# (45) Trend modeling

In this example show how the module trend1d is used to fit the CO2 data set collected from the top of Mauna Loa. This yields the famous Keeling curve.

    #!/usr/bin/env bash
    #               GMT EXAMPLE 45
    #
    # Purpose:      Illustrate use of trend1d mixed models
    # GMT modules:  text, plot, trend1d
    # Unix progs:   rm
    #

    gmt begin ex45
    	gmt set FONT_HEADING 24p
    	gmt subplot begin 4x1 -Fs15c/4.8c -R1958/2016/310/410 -Bxaf -Byaf+u" ppm" -BWSne+gazure1 -T"The Keeling Curve [CO@-2@- at Mauna Loa]"

    		# Basic LS line y = a + bx
    		gmt subplot set 3
    		gmt trend1d -Fxm @MaunaLoa_CO2.txt -Np1 > model.txt
    		gmt plot -Sc0.05c -Gred @MaunaLoa_CO2.txt
    		gmt plot -W0.5p,blue model.txt
    		gmt text -F+f12p+cTL+t"m@-2@-(t) = a + b\267t" -Dj8p -Glightyellow

    		# Basic LS line y = a + bx + cx^2
    		gmt subplot set 2
    		gmt trend1d -Fxm @MaunaLoa_CO2.txt -Np2 > model.txt
    		gmt plot -Sc0.05c -Gred @MaunaLoa_CO2.txt
    		gmt plot -W0.5p,blue model.txt
    		gmt text -F+f12p+cTL+t"m@-3@-(t) = a + b\267t + c\267t@+2@+" -Dj8p -Glightyellow

    		# Basic LS line y = a + bx + cx^2 + seasonal change
    		gmt subplot set 1
    		gmt trend1d -Fxmr @MaunaLoa_CO2.txt -Np2,f1+o1958+l1 > model.txt
    		gmt plot -Sc0.05c -Gred @MaunaLoa_CO2.txt
    		gmt plot -W0.25p,blue model.txt
    		gmt text -F+f12p+cTL+t"m@-5@-(t) = a + b\267t + c\267t@+2@+ + d\267cos(2@~p@~t) + e\267sin(2@~p@~t)" -Dj8p -Glightyellow

    		# Plot residuals of last model
    		gmt subplot set 0
    		gmt plot -R1958/2016/-4/4 -Byafg10+u" ppm" -Sc0.05c -Gred model.txt -i0,2
    		gmt text -F+f12p+cTL+t"@~e@~(t) = y(t) - m@-5@-(t)" -Dj8p -Glightyellow

    	gmt subplot end
    	rm -f model.txt
    gmt end show

Trend Modeling.