I'm running a game in which the party goes to the plane of Smoke, so I figured I should write up something for travelling in infinite and mostly undifferentiated spaces. Some of this stuff may work for the Outer planes as well, but many have their own special means of locomotion.
So here it is:
Travel on Infinite Planes (with a focus on the Inner planes)
Even though there are quantitative gradients of temperature, pressure, etc in infinite planes, these do not map onto physical dimensions. Rather, any well-defined region occupies a point in the infinite expanse of the plane and its borders are defined by the range of perception of people travelling through it. Once travellers leave the borders of a fixed region, they may make progress 'towards' any of the extremes or to the center at a rate defined by the conflict created in their minds by observing rapid changes of the properties of the plane. That is, someone who is well-versed with the plane may generate changes of an atmosphere of pressure, about 20K, a few hours worth of lighting, a significant change in humidity, etc every hour of travel. Someone new to the plane might navigate the same variations within a day or even a week. In general, those with higher perception have problems with travel as they notice the structure of a location. Of the elemental planes, the traverse difficulty is ordered Earth> Air=Fire > Water, as Air and Fire both have some structure but far less than Earth and far more than Water. Someone aware of the nuances of travelling an infinite plane can act to limit their perception to improve travel time - by wearing shaded goggles or rubbing their skin with an insulating layer of fat to prevent perception of temperature variations. Someone totally senseless cannot travel however as they need to subconsciously direct the changes in their environment - someone with no sense of balance and no subjective up or down cannot travel to areas associated with significantly different values of those coordinates (if we say that 'down' is towards Void and there is a location associated with the more Void-like areas of the Air...).
The mapping of Air (for sake of a concrete example) can only be done in a coarse fashion, associating each location with a coordinate in the +/-, Fire/Water, Void/Antivoid directions. Travel to such a location is best done by moving towards the conditions marked by that coordinate (temperature, pressure, etc) and then having some signature of the location (a physical object from the place) or a mental image. Homing in on the location will take a week for a vague hint of its nature, a few days from a memory of the place, a day if there is someone in the group very familiar with it, and down to an hour or so if that is combined with an object from the location.
If the place being travelled to is completely unfamiliar, there is a chance
of ending up elsewhere within the same environmental conditions. This chance depends on the plane:
Air 80%
Smoke 80%
Fire 80%
Magma 70%
Earth 50%
Ooze 80%
Water 90%
Ice 70%
Extra pieces of information that do not specify an image but rather environmental conditions lower the chance by 10% each (for instance, if something is known about the weather patterns of the area, the presence of thermals, the detailed composition of the air, etc)
Inclusion of people unfamiliar to the target location will adjust the 'travel score' in the following way.
Define S_i to be the travel score of the ith person, determined as follows:
S, Description
1,Totally clueless to the destination.
2,Non-sensory information (a name, a story about the place, etc)
5,Sensory description (it's a city made of glass...)
10,An image or object from the target place
20,A memory of the place, having been there once
50,Familiarity with the place (been there several times)
100,Native to the destination (city he/she grew up in)
+2, Perceptual details of the environment of the target location.
Each person contributes to the travel according to a perceptual index P_i.
P,Description
0, Unconscious. Inert. Inanimate. Total sensory deprivation
1, Only has a tertiary sense (temperature, feeling of static in the air, etc)
2, Only has secondary senses and below (sound, smells, indistinct colors)
3, Only has a primary sense (sight, but no sound)
4, Has all senses
+1,For making a DC 20 Listen check (if can hear)
+1,For making a DC 20 Spot check (if can see)
+1,Any preternatural sense that is in line with the plane.
The net S value of the party is: S = sum { P_i S_i } / sum { P_i }
The net P value of the party is: P = sum { P_i } / sum { 1 }
If the net P value is zero travel cannot occur.
If the net S value is less than 10 then there is a chance of ending up in the wrong place. Roll percentile dice as described earlier to figure out if the
party ends someplace strange.
The travel time to home in is:
S,Time
1-2, 1d4 weeks
3-5, 1 week
6-10, 1d6 days
11-15, 1d4 days
16-20, 1 day
21-30, 3d6 hours
31-40, 2d6 hours
40-50, 1d6 hours
50+, 1 hour
Choosing to travel to a random location takes 3d6 hours and
automatically trips the 'wrong place' criterion.
If there is some anchor to the region which people are leaving, that will prevent such travel. Each location in an infinite plane cannot be at more than a point, though it is quite possible to have points where the elements of neighboring planes meet (even though this meeting region is a geometric plane, its still a zero-volume structure though also infinite in extent)
This means that someone can create trapping regions which have extremely strong and hard-to-ignore signals. If someone wanders in by accident they may be unable to leave due to a loud piercing sound that travels for miles, a giant glowing light, or even a stench that carries for a long distance. If these signals can be blocked, then the region can be escaped. Or one can simply travel 'away' until the distance is enough to attenuate the signal. Of course, a 100 mile long glowing web of light might require 100 miles of travel to escape. One never travels to a point deep within the context of a region so there is some chance to detect the danger. However, a temporarily blinded man, say from Radiance, might end up travelling to the center of a visually rich area in the Air straight off, as he is not using sight to travel. This can pose some dangers... (emerging into the midst of some battle because you're blind and deaf at the time...)
Accidental encounters on infinite planes are almost impossible. But one can choose to 'seek other people' or 'seek a Djinn' and so on. The S values are calculated in a similar way, except that instead of sensory information about a place its sensory information about the desired type of target. If a specific person or group of people is sought, they can be found if they are within the same region of the plane. Scrying on the target gives a +20 bonus to the resultant S values, but if false information is obtained it will direct the seeker to the closest location to the false place. By performing general seeking, creatures on the various elemental planes can hunt for prey, and some forms of prey are simply environmental (i.e. they're part of the plane's nature and thus present at a non-zero density) - this applies to, for instance,
the prey of the Sootbeast on the plane of Smoke.
Some Guvners are involved in experiments to find out if the Inner planes are really infinite in extent or just really really big with a strange form of locomotion. One chosen method is to carry a long spool of wire and march off into the Air laying wire behind them. They take regular data on temperature, pressure, positive or negative elemental bias and try to find if there's any regular deviation from the original spot. Thus far, large fluctuations have been measured in local quantities - up to 10 or 20 K and a half atmosphere of pressure up or down but the fluctuations have always reversed at some point, averaging to a consistent value between groups that have gone off in different directions.