Plotting trend

[Arindam Chakraborty]

dear Chuck,
        both the methods should produce the same result.
In fact, m of y = mx + c is equal to the returned parameter
(coeff) of tregr(). The last example in the tregr()
documentation shows how to use the coeff to plot the
linear trend.

regards,

arindam
----------------------------
 ARINDAM CHAKRABORTY
 Department of Meteorology
 Florida State University
 Tallahassee, FL-32306, USA
 Tel: +001-850-6443524 (Off)
      +001-850-5758550 (Res)
 Fax: +001-850-6449642
____________________________

On Tue, 28 Jun 2005, Charles Seman wrote:

> Hi Everyone,
>
> I'm confused and have a question... how does what Arindam sent below
> differ from the GrADS "tregr" function?
> http://grads.iges.org/grads/gadoc/gradfunctregr.html
>
> Thanks for all your help in advance,
> Chuck
>
> Arindam Chakraborty wrote:
>
> >hi Shailendra,
> >        to plot a linear trend, you need to compute the slop and
> >y-intercept of the time series. A least square fit can be done
> >to obtain those two parameters. Here is an example to calculate
> >those and plot the linear trend.
> >
> > 'open file'
> > 'set lon 88'
> > 'set lat 0'
> > 'set t 1 24' * assuming this is the time limit
> > 'define yvar = aave(...)'
> >
> > 'd yvar'
> >
> >* make a variable 1, 2, ..., 24 to calculate m and c of y = mx + c
> > 'xvar = yvar*0 + 1'
> > 'xvar = const(xvar,1,-u)'
> > 'xvar = 25 - sum(xvar,t+0,t=24)'
> >
> > 'set t 1'
> >
> > 'define p = sum(xvar,t=1,t=24)'
> > 'define q = sum(yvar,t=1,t=24)'
> > 'define r = sum(xvar*yvar,t=1,t=24)'
> > 'define s = sum(xvar*xvar,t=1,t=24)'
> >
> > 'define rn = ave(xvar*yvar,t=1,t=24)'
> > 'define n = r/rn'
> >
> > 'define amp =  (n*r - p*q)/(n*s - p*p)' * this is m
> > 'define ave = (q - amp*p)/n' * this is c
> >
> > 'set t 1 24'
> > 'd amp*xvar + ave'
> >
> >
> >
> >Hope this helps,
> >----------------------------
> > ARINDAM CHAKRABORTY
> > Department of Meteorology
> > Florida State University
> > Tallahassee, FL-32306, USA
> > Tel: +001-850-6443524 (Off)
> >      +001-850-5758550 (Res)
> > Fax: +001-850-6449642
> >____________________________
> >
> >
> >On Sat, 25 Jun 2005, Shailendra Rai wrote:
> >
> >
> >
> >>Hello,
> >>
> >>
> >I have plotted a time series of area averaged variable variable with time. Is their any way in gradsc to plot the trend as a straight line ???
> >
> >With regards
> >Shailendra Rai
> >K. Banerjee Centre of Atmospheric and Ocean Studies
> >University of Allahabad
> >Allahabad
> >INDIA
> >
> >
>
> --
>
> ********************************************************************
>  Charles Seman                                charles.seman at noaa.gov
>  U.S. Department of Commerce / NOAA / OAR
>  Geophysical Fluid Dynamics Laboratory         voice: (609) 452-6547
>  201 Forrestal Road                              fax: (609) 987-5063
>  Princeton, NJ  08540-6649            http://www.gfdl.noaa.gov/~cjs/
> ********************************************************************
>
> "The contents of this message are mine personally and do not reflect
> any position of the Government or NOAA."
>
>