SC2Mapster

@SCMapper:

Since I use a top view camera that simply follow the unit, I use basic angles.

D = 0
W = 90
A = 180
S = 360

From a world perspective, pressing W would make the unit move toward the north, pressing D would make the unit move toward the east, etc.
I prefer this over a "turn unit" with A&D and move forward with W but I guess its based on tastes.

Are you using the default camera, a top view or a "behind the unit" view?
What is the value of Run_Angle[1]?

If you're using the default camera or a top view, I would suggest using a fix angle (D == 0 degrees, W == 90 degrees, A == 180 and so on).

There is various ways to do this.

Add a custom/copy of a siege attack to your unit.
Create a dummy unit.
Create/copy a custom ability with an AoE cursor (like EMP).
Make a trigger similar to this :

AttackGround
    Events
        Unit - CustomUnit is issued an order to Ghost - EMP Round
        ------- EMP Round is used here because my custom ability would based on EMP and would have the command EMP Round to be used.
    Local Variables
    Conditions
    Actions
         ------- The Dummy is a unit that can't be seen/selected in game and doesn't have any model or sounds.
       Unit - Create 1 Dummy for player 0 at (Target point for (Triggering order)) facing (Position of (Triggering unit)) (Ignore Placement)
       ------- The dummy should be created at the center of where your ability has been ordered ("casted").
        Unit - Add 1 Infestor - Infested Terran Timed Life to (Last created unit) from 1
         ------- This trigger add a timed life to the dummy unit, this way you won't have multiple dummy unit standing around.
        Unit - Order CustomUnit to ( Attack (Last created unit)) (Replace Existing Orders)
         ------- This order the unit to attack the invisible dummy creating a smiliar effect to shooting the ground.

I used something similar once and worked perfectly.
NB: Using multiple weapons might get weird results.