Accuracy of VB calculations (again...)

[MeasureMan]

I need to calculate the angle subtended between the start and end point of a spiral and the entry tangent of the spiral. The spiral starts as a straight line and ends with a known radius.

As an example, you can use a spiral with a length of 156.025, and a spiral parameter of 395 (the radius can be calculated: Radius = Parameter^2/Length).

I can calculate an almost correct answer with the following code:

Dim dTheta As Double           'Exit tangent of clothoide
Dim dAlpha As Double           'Starting tangent
Dim dX As Double               'Opposite side of triangle formed by Start/End points and entry tangent
Dim dY As Double               'Adjacent side of triangle formed by Start/End points and entry tangent
Dim dRadius As Double          'Clothoide radius
 
'Calculate end radius of clothoide
dRadius = dParameter ^ 2 / dLength
 
'Calculate end tangent angle
dTheta = dLength / (2 * dRadius)
 
'Calculate sides of triangle
dX = dLength - (dLength ^ 5 / (10 * ((2 * dLength * dRadius) ^ 2))) _
             + (dLength ^ 9 / (216 * ((2 * dLength * dRadius) ^ 4))) _
             - (dLength ^ 13 / (9360 * ((2 * dLength * dRadius) ^ 6))) _
             + (dLength ^ 17 / (685440 * ((2 * dLength * dRadius) ^ 8)))
 
dY = (dLength ^ 3 / (3 * (2 * dLength * dRadius))) _
     - (dLength ^ 7 / (42 * ((2 * dLength * dRadius) ^ 3))) _
     + (dLength ^ 11 / (1320 * ((2 * dLength * dRadius) ^ 5))) _
     - (dLength ^ 15 / (75600 * ((2 * dLength * dRadius) ^ 7))) _
     + (dLength ^ 19 / (6894720 * ((2 * dLength * dRadius) ^ 9)))
 
'Calculate missing angle
dAlpha = Atn(dY / dX)

But this solution gives an answer with an accuracy of +/- 0.001 gon and I need one order of magnitude more (ie. +/- 0.0001 gon or 0.0000015 radians).

I have done a search on the forum and tried changing the doubles to variants and using the cdec() function according to various posts but I always get the same answer.

Can anybody see a mistake in the code or advise as to the best way to acheive the required accuracy in the calculation?

Many Thanks

[Evil_Giraffe]

Well, ont that it would really belong in this forum, but have you tried writing your own data type? If you have a data type that holds several longs or doubles, then you can put it in a class and write functions to do all the operations you need.
Make sense?

[RAEsquivelC]

Funny that you say you only get a +/-0.001 precision when using your code. I loaded it in VB6 and got the angle as:
Angle = 2.60028267012505E-02, or, without the *10^-2, is:
Angle = 0.0260028267012505, certainly with precision much, much larger than your reported +/-0.001, I would think. Don't you agree?