use of function cdiff

[mario frangipane]

Dear Grads Users,
I do not understand the example on using the cdiff function at page 76 of the Grads manual.
For example, if (u,v) is the wind, to calculate du/dy I believe that the calculation should be:
define du=cdiff(u,y)define dy=cdiff(lat,y)*(3.1416/180)*(6.37e6)  (in meters)define dudy=du/dy
Instead, in the Manual, at pag. 76, I find (example 1):
define du=cdiff(u*cos(lat*3.1416/180),y).
Anyone can help me to understand?




      
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://gradsusr.org/pipermail/gradsusr/attachments/20100201/91003f44/attachment.html