Visual basic,atn2 converting to compass bearing

Question

Since atan2 returns values in the range -π ... +π (that is, -180° ... +180°), to normalise the result to a compass bearing (in the range 0° ... 360°, with -ve values transformed into the range 180° ... 360°), convert to degrees and then use (θ+360) % 360, where % is modulo.

Above para i found through the net.i'm using VB6 and the my codes are like this

brng=(θ+360) Mod 360

(where 0= - 68) I'm getting the answer 292°,but expected answer is 248°.

am i committing a mistake..? or am I missing something.? Please help me.

Update:

I'll further explain my question,

Dim b As Double
Dim x As Double 
Dim y As Double 
Dim Dlat As Double 
Dim DLon As Double

Private Const pi As Double = 3.14159265358979

Public Function Atn2(ByVal y As Double, ByVal x As Double) As Double
    On Error GoTo DivideError
    Atn2 = Atn(y / x)
    If (x < 0) Then
        If (y < 0) Then Atn2 = Atn2 - vbPI Else Atn2 = Atn2 + vbPI
    End If
    Exit Function

DivideError:
    If Abs(y) > Abs(x) Then   'Must be an overflow
        If y > 0 Then Atn2 = vbPI / 2 Else Atn2 = -vbPI / 2
    Else
        Atn2 = 0   'Must be an underflow
    End If
    Resume Next
End Function


Public Sub AFAMP()
    lat1 = Val(Text1.Text) * pi / 180 'conveting to radians
    lat2 = Val(Text2.Text) * pi / 180
    Long1 = Val(Text3.Text) * pi / 180
    Long2 = Val(Text4.Text) * pi / 180
    Dlat = (lat1 - lat2)
    DLon = (Long1 - Long2)
    y = Math.Sin(DLon) * Math.Cos(lat2)
    x = Math.Cos(lat1) * Math.Sin(lat2) - Math.Sin(lat1) * Math.Cos(lat2) * Math.Cos(DLon)
    b = Atn2(y, x) * (180 / pi)
    Text5.Text = (b +360) mod 360
End Sub

coordinates are as below in decimal degrees

lat1 = Val(7.337361) * pi / 180 

lat2 = Val(7.000667) * pi / 180

Long1 = Val(81.626198) * pi / 180

Long2 = Val(80.773737) * pi / 180

I have checked with an online coordinate calculator, the answer is 248. In my case it is 292 degrees. Can anyone please help me? I'm stuck.

Answer 1

As Deanna said in a comment, 292 is the correct answer. -68 + 360 = 292

Answer 2

This code is JavaScript because I can't remember VB syntax.

Convert to degrees. Don't repeat this code, make it a method on Number. Or a first class function with two parameters, whatever your target language supports.

  Number.prototype.toDegrees = function (unit) {
    const u = (unit || "d").toLowerCase().charAt(0);
    switch (u) {
      case 'r': //radians
        return this * 180 / Math.PI;
      case 'g': //grads
        return this * 200 / Math.PI;
      case 'd': //degrees
        return this;
      default: //radians
        return this * 180 / Math.PI;
    }
  };

Normalise the range.

Number.prototype.toNormalisedDegrees = function () {
  return (this - 360 * (Math.trunc(this / 360) - 1)) % 360;
}

Rotate 90 degrees and reverse the direction.

  Number.prototype.toCompassBearing = function (unit) {
    const degrees = unit && unit.toLowerCase().charAt(0) === "r" ? this.toDegrees() : this;
    return (90 - degrees).toNormalisedDegrees();
  }

Test with some known values

Number.prototype.toDegrees = function (unit) {
  const u = (unit || "d").toLowerCase().charAt(0);
  switch (u) {
case 'r':
  return this * 180 / Math.PI;
case 'g':
  return this * 200 / Math.PI;
default:
  return this;
  }
};
Number.prototype.toNormalisedDegrees = function () {
  return (this - 360 * (Math.trunc(this / 360) - 1)) % 360;
}
Number.prototype.toCompassBearing = function (unit) {
  return (90 - this.toDegrees(unit)).toNormalisedDegrees();
}

console.log(`0 cartesian (E) => ${(0).toCompassBearing("d")} compass bearing`);
console.log(`90 cartesian (N) => ${(90).toCompassBearing("d")} compass bearing`);
console.log(`180 cartesian (W) => ${(180).toCompassBearing("d")} compass bearing`);
console.log(`270 cartesian (S) => ${(270).toCompassBearing("d")} compass bearing`);
console.log(`0 cartesian (E) => ${(0).toCompassBearing("r")} compass bearing`);
console.log(`PI/2 cartesian (N) => ${(0.5 * Math.PI).toCompassBearing("r")} compass bearing`);
console.log(`PI cartesian (W) => ${Math.PI.toCompassBearing("r")} compass bearing`);
console.log(`1.5 PI cartesian (S) => ${(1.5 * Math.PI).toCompassBearing("r")} compass bearing`);