Dead reckoning means advancing one's position from the last determined position using the direction and distance of subsequent movement. Each completed leg supplies another calculated change in position. The resulting location is a propagated position carrying the uncertainties of the starting point and movement data. During military isolation, this method can preserve the spatial relationship between the last known location and one's own movement when electronic positioning is unavailable or fails.
What the propagated position tells you
A determined starting location rests on a position observation, such as an unambiguously identified point on the ground whose location is known. That observation also has limited accuracy. The subsequent dead reckoning position is obtained by applying the distance travelled in each direction to that starting point. If the calculation begins at an assumed location, every subsequent point remains tied to that assumption. Extra digits in a coordinate cannot remove an incorrectly assumed starting location.
A change of direction begins a new leg with its own direction and distance. Someone who detours around impassable ground will have travelled farther than they have advanced towards the original destination. Adding the entire walking distance along the first course would place the calculated location too far along that line. The position must therefore reflect the movement that actually occurred, including departures from the plan.
Direction, distance and missing means
Direction and distance need an identifiable basis. A usable direction instrument can supply a reading; a remembered general direction of movement remains coarser. With pace counting, the distance estimate comes from the counted movement and the personal pace length established for it. A vehicle distance indicator, if available, supplies another distance basis. The feeling that one has now walked roughly far enough already contains an estimate, which must also enter the position as an estimate.
A watch supplies time. Deriving distance from it also requires a speed appropriate to the movement that actually occurred. Someone who was away for an hour but spent a substantial part of it stationary in concealment must not count the entire hour as walking time at constant speed. Soft ground, uphill travel and exhaustion change speed and pace length. When reliable direction or distance data are missing, the result remains a rough idea of location; precise arithmetic using assumed values does not improve those inputs.
How errors propagate
A direction error shifts the endpoint sideways; for the same angular error, this lateral discrepancy increases with the length of the leg. A distance error places the endpoint too near or too far along that direction. Calculating onward from this erroneous point carries its discrepancy into later points. Errors in the new legs are added as well. Individual discrepancies can partly cancel, but the calculation itself supplies no evidence that they have done so.
Lost pace totals, unrecorded changes of direction and additional detours break the reliable progression of the estimate. When relating movement to a map, there is another issue: map distance is horizontal, so the extra distance walked over a slope is not an equal displacement in the map plane. Such gaps or changed movement conditions require allowing for greater uncertainty. Keeping the last confirmed location separate from the subsequent movement that can only be roughly reconstructed preserves the information still available.
What independent position evidence changes
A newly recognised location can correct the calculation when its identity and position are actually established. The observation must supply more than “The junction ought to be here now.” An unambiguously identified junction can provide a new starting point; an arbitrary junction in the expected area cannot. A recognised terrain feature may initially only narrow the possible area of location. What it establishes depends on how clearly the feature, its location and one's own relationship to it have been determined.
If the actual observation conflicts with the calculated point, the starting assumption and movement data need to be checked. Declaring the expected terrain feature found merely because the walking time fits would confirm the estimate with its own output. An independently determined new location instead establishes a new reference for further dead reckoning; its remaining uncertainty is retained.
Significance during isolation and recovery
For an isolated person, the dead reckoning position indicates where they probably are on the basis of the movement recorded so far. It can help them assess their location relative to the intended recovery area. An agreed point or calculated arrival time, however, remains part of the plan. Walking the planned distance confirms neither arrival at that point nor the possibility of pickup by friendly forces. If actual PR contact becomes possible through available means, the last confirmed location, the time it was established, subsequent estimated movement and new position evidence are distinct items of information. They allow the recipient to recognise the basis of the reported position and what still needs verification.
US nautical terminology describes dead reckoning more narrowly through the last position fix, the vessel's course, speed and elapsed time. Adding an allowance for an estimated current can produce an estimated position. This matters during isolation at sea: current and wind can displace a craft relative to the position calculated from its own movement. The land definition concerns the direction and distance of completed legs; nautical labels for supplemented estimates must therefore not be understood as additional confirmation of location.