I'm seeking some advice on using Spot and Listen in my campaign.
In the players handbook it says to apply a -1 modifier on a spot check for each 10 feet of distance between two groups.
I've got long discussions of this elsewhere, but in short, "No, don't do that".
The DC of spotting something increases linearly not with a linear increase in distance but with something closer to each doubling of the distance. (It's actually closer to every 30% increase in distance is another +1, but anyway, the point is that a table is required if you want both reasonable versimilitude and ease of play.).
So if 10 feet implies -1, then 20 feet imples -2, but 40 feet (and not 30 feet) implies something more like -3 and 80 feet (and not 40 feet) implies something more like -4. (Those aren't the actual values I use, but you get the idea.)
If you don't do this, mountains will be impossible to spot. That sounds like a ridiculous case, but it becomes absolutely essential if you run combat at longer than normal dungeon distances (even just at 300').
Listen on the other hand doesn't have any modifiers listed for distances. At what distance would you start applying modifiers for listen checks? -1 for every 10 ft? Or would the distance be greater? In my mind you could potentially hear something easier than you could spot it at a great distance.
On my table, the difficulty in hearing something at range is twice that of seeing at range. So you have the same penalty at 50' to hear as you would to see at 100'. This is because human vision is much more acute than hearing, and light travels more freely than sound. On the other hand, hearing is much less effected by available concealment than sight.
In my game, the DC of spotting something that doesn't have cover and isn't actively hiding is -5 (DC 0 if you are distracted). Since this roll is made passively every few seconds, in general if vision is unobstructed and you aren't near sighted, you'll typically see it when it is still VERY far away. In fact, the DC of spotting a medium sized creature that is moving is so low, that typically you will be limited by natural obstructions well before you are limited by anything else. If you are out on a flat plain, you'll see any medium sized creature that isn't trying to hide when it is still miles away. You probably won't be able to make out in details at that distance, but you'll see it.
For example, on my table the DC of seeing something that is 2200' away, moving, and not concealed is only 7 if you are alert and only 12 if you are distracted (by a conversation or the like). It's more than possible to see someone miles away under these conditions, especially if you are sharp eyed (or heroically sharpeyed like Legolas in LotR, keeping in mind that 'Gandalf is a 6th level Wizard').
