is there a better way...?

is there a faster (better) way of doing this?

VB Code:

Private Function GetAngle(X1 As Single, Y1 As Single, X2 As Single, Y2 As Single) As Single
    On Error Resume Next
    Dim XDiff As Long
    Dim YDiff As Long
    Dim Angle As Single
    XDiff = Abs(X1 - X2)
    YDiff = Abs(Y2 - Y1)
    Angle = Atn(XDiff / YDiff)
    Angle = Angle * 180 / Pi
    If Y2 > Y1 Then Angle = 180 - Angle
    If X2 < X1 Then Angle = 0 - Angle
    
    If Angle < 0 Then Angle = 360 + Angle
    
    If YDiff = 0 And X1 - X2 < 0 Then
        Angle = 90
    End If
    
    If YDiff = 0 And X1 - X2 > 0 Then
        Angle = 270
    End If
    
    GetAngle = Angle
End Function

Did you want to get the Reference Angle?

yep

Hi, my function also uses 5 "IF'S" but I arranged them a bit different, faster ?? I don't know. But for sure I don'T have a division by 0 if dy=0 ( in yours YDif=0)!
Sorry for any typos, I had to change some GERMAN Variable-names to ENGLISH.

VB Code:

Public Function Bearing(X1 As Single, Y1 As Single, X2 As Single, Y2 As Single)
'Input X1, Y1, X2, Y2
'Calculates Bearing  from X1,Y1 to X2,Y2 (0-360)
Const PI = 3.141592654
Dim dx As Single
Dim dy As Single
dx = X2 - X1
dy = Y2 - Y1
If dy <> 0 Then
        Bearing = (Atn(dx / dy) * 180 / PI)
        If X2 > X1 Then
                If Y2 > Y1 Then
                        Bearing = Bearing
                Else
                        Bearing = 180 + Bearing
                End If
        Else
                If Y2 > Y1 Then
                        Bearing = 360 + Bearing
                Else
                        Bearing = 180 + Bearing
                End If
        End If
Else
        If X1 > X2 Then
                Bearing = 270
        Else
                Bearing= 90
        End If
End If
End Function

I've made a sample project. It has only one form and one command button. The form code is shown here.
If you convert all parameters and the return value to double floating point values, it will give a performance boost of about 20 %, when run in the VB IDE (see function GetAngle1). The reason for this is that most functions use doubles, instead of singles. So there is less type conversion. Another strange thing in your code is that you dimmed XDiff and YDiff as longs. All decimals will then be cut off.
The best performance gives the last function (GetAngle2), which also uses arctan extensively. The extra code in the arctan2 function is to guarantee that opposite directions will give opposite angles. The extra performance boost between this function and GetAngle1 is not much, but the code is much simpler.
Good luck!

VB Code:

Option Explicit
 
Private Const m_iTimes = 100000
Private Const PI = 3.14159265358979
Private Const HALFPI = 1.5707963267949
 
Private Declare Function GetTickCount Lib "kernel32" () As Long
 
Private Sub Command1_Click()
  Dim l As Long
  Dim ang As Single
  Dim t1 As Long, t2 As Long, t3 As Long, t4 As Long
  
  Randomize Timer
  
  t1 = GetTickCount()
  For l = 1 To m_iTimes
    ang = GetAngle(Rnd, Rnd, Rnd, Rnd)
  Next l
 
  t2 = GetTickCount()
  For l = 1 To m_iTimes
    ang = GetAngle1(Rnd, Rnd, Rnd, Rnd)
  Next l
 
  t3 = GetTickCount()
  For l = 1 To m_iTimes
    ang = GetAngle2(Rnd, Rnd, Rnd, Rnd)
  Next l
 
  t4 = GetTickCount()
  
  MsgBox "First function: " & (t2 - t1) & vbCrLf & _
    "Second function: " & (t3 - t2) & vbCrLf & _
    "Third function: " & (t4 - t3)
 
