The outer detection zone is only worth what it buys in warning time, and that is distance divided by the intruder’s pace: a person walking a flat path at about 5 km/h crosses 300 m in roughly 3.6 minutes, while the same person in brush takes about 6 minutes. Size the zone from the time your household needs to reach safety, then check that the ground does not let a walker arrive faster than you assumed.
Part of Perimeter Defense Design
The outer detection zone is the band from roughly 100 m to 300 m out from a dwelling, where cleared lanes, noise devices, lookouts and animals exist to notice approach rather than to stop it.
Warning Time Is Distance Over Pace, And Pace Depends On The Ground
The parent article gives the outer zone as 100-300 m and the sizing rule of Early Warning Systems: thirty seconds is barely enough to grab a weapon or a bag, five minutes lets you organise, thirty minutes lets you leave entirely. Those thresholds are in time. A zone is in metres. The bridge between them is the pace of whoever is approaching, and it varies by a factor of more than two with terrain.
I am recalling, not reading, Tobler’s hiking function (W. Tobler, 1993, a technical report of the US National Center for Geographic Information and Analysis): walking speed in km/h is W = 6 × e^(-3.5 × |S + 0.05|), where S is slope as rise over run, positive uphill. It peaks at about 6 km/h on a gentle downhill of 5%. Tobler also gave a factor of 0.6 for walking off a path or road. Treat these as the typical walking pace of a fit adult and test them on your own ground (see below).
What You Need
- A measured distance on your site: pace out 100 m and 300 m and mark them.
- A watch or a counted rhythm for timing walks.
- The time your household needs to reach its safe room or exit, measured in a drill. Without that number the zone cannot be sized.
Method: Size The Zone From The Time You Need
Step 1: Measure the household’s response time T. Run an unannounced drill and time it from the alarm to everyone being in the safe room or at the exit. Suppose, as an example input only, it takes 90 s.
Step 2: Pick the pace for each approach. Use the slowest realistic terrain only for the arrival you are willing to rely on, and the fastest for the one you must survive.
Step 3: Compute the minimum detection distance d = v × T.
Step 4: Compare with where detection actually happens. Dogs, a lookout and a noise line each trigger at a different distance. If the nearest reliable trigger is closer than d, move it out or add delay behind it.
Worked Example: The Same 300 m Takes Three Different Times
From Tobler’s function (recalled, see above):
- Flat path, S = 0: W = 6 × e^(-3.5 × 0.05) = 6 × e^(-0.175) = 6 × 0.8395 = 5.04 km/h, about 5 km/h, = 5,040 m ÷ 3,600 s = 1.40 m/s.
- Flat, off the path: 5.04 × 0.6 = 3.02 km/h = 0.84 m/s.
- Uphill 10%, S = 0.1, on a path: W = 6 × e^(-3.5 × 0.15) = 6 × e^(-0.525) = 6 × 0.5916 = 3.55 km/h = 0.99 m/s.
Time to cross the zone edge to the dwelling, t = d ÷ v:
- 300 m on a flat path: 300 ÷ 1.40 = 214 s, about 3.6 min.
- 300 m off the path: 300 ÷ 0.84 = 357 s, about 6.0 min.
- 300 m uphill on a path: 300 ÷ 0.99 = 303 s, about 5.1 min.
- 100 m on a flat path: 100 ÷ 1.40 = 71 s.
Reading the five-minute threshold the other way: 5 min = 300 s, and 300 s × 1.40 m/s = 420 m. A flat open path therefore needs detection at about 420 m, beyond the 300 m edge of the zone, to give five minutes. A detector at the 300 m edge gives about 3.6 min, which is between the thirty-second and five-minute thresholds. Thirty minutes is 1,800 s × 1.40 m/s = 2,520 m, which only a lookout with a long view or a signal from a neighbouring post can supply.
For the example drill time T = 90 s: d = 1.40 × 90 = 126 m on a flat path, and 0.84 × 90 = 76 m off a path. Detection at 100 m is enough through brush and not enough on an open path.
Clearing The Zone Speeds The Intruder Up
Cutting brush so that you can see makes the intruder faster as well. The pace ratio is 1.40 ÷ 0.84 = 1.67, so a cleared 300 m is crossed in 214 s instead of 357 s, a loss of 143 s. To win that back by seeing earlier you would need 143 s × 1.40 m/s = 200 m of extra sight distance. The answer is not to leave everything wild but to clear lanes for seeing and keep tangled growth across them, so sight is long and walking is slow. See Clearing Fields of Observation.
Layers Add Only If They Fail Independently
If six systems each independently caught 50% of approaches (an illustrative input, not a measured one), the chance all six miss is 0.5^6 = 0.0156, about 1.6%. The assumption is the weak point: fog, a quiet wind and a night with the dog asleep affect several layers at once, so real misses are correlated and the true figure is worse. Place layers so they do not share a failure mode, for example sound-based and sight-based.
Common Mistakes
- Using one pace for every direction. A route uphill through trees and a track downhill differ by more than a factor of two.
- Counting the dog’s range as a detection distance. A dog hearing something at 300 m is not the same as a dog alerting you at 300 m.
- Assuming warning ends at the first trigger. Wake, dress, gather the children and reach the exit all take time. Measure them.
Safety
A detection plan that keeps people out at night also puts them in the dark
Night movement by your own household over rough ground, with lanes and lines of cord, causes falls. Mark safe paths for your own people with something they can feel by foot or hand, and do not run cords across routes your household uses after dark. A rope guide at hand height is safer than a trip line at ankle height.
What’s Next
- Learn next: Middle Delay Zone — turning detection time into delay time
- Combine with: Observation Posts — the one layer that extends detection past 300 m