Page 170 - MATLAB Recipes for Earth Sciences

7.7 Gridding Example 165

maximum two digits.

labels = num2str(data(:,3),2);
The 2D plot of our data is generated in two steps. Firstly, the data are dis-
played as empty circles by using the plot command. Secondly, the data
are labeled by using the function text(x,y,’string’) which adds text
contained in string to the xy location. The value 1 is added to all x coor-
dinates as a small offset between the circles and the text.

plot(data(:,1),data(:,2),'o')
hold on
text(data(:,1)+1,data(:,2),labels);
hold off

This plot helps us to define the axis limits for gridding and contouring,

xlim = [420 470] and ylim = [70 120]. The function meshgrid transforms
the domain specified by vectors x and y into arrays XI and YI. The rows of

the output array XI are copies of the vector x and the columns of the output
array YI are copies of the vector y. We choose 1.0 as grid intervals.

x = 420:1:470; y = 70:1:120;
[XI,YI] = meshgrid(x,y);

The biharmonic spline interpolation is used to interpolate the irregular-
spaced data at the grid points specifi ed by XI and YI.

ZI = griddata(data(:,1),data(:,2),data(:,3),XI,YI,'v4');
The option v4 depicts the biharmonic spline interpolation, which was the
sole gridding algorithm until MATLAB4 was replaced by MATLAB5.
MATLAB provides various tools for the visualization of the results. The
simplest way to display the gridding results is a contour plot using con-
tour. By default, the number of contour levels and the values of the contour
levels are chosen automatically depending on the minimum and maximum
values of z.

contour(XI,YI,ZI)

Alternatively, the number of contours can be chosen manually, e.g., 10 con-
tour levels.

contour(XI,YI,ZI,10)

Contouring can also be performed at values specified in a vector v. Since the
maximum and minimum values of z is