Calculate Direction from U,V

Hi
Personally, I have no idea what you are talking about. U, v are just letters so you'll have to explain some more.
If you don't know what formula to use, how can an Excel forum be of help ? Perhaps first find the formula describing this physical property, we will be glad to translate it to Excel

Like pepe, I have no idea what this calc would look like If you were doint this by hand, what would the forumula look like?

It sounds to me like a basic "how to convert rectangular to polar coordinates" type problem. (basic tutorial with a calculator here http://www.analyzemath.com/Calculators/Rect_Polar.html).

You are correct that this usually involves the arctan function (=ATAN() and ATAN2() functions in Excel see help file here http://office.microsoft.com/en-us/ex...in=HP010342680). When you say you " haven't had any success." in what specific ways has it failed? Have you gotten error values back? Incorrect values? Excel's trig functions operate on angles in radians, are you looking for results in degrees and did not realize that Excel used radians? Perhaps if you can explain to us what you have done that has not worked, we can better help.

On edit: Another thing to consider - by convention, the arctan function usually returns angles in the 1st and 4th quadrant (-pi/2 to +pi/2). Your directions can probably cover the entire circle, so you may need an additional step in the algorithm if the desired angle is in the 2nd and 3rd quadrants.

What value is it returning for a given V,U and what value do you expect?

Brief trig lesson for today. Recall from the very beginning of trigonometry (probably even studied in geometry) the concept of the unit circle, from which we derived the trig functions. Recall that all angles were measured from the positive x axis. positive angles were CCW from the x axis, negative angles are measured in a CW direction from the positive x axis.

So the first step in understanding/developing/debugging this is to identify the direction of our unit vectors. According to Wikipedia, U is east-west and V is north-south. Is positive V due north or south? Is positive U due west or east?

Remember that angles from the trig functions are increasing CCW. Compass directions, of course, are just the opposite. Compass directions start with due north being 0 and increase CW from there. Once we know which direction U and V are, then we can look at the result and see if we need to apply a transformation to the result to get the trig angle in radians converted to a global direction in degrees.

So, we are looking at a polar/rectangular coordinate system with the following characteristics and analysis:

1) we are treating V as X and u as Y.
2) +V is due south, +U is due west.
3) Constructing a unit circle in this system yields the following analysis. Recognizing that the ATAN2() function returns values between pi and -pi (explained in the help file linked to above), we can see that
a) 0 is due south (180 compass)
b) pi/2 (90) is due west (270 compass)
c) pi (180) is due north (360 compass)
d) -pi/2 (-90) is due east (90 compass)
e) -pi (-180) is due north (0 compass)

From this, it looks like a function of compass direction=180 +degrees(atan2(v,u)) should return the correct compass direction for a given V,U. If this isn't obvious, put the atan2() outputs and the corresponding compass points into a table and regress a straight line for them.

The only hiccup I see is that the atan2() function will probably prefer to return +pi rather than -pi for due north (v<0, u=0), which means the function will prefer to return 360 rather than 0 for north. An IF function should be able to test and correct this condition if it is a real problem.