At the 200-500 meter ranges that matter for a settlement, the curve of the Earth is irrelevant (it drops only about 2 centimeters at 500 meters), so what you can see is decided by terrain and plants in the way. Check each approach with a line-of-sight calculation over the highest intervening ground: from an eye height 12 m above the valley floor, a knoll 8 m high and 100 m away hides a standing person until they are about 258 m away.
Working out what a settlement can and cannot see of its approaches, including the dead ground that hides movement from a lookout.
Height Helps Because It Lifts the Line of Sight Over Obstacles
The carrier asks whether you can see people approaching from at least 200-500 meters in the most likely approach directions, and says a modest elevation advantage of 10-20 meters dramatically extends visual range. The reason is not the curvature of the Earth. Distance to the horizon from eye height h is d = sqrt(2 R h), with R = 6,371 km (the Earth’s radius), ignoring atmospheric refraction:
- Standing eye height 1.7 m: d = sqrt(2 x 6,371 x 0.0017) = 4.65 km.
- Eye height 12 m: d = sqrt(2 x 6,371 x 0.012) = 12.4 km.
Both are far beyond 500 m. At 500 m the ground curves away by 500 x 500 / (2 x 6,371,000) = 0.0196 m, about 2 cm. So at defensive ranges nothing is hidden by the planet. What hides people is dead ground: a knoll, bank, wall, hollow or crop that blocks the line of sight, and trees and undergrowth. Elevation helps because a higher eye sees over low obstacles.
What You Need
- A sketch map or profile of each approach — heights at the lookout, at any intervening rise and at the target.
- A level or water level — to measure heights. See Water Level.
- An hour in each season — leaf cover changes sightlines completely.
Method: Line-of-Sight Check
Step 1: Choose the lookout and the approach line Measure the eye height of the lookout above the ground at the target end (the valley floor, say).
Step 2: Find the controlling obstacle The obstacle that blocks the view most is the one that rises highest into the line from eye to target. Measure its height above the same reference and its distance from the lookout.
Step 3: Draw the line The line from the eye grazing the top of the obstacle continues downward. Where it meets the ground, or reaches a target’s head height, is the limit of what is hidden.
Worked Example
Eye height above the valley floor 12 m. A knoll with crest 8 m above the floor, 100 m from the lookout. Target: a standing person 1.7 m tall at 300 m.
- Slope of the line over the crest: (12 - 8) / 100 = 0.04 m of drop per metre of distance.
- Head height of the person is reached by the grazing line at (12 - 1.7) / 0.04 = 257.5 m.
- Ground level is reached at 12 / 0.04 = 300 m.
So everyone standing between the knoll (100 m) and about 258 m is completely hidden, and from 258 m to 300 m only the upper body shows. The person at 300 m is seen from head to toe, with the feet just grazing the line. Without the knoll the lookout would see all of them. The dead ground is the 100 m to 258 m band. Check the arithmetic against a direct test: at 300 m the line to the head is at 12 - 10.3 x (100 / 300) = 8.57 m at the knoll, higher than the 8 m crest, so the head is visible.
Practical Rules That Follow
- Walk the approach in reverse. Stand where an outsider would, at 1.7 m eye height, and see what shows of your settlement: smoke, lights, roofs. Visibility works both ways.
- Place watch points where they overlook dead ground, not just where they have the longest view.
- Combine watching with work, as the carrier advises: put the lookout where daily work happens so that watching and working are the same activity. See Observation Fundamentals and Watch & Patrol Scheduling.
- Re-check in summer and winter, because leaves hide movement in the first and not the second.
Common Mistakes
- Choosing the highest point — highest is exposed to wind and weather, and visible from everywhere.
- Forgetting the lookout’s own height — a person in a trench or behind a parapet sees less.
- Measuring in winter and trusting it in summer.
Safety
Lookouts fall and freeze
Elevated platforms and cliff-edge posts need rails or a rope, and watchers on exposed ridges need shelter from wind and cold. A tired watcher on a high platform is a falling risk.
What’s Next
- Learn next: Retreat Routes — leaving when watching is not enough
- Combine with: Contour Mapping — drawing the ground that hides things