Units of Measurement in Surveying
In surveying, we use specific units for length, area, volume, and angle. Two main systems are in use: the English System (feet, inches) and the International System of Units (SI) (meters, centimeters).
English System (Foot)
The basic unit of length is the foot (ft). Historically there were two definitions:
- Pre-1959 U.S. Survey Foot: 1 m = 39.37 in → 1 ft ≈ 0.3048006 m.
- International Foot (since 1959): 1 in = exactly 2.54 cm → 1 ft = exactly 0.3048 m.
Table 2.1: Common Length Units
| Unit | Equivalent |
|---|---|
| 1 foot | 12 inches |
| 1 yard | 3 feet |
| 1 inch | 2.54 cm |
| 1 rod (pole, perch) | 16.5 feet |
| 1 Gunter’s chain (ch) | 66 feet = 4 rods |
| 1 mile | 5,280 feet = 80 chains |
| 1 nautical mile | 6,076.1 feet |
Note: Many states still use the U.S. survey foot; others use the international foot.
Angle Units
Surveying angles are measured in degrees (°). 1° = 60 minutes (′), 1′ = 60 seconds (″). Subdivisions include grads (400 grads = full circle) and mils (6,400 mils).
International System of Units (SI)
The basic SI unit of length is the meter (m). Subdivisions: millimeter (mm = 0.001 m), centimeter (cm = 0.01 m), kilometer (km = 1,000 m). Areas use square meters (m2) or hectares (ha = 10,000 m2). Volumes use cubic meters (m3). Angles use degrees or radians.
Simple Explanation: SI units are based on decimal factors.
The meter is now defined by the distance light travels in 1/299,792,458 sec, making it very precise.
