Dynamic Environment Mapping is a posh name for "making it up as you go along".  Specifically, it means drawing the map as the players explore, rather than drawing the map beforehand.  This is useful because it means you don't have to do so much preparation before a mission.  It's also useful if your players decide to go and do something totally unexpected instead of what you had planned.

Another benefit of this system is that once the players have explored an area, you can keep the map you generated and use it again later.

This system was originally invented for the autocar chase described at the end of this page, but it can be adapted for most exploration - corridors, ventilation ducts, transit tubes, etc.  Much of Alpha Complex is based on a grid pattern, so the same system can be used with minimal modification.

What you will need

• Graph paper (big squares are best)
• Pens/pencils
• D20 die

The first thing you need to do is mark out the boundary of your map.  Any area explored by the players will be of finite size, so you should start by drawing a box to show the outer walls of the area.  This box doesn't have to be a regular shape, it's really up to you.

Having decided on the shape of the area, you should next mark out the start point (where the Troubleshooters enter the area) and any other important places.  If they are trying to navigate through the area, you should mark their exit point.  If they are trying to find something, mark where it is.  You can also mark out some interesting encounters which they may or may not go anywhere near, some corridors, some rooms.  It's really up to you how much or how little you put on the map at this stage.  It's probably a good idea to have a few encounters lined up beforehand.

Having marked these points, draw corridors into each of these features, which will link up with corridors the players discover as they explore.  These lines should only go HALFWAY along the side of a square, since at present they're not linked to anything that has been discovered.

Letting the players loose

Now you're ready to let the players loose.  They'll wander up the corridor and reach an intersection which isn't currently mapped.  Roll a D20 and use the lookup table below to determine what directions the intersection allows:

 

1-5	Forward, left and right
6-9	Forward only
10-12	Forward and left
13-15	Forward and right
16-17	Left and right
18	Left only
19	Right only
20	Dead end

As you get further in to the map you will find that sometimes the die roll indicates an invalid direction (either to an area outside the bounds of the map, or it points towards an intersection which you've already shown doesn't have a corresponding corridor).  In these cases, the result is left to the GM.  As a rule of thumb, ignore the corridor if it heads towards an external wall, and modify the roll if the corridor heads towards an intersection that can't link up.

For example, the dice shows forward and left, but you can't go left because the intersection to the left has already been mapped and has no corresponding corridor. You could modify this to forward and right instead.

Feel free to modify dice rolls if you need to.  Sometimes it's unavoidable because the players are going to run out of corridors.  (A worst-case scenario would be if the first roll of the area was a Dead End).  Sometimes you may have to modify a roll because the map is about to cut off the last remaining way into the target area.

If the players want to know what lies down a corridor, just do a few rolls to work it out.  It's a good idea to decide beforehand how far the players can see in any one direction.  (For example, two intersections while in the sewers).  Draw what you've rolled on the map, but don't forget where the players are (since they've only looked down the corridor, not actually walked down there).

Confused?

	This is the map to start with.  The thick black line around the outside represents the boundary of the map.  The blue line at the bottom represents the corridor where the players will enter the area.  The red marker represents the target the players are aiming for and the two blue lines protruding from it are two corridors.
	The GM rolls a 10, which means the corridors go forward and left.  These two corridors are drawn on the map, but only halfway towards the next intersection (since they currently don't connect to anything).
	The players continue going north.  The GM rolls a 12 (forward and left) followed by a 20 (dead end).  The players decide they want to backtrack and take the upper west-facing corridor.
	Now the GM rolls a 16 (left and right).  Remember that the players are currently facing west, so the resulting corridor runs north/south.  At this point the corridor heading south has not been linked up with the lower one.  That's because the GM hasn't yet determined whether the next intersection southwards also has an exit to the west.  (If that intersection was in the corner you could draw it in now, since there wouldn't be any other ways out).
	The players decide to head north again.  The GM rolls a 13 (forward and right).  But according to his map the corridor can't go right, because there's a dead end there.  So instead he modifies the roll into forward and left.
	This is an example of an instance where you would want to modify a dice roll.  The players have reached the position indicated by the yellow marker, and the GM rolled a 19 (right only).  This would leave them with no way of reaching the target (since the map has determined that the west-facing corridor from the target is a dead end).

Using Dynamic Mapping

Autocar chase

The Troubleshooters have to catch some clones in an autocar.  The PC's car is faster than the NPC's car, but only just.  The only way the NPCs can hope to lose the players is by driving at speed around the streets of Alpha Complex.

Reroll any Dead End rolls (since you want to keep the pace fast).  Make Autocar Op rolls whenever the NPCs or players have to make a sharp turn - these become harder for the PC driver as they get closer to the NPCs (but not for the NPC driver, since he's the one who intiates the turn).  If the driver fails the roll slightly, he loses a bit of distance (and perhaps damages the autocar too).  If the driver fails the roll badly, either the autocar crashes or he misses the turn.

In the end they'll either catch up with the NPCs and ram them off the road, or crash their own autocar, or the NPCs will get far enough in front that they lose the players.

Or the sewage system.  Limited visibilty and space.  Ask the players what order they are marching in.  Ask them this a lot - let them all try to be the one in the middle.  ("I've seen that vidtape - it's always the one at the back who gets picked off first, so the team doesn't know they're under attack").  Who knows what might be lurking in all those pipes?  You can turn this into a classic dungeon crawl if you so wish.

Issue the Troubleshooters with a tracking device.  This can either tell them the distance to the target, or the direction of the target.  (Telling them both would make things a bit too easy).  The target can be anything you like.  Of course there's no guarantee that the unit works, or the target is stationary, or the readout is of any use (eg. distances measured in hundredths of a furlong), or that it's even the right target.