Dead Reckoning (DR) is a foundational maritime navigation technique used to estimate a vessel's current position by projecting its course, speed, and time elapsed from a last known position (or "fix"). It allows sailors to plot their position on a chart without external references like GPS or landmarks.

Key Components and Process:
Starting Point: Initiated from a known fix.

Variables: Uses compass heading, speed through the water (log), and time.

Calculation: Formulaic approach: The navigator moves their position forward along the steered course.

Limitation: It is prone to cumulative errors because it often does not account for external factors like wind, leeway, or currents.

Why It Is Used:
Backup Method: Serves as a crucial backup for when electronics (GPS) fail.

Visibility Issues: Reliable in fog or at night when landmarks are invisible.

Situational Awareness: Helps sailors develop a better understanding of their progress.

Origin:
The term is believed to be a contraction of "deduced reckoning" (or "ded-reckoning"), stemming from the Age of Exploration when navigators would, as described by Teletrac Navman, calculate their position to determine the day's ending point and use it as a starting point for the next.