Visibility rules
 

This tread is intended to gather some advice and insights on guidelines for general visibility rules, that are dispersed throughout rules books. I miss a structured set of instructions when planning realistic patrols, search operations, castle walls manning, etc.

I don't know if this has been discussed elsewhere, but I could not find it.

These general visibility rules could address some of the following questions:

1) How far can a man spot another man moving on the grass, assuming he is on a castle wall, like 15ft above the ground, and under perfect light and weather conditions? Or, put in other words, on average, how far a person with Per of 10 could roll against 12?

2) How far can a person recognize a friend in the same conditions?

3) How far can a soldier keep track of another one in differente environments, like a deciduous forest, or a night grass field?
It would help to answer questions of how much land ten man can sweep with a search team in one hour.

Thanks in advance!

Re: Visibility rules
 

For GURPS, you're mostly looking for the Plain Sight rules. If you want realism, this is the kind of thing the military has studied, you should be able to find papers on the internet (I have no links at the moment; haven't looked at this in quite some time).

Re: Visibility rules
 

The distance at which you can spot someone boils down (all else being equal) to how much of a range penalty you can soak on your vision roll. There are other modifiers to that Perception roll, of course -- things like darkness, camouflage, or those trees in the way. Being "in plain sight" is a +10, so that makes things much more visible at a distance.

This past thread has some useful discussion.
http://forums.sjgames.com/showthread.php?t=130631

Realistically speaking, you'll also have to consider terrain. Also the horizon distance, if you're talking really long distances and "flat" terrain.

(The horizon on Earth from a 6' / 2m height -- human eyeballs -- is about 3 miles / 5 km. Further than that, and the object is sliding down behind the curve of the planet. Increased height and tall objects to view can greatly increase the horizon distance; you can find calculators on the web it if matters.)

Re: Visibility rules
 

So, in raw, the answer to question #1 would be the distance that makes the perception +10 of an average man drop to 3. In this case, -17, or around 1,500 yards.

If by any chance you remember of any of these links or papers, I would really appreciate. Maybe it is my Research Skill fault, but I could not find anything in Google.

Edited for correcting:

Re: Visibility rules
 

If you want to see them, yes. It's more complex for recognizing them. On one hand, once you've spotted them, discerning their features takes a +20 bonus (see GURPS Powers: Enhanced Senses). But on the other, you have to discern features that are a lot smaller than an SM 0 human!

Re: Visibility rules
 

Actually that was Anthony's comment. But if it helps:

Basics of squad/platoon level infantry operations are in US Army Field Manual 7-8. They have a few diagrams of typical spacing, though the days of drilling to precise regulation distances are long gone. For other tasks, you might try searching for "search theory" and "search and rescue". Various organizations have plans for categories of search, depending what you're looking for, all the way down to those searches you've seen on cop shows, where llines of people a yard apart are sweeping a field and looking for evidence.

Re: Visibility rules
 

If we consider that a character knows well someone, a GM could let the player use proportions of parts of the body to spot him or her. Like the head, thigh or forearms measures crossed against each other. That would give a -6, and as a rule, an average person would recognize someone he knows at 150 yards.

http://www.telegraph.co.uk/news/scie...s-no-help.html

To tell one face from the other, a person would have to compare the lenght of facial features with one another (2 or 3 inch). That would give like -10, or, 30 yards.

That would be a reasonable answer for #2.

Re: Visibility rules
 

Let's see. If you have +20 for seeing details of an object you've already spotted, then making out a single human-sized object in something you'd seen as a larger mass would have SM 0, and range +20, which is 5,000 yards or about 3 miles. The -6 for relative size of body parts gets you to 500 yards.That's with 20/20 vision, of course.

Re: Visibility rules
 

Realistically, human 20:20 vision is a resolution of about 1"/100 yards, so 200-300 yards.

Re: Visibility rules
 

A letter on the Snellen chart is five arcmin, and the single lines and spaces are 1 arcmin. A circle with radius 100 yards has circumference 1885'; one degree is about 5', and one minute then is 1". At 500 yards, you'd be able to see body parts that were 5" across, which is about the right order of magnitude for head, arms, or legs; since SM -6 corresponds to 7", it's roughly correct. You'd need to be significantly closer to recognize facial features, of course.