国平面直角座標系（SPCS）変換 & ガイド
各州固有の正角投影グリッドからGPS WGS84への高精度測量・工学座標変換。
地殻変動と再施工リスク：NAD83 vs WGS84の累積誤差
北米プレートは年間約2cm移動するため、NAD83(2011)とWGS84(G1762)は現在2.2メートル以上乖離しています。 高精度なNAD83地籍杭打ちに「標準的な」GPS WGS84座標を使用したことで、基礎のやり直しやインフラの配置ミスによる5万ドルの専門家責任賠償請求が発生しています。