Place observation posts by measuring how far a sentry can really spot a person, then checking that the posts’ covered arcs overlap around the whole ring you need to watch, with no gap and no post blind to its own surroundings. If a sentry spots a walking person at 500 m, a post on the edge of a 300 m radius settlement covers about 162 degrees of a ring 450 m out, so two posts leave a gap and three are needed; at 600 m two posts do.
Part of Watch and Patrol Scheduling
Post placement is the choice of where sentries stand so that together they see every approach, are covered by each other and cannot be seen or bypassed from outside.
Start From The Range A Sentry Really Has
The geometric horizon of a post is derived in Observation Posts and is an upper bound; nobody sees a person at 9 km. The working figure is the distance at which a sentry in typical light and weather first notices a person walking. It must be measured: send someone to walk away along several approach routes, by day and at dusk, and record how far they get before the sentry reports them. Use the shortest of those distances for planning, not the best.
Worked Example: Two Posts Or Three?
The sizes are example inputs. A settlement of radius a = 300 m, posts on its edge, and a warning ring b = 450 m from the centre, 150 m outside the edge, that must be watched all the way round. A post at distance a from the centre sees a point on the ring at angular separation delta when the distance between them is at most the working range r.
By the cosine rule, r squared = a squared + b squared - 2ab cos(delta). Solving for delta:
cos(delta) = (a squared + b squared - r squared) / (2ab).
| Working range r | cos(delta) | Half-angle delta | Arc covered per post | Posts needed |
|---|---|---|---|---|
| 400 m | (90,000 + 202,500 - 160,000) / 270,000 = 0.491 | 60.6 degrees | 121 degrees | 3 (3 x 121 = 364) |
| 500 m | (90,000 + 202,500 - 250,000) / 270,000 = 0.157 | 80.9 degrees | 162 degrees | 3 (2 x 162 = 324, a gap of 36 degrees) |
| 600 m | (90,000 + 202,500 - 360,000) / 270,000 = -0.25 | 104.5 degrees | 209 degrees | 2 (2 x 209 = 418) |
The parent’s minimum of two posts covering 360 degrees holds only for a working range near 600 m with this layout. At 500 m, two posts leave a 36-degree gap on the ring, and at 400 m, three posts only just close it with no margin for dead ground. Dead ground, weather and darkness shrink r, so measure before deciding.
What Else Decides A Site
- Overlap. Each approach should be seen from two posts, or from one post and covered by an alarm.
- Dead ground. Every post has areas it cannot see; map them with the formula in the observation-posts article, and cover them with a noise line or another post (Early Warning Systems).
- The foot of the post. A tall post looking far may be blind to the ground at its own base; check by walking up to it.
- Sun and wind. A sentry facing the low sun is dazzled, so avoid making the main approach the one the sun sets over, and shade the sentry. A post in the lee of a wall hears better at night than one in the wind.
- The silhouette. A post against the skyline is visible from outside (Camouflage and Concealment); set it below the crest.
- A hidden approach and a fallback path known by heart, as the parent says.
- Mutual support. Posts within hearing of each other, or linked by line of sight, can pass an alarm (Communication Between Posts).
Method: Choosing Post Sites
Step 1: Draw the ring or the lanes you need to watch, on a sketch map (Sketch Mapping).
Step 2: Measure the working range with a walking test.
Step 3: Compute the arc covered per post with the cosine rule, or lay a string of length r on the map.
Step 4: Place posts so arcs overlap, then adjust for dead ground, silhouette and sun.
Step 5: Walk each site at dawn, dusk and night, and from outside, looking back at the post.
Common Mistakes
- Siting by the best view. The best view often has the worst cover and the longest climb.
- Using the horizon figure as the range. It is a ceiling.
- Posts all on the same side. Coverage is a ring, not a line.
- Forgetting the staffing cost. Each post adds roughly four or five people for round-the-clock watch (Watch Rotation Systems).
What’s Next
- Learn next: Handover Procedures — keeping a post manned between shifts
- Combine with: Clearing Fields of Observation — improving what a post can see