End Sub
 
 
Private Function GetAngle(X1 As Single, Y1 As Single, X2 As Single, Y2 As Single) As Single
    On Error Resume Next
    Dim XDiff As Long
    Dim YDiff As Long
    Dim Angle As Single
    XDiff = Abs(X1 - X2)
    YDiff = Abs(Y2 - Y1)
    Angle = Atn(XDiff / YDiff)
    Angle = Angle * 180 / PI
    If Y2 > Y1 Then Angle = 180 - Angle
    If X2 < X1 Then Angle = 0 - Angle
    
    If Angle < 0 Then Angle = 360 + Angle
    
    If YDiff = 0 And X1 - X2 < 0 Then
        Angle = 90
    End If
        If YDiff = 0 And X1 - X2 > 0 Then
        Angle = 270
    End If
        GetAngle = Angle
End Function
 
Private Function GetAngle1(X1 As Double, Y1 As Double, X2 As Double, Y2 As Double) As Double
    On Error Resume Next
    Dim XDiff As Double
    Dim YDiff As Double
    Dim Angle As Double
    XDiff = Abs(X1 - X2)
    YDiff = Abs(Y2 - Y1)
    Angle = Atn(XDiff / YDiff)
    Angle = Angle * 180 / PI
    If Y2 > Y1 Then Angle = 180 - Angle
    If X2 < X1 Then Angle = 0 - Angle
    
    If Angle < 0 Then Angle = 360 + Angle
    
    If YDiff = 0 And X1 - X2 < 0 Then
        Angle = 90
    End If
        If YDiff = 0 And X1 - X2 > 0 Then
        Angle = 270
    End If
        GetAngle1 = Angle
End Function
 
Private Function GetAngle2(X1 As Double, Y1 As Double, X2 As Double, Y2 As Double) As Double
  Dim XDiff As Double, YDiff As Double
 
  XDiff = X2 - X1
  YDiff = Y2 - Y1
  
  GetAngle2 = Arctan2(YDiff, XDiff)
  If GetAngle2 < 0 Then GetAngle2 = GetAngle2 + 2 * PI
  GetAngle2 = GetAngle2 * 180 / PI
 
End Function
 
Public Function Arctan2(y As Double, x As Double) As Double
 
    If x = 0 Then
        If y > 0 Then
            Arctan2 = HALFPI
        Else
            Arctan2 = -HALFPI
        End If
    ElseIf x > 0 Then
        Arctan2 = Atn(y / x)
    Else
        If y < 0 Then
            Arctan2 = Atn(y / x) - PI
        Else
            Arctan2 = Atn(y / x) + PI
        End If
    End If
    
End Function

Thanks riis, I think i'll use your code (GetAngle2) instead of mine.
Anybody using GetAngle or GetAngle1 should keep in mind, that the division by Zero is only stepped over by:

VB Code:

On Error Resume Next

Or after testing my function with your "test-code" (using it in VBA-EXCEL) I think I stick to my approach.

GetAngle2 290 vs. Bearing 280
And I didn't change my single declaritions!

Looked at it more closely.
The difference between GetAngle2 and Bearing is the most if the Division by Zero comes into the game.
In my Function, this event is handled earlier, i.e. faster.

The disadvantage of the GetAngle2 function is that it uses an extra function call, i.e. ArcTan2. Of course it is possible to integrate the ArcTan2 function into the GetAngle2 function.
By the way, it looks like your and Cyborg's function has North as 0 degrees and the angle increases clockwise. By using Arctan2 the way I did East is 0 degrees and the angle increases counterclockwise. If anyone is going to work with it, he should keep this in mind!

Yes Riis,
you are using this function in the more "technical" way.
Since I'm using it more for navigational stuff, I need North to be Zero and East to be 90.

You're right, anybody who is using one of this functions should keep that in mind.
I had to learn it the hard way, on work I have to use a simulation-system build by people who think more in the "technical" way. So they are using the Degrees like you (0 is East and counting counterclockwise).
That was no problem as long they didin't show anything on the UserInterface.
But that system is a simulation for Naval Things, so they have to show bearings, and these have to be "understandable" be Navigators.
So far I found about 5 places in the system that they had to change (soetimes more than once), and I'm still looking.............