Thanks all.

Some time ago I was looking for real world research information on spotting distances in order to translate them to a game. Well I finished that, mostly through US military basic research documents from the 60s and 70s.

My conclusion is interesting and not something I've seen in any RPGs. Details vary but all the final graphs I needed had the same basic shape, a sort of stretched out Z. While the exact percentages and distances depend on many factors everything from spotting people in the jungle, to jets in the air, to trucks in the desert from a jet, all ended up having that sort of stretched Z shape. The first 1/3rd to a bit more of the zero to whatever distance it goes from 100% down to 80% or 75%, then in the last 1/2 to 1/3rd of the distance it's going from 25% or 20% down to 0%, and in the middle there's something that generally looks like a cliff.

Anyway, thanks for the initial direction of searching. Partial bibliography to follow and then the typed up data I used in a spoiler for the morbidly curious. There's a pile of handwritten notes too but I'm not transcribing those.

AD-753 600 TARGET DETECTION AND RANGE ESTIMATION
James A. Caviness, et al
Office of the Chief of Research and Development (Army)
November 1972

https://apps.dtic.mil/sti/pdfs/AD0438001.pdf
RESEARCH MEMORANDUM, MOONLIGHT AND NIGHT VISIF1LITY
by Thomas F. Nichols and Theodore R. Powers, USAIHRU, January 1964

Dobbins, D.A. et al. Jungle Vision IL: Effects of Distance, Horizontal Placement, and
Site on Personnel Detection in an Evergreen Rain forest, U.S. Army Tropic Test Center,
Fort Clayton, Canal Zone, March 1965.

Louis, Nicholas B. The Effects of Observer Location and Viewing Method on Target
Detection with the 18-inch Tank-Mounted Searchlight, HumRRO Technical Report 91,
June 1964.

ASTIA, Report Bibliography on Target Detection and Range Estimation by
Humans, Armed Forces Technical Information Agency, Arlington, Virginia,
November 19 60.

Louis, Nicholas B. The Effects of Observer Location and Viewing Method on Target
Detection with the 18-inch Tank-Mounted Searchlight, HumRRO Technical Report 91,
June 1964.

Spoiler: raw data stuff
Show

open terrain 95%
middle terrain 88%
heavy foliage 55%

100m 90%
200m 90%
300m 55%

AD-753 600 TARGET DETECTION AND RANGE ESTIMATION
James A. Caviness, et al
Office of the Chief of Research and Development (Army)
November 1972

------------------------------------------------------------------------------------------------------------
https://apps.dtic.mil/sti/pdfs/AD0438001.pdf
RESEARCH MEMORANDUM, MOONLIGHT AND NIGHT VISIF1LITY
by Thomas F. Nichols and Theodore R. Powers, USAIHRU, January 1964

TABLF 4
VISIBILITY AT NIGHT
Moon Age Ground and Background
Object SingleSoldier(meters) Patrol Unit
Starlit Night
Level, Grassy Ground
25 30 40
Level, Rare Ground
30 40 45
Dark Background
10 10 15
Silhouetted Against Sky
35 55 80

12th Day from the Full Moon (Crescent)
Level, Grassy Ground
30 60 75
Level . Bare Ground
30 45 50
Dark Background
10 15 20
Silhouetted Against Sky
130 140 180

7th Day from the Tull Moon (Half)
Level, Grassy Ground
60 70 80
Level, Bare Ground
35 50 55
Dark Background
10 15 20
Silhouetted Against Sky
140 170 230

3d Day from the Full Moon (3/4)
Level, Grassy Ground
70 75 120
Level, Bare Ground
40 50 70
Dark Background
15 20 25
Silhouetted Against Sky
160 220 280

15th Day of the Moon (Full)
Level, Grassy Ground
75 100 150
Level , Bare Ground
50 80 100
Dark Background
15 20 25
Silhouetted Against Sky
180 250 300
50

1. Patrol is three or four men. Unit is a platoon in column. The above figures show the visible range when the object is not in motion. It is easier to Identify an object in motion especially when it is moving crosswise.
2. This experiment was conducted on clear nights during January and February, [t can be assumed that the brightness is practically equal for two or three nights before and after each age of the moon in the chart.

IDENTIFICATION of person size object
FULL MOON %% NO MOON
20m 95%
40m 90% 5m
55m 80% 12m
60m 70% 15m
70m 60% 18m
85m 50% 27m
100m 40% 33m
110m 30% 40m
120m 20% 50m

TABLE 7 -- moonless & cloudless
RECOGNITION RANGES (YARDS) FOR THE
M-48 TANK AND THE 2-1/2-TON TRUCK
recognition = identification
paths 1=60m, 2=60mstraight, 3=50m, 4=125m, noise/no-noise
drive towards observer, 'no noise' = white noise covering
PATH TANK TRUCK
4 111/210 86/77
3 126/192 100/95
2 184/155 83/108
1 152/173 91/97

TABLE 8 -- moonless & cloudless
VISIBILITY RANGES FOR TARGETS GROUPED ACCORDING TO SIZE
"Large" targets included such things as a 2-1/2-ton truck, "medium" targets included such +hings as a jeep, and 2-man tents and similar size material were grouped as "small" targets.
detect / recognize
Large Targets 73/45
Medium Targets 43/24
Small Targets 16/11

TABLE 9 -- full moon & 2minute time limit
VISIBILITY RANGES FOR HUMAN TARGETS
dist, stand, walk
250, 25, x
180, 50, 25
130, 75, x
100, 90, 45

---------------------------------------------------------------------------------------------------------------

Dobbins, D.A. et al. Jungle Vision IL: Effects of Distance, Horizontal Placement, and
Site on Personnel Detection in an Evergreen Rain forest, U.S. Army Tropic Test Center,
Fort Clayton, Canal Zone, March 1965.

TABLE II
Detection thresholds and 25-75% range at
each of three evergreen rainforest sites.
25% 50% 75%
82 62 47 - dim (50% others), wild palm, stilt palm, maquengue palms, *Geonoma decurrens*
92 80 55 - wild fig, stilt palm, wide leaf palm, *wide-leaf palms*
95 76 59 - Scheelea zonensis, broadleaf evergreens, Str6manthe lutea, *panama hat palm*
90 72 56 - averaged

%% feet time
95% 40
80% 50 25s
70% 60 33s
60% 68 34s
50% 75
40% 80 39s
10% 100 61s

--------------------------------------------------------------------------------------------------


Louis, Nicholas B. The Effects of Observer Location and Viewing Method on Target
Detection with the 18-inch Tank-Mounted Searchlight, HumRRO Technical Report 91,
June 1964.

Probability of Detecting Targets & observer distance from searchlight
yards time %%
0 10 .14
0 20 .23
10 10 .23
10 20 .35
20 10 .29
20 20 .37
40 10 .21
40 20 .35
80 10 .40
80 20 .54
160 10 .33
160 20 .41

Probability of Identifying Targets
dist %%
0 .20
10 .26
20 .26
40 .25
80 .37
160 .36

Performance on Targets Viewed
From 80-Yard Position by Observers Using Binoculars
type 655yd 780yd 900yd 1055yd
detect
tank 87 89 74 54
apc 96 83 81 60
jeep 90 66 59 32
ID
tank 84 72 46 39
apc 84 56 49 21
jeep 70 28 35 21