An observation post sees as far as the horizon allows: about 4.65 km for a standing observer, and about 9.2 km for a sentry on a 5 m platform, whose eyes are about 6.7 m above the ground. Haze, trees and dead ground cut the real range far below that, so site a post by what it can see out to the edge of the outer zone, where warning is still useful.
Part of Early Warning Systems
An observation post is a raised or well-sited position from which a sentry watches approaches to a dwelling or settlement, chosen for the area it overlooks, the dead ground it leaves and how well it conceals the sentry.
The Horizon Is A Geometric Upper Bound, Not A Range
The parent article says a person at ground level on flat terrain sees about 5 km, at 5 m elevation about 8 km, at 10 m about 11 km, and gives the advice that every metre of height extends the view, that the post should see without being seen, that two posts at minimum should cover 360 degrees between them, and that each post needs a way to signal the camp. This page derives the figures, finds a place where the parent’s use of them needs a correction and works out what height does and does not buy.
What You Need
- A measured eye height for the sentry: the platform height plus the distance from platform to eyes, about 1.7 m for a standing adult.
- A pace-measured sketch of the ground around the post, with dips and hills marked.
- Binoculars or a spy-glass, if available, and shade for the sentry.
- A concealed approach and a fast, safe way down.
- A signal to the camp.
Method: Compute The Horizon, Then Find The Dead Ground
Step 1: Compute the horizon distance from eye height h with d = √(2 × R × h), where R = 6,371 km is the mean radius of the Earth (a standard value I am recalling, not reading from this corpus). Convert units before multiplying.
Step 2: Add the target’s own height. A standing person of height p is visible beyond the observer’s horizon until the person’s own horizon distance is used up: maximum distance = √(2Rh) + √(2Rp).
Step 3: Find the dead ground near the post: dips and reverse slopes. Use the formula below.
Step 4: Check against the limiting range, which is usually haze, trees and buildings. Walk out and look back at the post.
Step 5: Decide height by cost. Every extra metre of platform is a longer fall and more exposure. Match the height to the dead ground you need to remove.
Worked Example: Horizon, Target And Dead Ground
Horizon. 2R = 2 × 6,371,000 m = 12,742,000 m.
- Eye height 1.7 m: d = √(12,742,000 × 1.7) = √21,661,400 = 4,654 m, about 4.65 km. Typical atmospheric refraction bends light around the curve and extends this by roughly 7%, as I recall from surveying practice, giving about 5 km, which matches the parent article’s ground-level figure.
- Eye height 5 m: d = √(12,742,000 × 5) = √63,710,000 = 7,982 m, about 8.0 km. This is the parent article’s 8 km, but note that it is the horizon of an eye 5 m up. A sentry standing on a platform 5 m high has eyes at 5 + 1.7 = 6.7 m.
- Eye height 6.7 m: d = √(12,742,000 × 6.7) = √85,371,400 = 9,240 m, about 9.2 km.
- Eye height 10 m: d = √127,420,000 = 11,288 m, about 11.3 km. For a platform 10 m high, eyes at 11.7 m give √149,081,400 = 12,210 m, about 12.2 km.
- A shortcut: d in km is about 3.57 × √h with h in metres. For h = 1.7: 3.57 × 1.304 = 4.65 km.
Target. A person 1.7 m tall seen by a sentry with eyes at 6.7 m: the person’s own horizon is 4.65 km and the sentry’s is 9.24 km, so the top of the head is visible out to 9.24 + 4.65 = 13.9 km on flat ground in perfect air. A sentry at ground level sees the same person to 4.65 + 4.65 = 9.3 km. In practice nobody sees a person at those distances. Visual acuity of about 1 arcminute, a standard figure for good eyes, means a figure 0.5 m wide stops being resolved as a shape at about 0.5 m ÷ 0.000291 = 1,720 m, and recognition needs far more detail than that. Binoculars magnify the angle by their power. The horizon figure is a ceiling, not a forecast.
Dead ground. Take a sentry with eye height H above the ground at the post and a dip whose near lip is x from the sentry. A person of height p stands in the dip at a distance y beyond the lip, at the bottom of a dip of depth z. The line of sight grazes the lip and then falls by H × y ÷ x below ground level at the person. The person is hidden if the top of the head is below that line: p - z < -H × y ÷ x, so z > p + H × y ÷ x.
- Eyes at 6.7 m, lip 100 m away, person 10 m beyond the lip: z > 1.7 + 6.7 × 10 ÷ 100 = 1.7 + 0.67 = 2.37 m.
- Eyes at 1.7 m: z > 1.7 + 1.7 × 10 ÷ 100 = 1.7 + 0.17 = 1.87 m.
Height helps with dead ground, but not very much: raising the eye from 1.7 m to 6.7 m only raises the depth of dip that can hide a standing person from 1.87 m to 2.37 m. A hollow of that size is the kind that needs a second post, a noise line or a dog, not a taller platform.
Watch Rota Arithmetic
One post watched round the clock on 2-hour shifts needs 24 ÷ 2 = 12 watches a day. How many people that takes is set by rest and other duties; the figures are in Watch and Patrol Scheduling.
Common Mistakes
- Quoting the horizon as the working range. Nobody sees a person at 9 km.
- Forgetting the sentry’s eye height on a platform, so the horizon is under-estimated.
- A silhouette on the skyline. A person on a roof or ridge is visible for kilometres; place the post slightly behind the crest.
- A post whose only way down is a pulled-up ladder. The sentry is trapped there in a fire, a collapse or an injury.
Safety
Falls, lightning and the sentry's way down
A tree platform 3-5 m up is a fall hazard in wind, ice and the dark: fit a railing and a rope handrail, and test the platform under load. In thunderstorms an exposed high post is the most dangerous place on the site; the sentry comes down. The sentry must have a way down that does not depend on someone else lowering a ladder. For the signalling at the top see Signal Systems.
What’s Next
- Learn next: Signal Fires and Beacons — passing the alarm beyond voice range
- Combine with: Night Observation — how to look when the light goes