Relative position of area of effect spells compared to the casting reference frame

[noob]

Let us say that we have a magician casting a prismatic wall on earth this prismatic wall is not supposed to move but it will be static compared to what?
We can say that the earth is moving relatively to anything you want but it is not like if you chose while casting the spell the reference frame(else I would choose a referential frame making my wall violently go everywhere relatively to earth and who would destroy it in two seconds and the wall would be static for my curiously chosen reference frame)
Now I ride a boat is it different?
And if the earth accelerate will my wall accelerate the same way(for example I make a prismatic wall who last one year the earth acceleration would be felt a lot if my wall did not accelerated in the same fashion as my wall would be probably in space following a different trajectory)?
I would like to know clear rules for how the referential frame is chosen for spells.

[Grooke]

I think you just killed a catgirl.

More seriously though, I see the problem but I'm really not sure if there is a clear-cut answer.

I'd try to go along with "whatever seems reasonable at the time" to make sure the wall stays "immobile". It would definitely remain Geo-stationary, probably follow a boat (though even there, casting it on a raft and sending it out to destroy stuff is abusive), but certainly wouldn't allow it to be cast on a piece of cardboard that can be chucked at unsuspecting enemies.

Edit: if I had to create a clear-cut houserule, I'd probably look into letting be anchored (and therefore immobile with regards to) anything X times larger than the wall itself (within range, LoS, and LoE).

[The Evil DM]

Let us say that we have a magician casting a prismatic wall on earth this prismatic wall is not supposed to move but it will be static compared to what?
We can say that the earth is moving relatively to anything you want but it is not like if you chose while casting the spell the reference frame(else I would choose a referential frame making my wall violently go everywhere relatively to earth and who would destroy it in two seconds and the wall would be static for my curiously chosen reference frame)
Now I ride a boat is it different?
And if the earth accelerate will my wall accelerate the same way(for example I make a prismatic wall who last one year the earth acceleration would be felt a lot if my wall did not accelerated in the same fashion as my wall would be probably in space following a different trajectory)?
I would like to know clear rules for how the referential frame is chosen for spells.

Reference frame is completely undefined in D&D and is something the GM needs to decide.

Personally I consider the planet or extra planar space to be the reference frame but I do not allow stationary effects to move with boats.

Two reasons,

1) It can create some instances where effect has application based definitions. For instance - I can use a wall of force to make a barrier in front of a ship that it crashes into or can I use it in the ship to create a barrier between spaces that moves with the ship. In a purely vancian system I want the effect to be singular. It also entices players to find or search for objects, components or alternative spells to do what they want.

2) It prevent the abuse mentioned by second poster about putting prismatic sphere on a raft.

[jiriku]

For a typical prime material plane, D&D assumes that the ground is not moving and spells use the reference frame of the stationary earth. Note that for many game worlds, the "earth" is in fact stationary, and is not rotating or translating through space. If the caster casts from a moving object, such as the deck of a ship, stationary spell effects that are attached to the moving object (such as a summoned monster standing on the deck of the ship) will move with the object. Stationary spell effects that are not attached to the moving object (such as a fog cloud will remain stationary with respect to the earth, and the ship may well sail through them and leave them behind (reference Stormwrack pp.29-30). Velocity and momentum do not exist in D&D in ways that you would recognize, so there is no concern about a monster falling over if it is summoned onto a rapidly moving object.

The above is valid for a generic prime material plane only. On other planes of existence, YMMV.

[Grooke]

1) It can create some instances where effect has application based definitions. For instance - I can use a wall of force to make a barrier in front of a ship that it crashes into or can I use it in the ship to create a barrier between spaces that moves with the ship. In a purely vancian system I want the effect to be singular. It also entices players to find or search for objects, components or alternative spells to do what they want. .

There is a flipside though. If it doesn't move with the ship, imagine fighting in the hull with this configuration:


-------Ship direction -------->
___________________________________
| Enemy Wall Caster
|__________________________________


The wall will "advance" on the enemy, crushing them against the back of the room, then bursting through the ship.

[The Evil DM]

There is a flipside though. If it doesn't move with the ship, imagine fighting in the hull with this configuration:


-------Ship direction -------->
___________________________________
| Enemy Wall Caster
|__________________________________


The wall will "advance" on the enemy, crushing them against the back of the room, then bursting through the ship.

Which is exactly how it works in my game. Walls of force on ship to ship combat get interesting.

[The Evil DM]

For a typical prime material plane, D&D assumes that the ground is not moving and spells use the reference frame of the stationary earth. Note that for many game worlds, the "earth" is in fact stationary, and is not rotating or translating through space. If the caster casts from a moving object, such as the deck of a ship, stationary spell effects that are attached to the moving object (such as a summoned monster standing on the deck of the ship) will move with the object. Stationary spell effects that are not attached to the moving object (such as a fog cloud will remain stationary with respect to the earth, and the ship may well sail through them and leave them behind (reference Stormwrack pp.29-30). Velocity and momentum do not exist in D&D in ways that you would recognize, so there is no concern about a monster falling over if it is summoned onto a rapidly moving object.

The above is valid for a generic prime material plane only. On other planes of existence, YMMV.

The following is my ruling any my ruling only

For summoning, teleportation and other effects, the magic is instantaneous and the object is just placed on the surface. At any given instant - any surface is stationary to all others. A spell that creates matter - such as a fog cloud - could create that matter inside a ship and then the matter will act as normal matter just like anything else.

But if the magic is a continuous stationary energy effect the energy must connect to the energy field magic comes from. This field is stationary relative to the "Planet" "Plane" or what ever the ground reference frame is.

So - Walls of Fire, Force, Prismatic Walls, and other energy type effects don't move.

But a wall of stone - permanent after casting - tossed onto a deck of a ship will move with that ship as long as it doesn't sink it first.

[Grooke]

In a purely vancian system I want the effect to be singular.

Which is exactly how it works in my game. Walls of force on ship to ship combat get interesting.

This again becomes an issue of casting reference. The effect is singular with regards to the plane, but neither strategically, nor from a battlefield point of view.

Just to clarify, I'm not bashing your system and find it perfectly acceptable and interesting.

[The Evil DM]

This again becomes an issue of casting reference. The effect is singular with regards to the plane, but neither strategically, nor from a battlefield point of view.

Just to clarify, I'm not bashing your system and find it perfectly acceptable and interesting.

That is what I meant by effect singular.

It is either stationary or mobile relative to the ground plane reference frame.

Tactically or strategically relies on how you use this effect versus an other opponent that may have many options to deal with this new piece of information. A stationary opponent firing ranged weapons may have to move to circumvent a wall of force. A mobile opponent my have to (or be unable to) change direction to avoid the wall of force. In either case - its effect is a stationary invisible indestructible wall.

[Telok]

But if the magic is a continuous stationary energy effect the energy must connect to the energy field magic comes from. This field is stationary relative to the "Planet" "Plane" or what ever the ground reference frame is.

I used this sort of thing with the Force Ladder spell to sweep the decks of a ship once. Well, the fore deck. Force Ladder isn't unstoppable and the spell ended when the mast hit it.