departure

In surveying, departure is the horizontal distance traveled in the east-west direction along a survey line or traverse. If you walk from point A to point B and measure how many meters you moved toward the east (positive departure) or west (negative departure), that measurement is your departure, regardless of how far north or south you also traveled.

Departure is one component of a traverse calculation; the other is latitude, which measures north-south movement. Together, these two values define the position of one point relative to another. A surveyor measures the bearing and distance of each line segment in the field, then calculates departure and latitude in the office using trigonometry: departure equals the distance multiplied by the sine of the bearing angle.

Closure and error detection

When a traverse returns to its starting point, the sum of all departures should equal zero, and the sum of all latitudes should equal zero. If the numbers do not close, an error exists in the measurements or calculations. A closure error in departure suggests either a mistake in bearing measurements or a computational error. Surveyors use the total closure error to adjust individual measurements proportionally across the traverse, a process called balancing.

The term departure derives from the verb depart, meaning to leave or go away. In the context of a survey line, the departure describes how far the line has departed from a true north-south reference line. The concept has roots in navigation and astronomy, where pilots and astronomers needed to track east-west drift separately from north-south progress.

Departure calculations are essential for converting field observations into coordinate systems. Modern total stations and GPS equipment automate these calculations, but surveyors must still understand the concept to verify results, detect blunders, and interpret closures on plans and reports.