From which axes does start to read the angle (.nc file from)

[perezro]

Helloo!!!

When My supervisor try to use the .nc file I got...and he ask me from which position the angle of each cell is readed... I didn't know what to answer...

I mean the axe 0->x (hotizontal=east) is equivalent to 0 degree and 0->y (vertical=north) 90 degrees...or there is other convention.. like start reading the angles of the cell distorsion from the left (0 -> -x) axe...

Thank you: r

[kate]

The grid angle is the angle between the positive i direction and due east. A grid aligned with i increasing eastward will have a zero angle. My Northeast Pacific grid is tilted so that positive i is heading ENE at the southern end, pointing nearly northward at the northern end. The value of angle will be nearly 90 degrees (pi/2) at the 'northern' end (which is actually facing west). Perhaps a picture would help:

[perezro]

Thanks Kate...
Even if the pic is not visible I believe I got it...
RRR

[wilkin]

How to interpret the sense of the rotation is sometimes confusing. i.e. do you rotate a vector in roms coordinates by *angle* to place it in earth coordinates for plotting, or rotate data in earth coordinates by *angle* for input?

I sometimes add an "equation" attribute to angle in my grid file to remind myself:

angle:long_name = "angle ROMS xi axis is rotated anticlockwise from due east" ;
angle:equation = "u(roms)=Real((Ueast+i*Vnorth)*exp(-i*angle)) and v(roms)=Imag((Ueast+i*Vnorth)*exp(-i*angle))" ;
angle:units = "radian" ;

[perezro]

ok!! really good idea...
Thank you Wilkin: R