How to Extrapolate griddata for contour plot?

I have xy-grid with corresponding contour values which I intend to plot in a contour.

my x-grid and y-grid values are at dimensions 0:20:120 and 0:20:220, respectively.

load('xyref.mat')

I interpolated these values at grid of 10 units in xy direction using the contour value using meshgrid

load('contr.mat')
[x1,y1] = meshgrid(0:10:120, 0:10:220); 
za = griddata(x,y,contr,x1,y1);
figure
[c,h] = contourf(x1,y1,za,'ShowText','on',FaceAlpha=1,EdgeAlpha=0.5,EdgeColor='k',LevelStep=5);
grid on

Now, at this point, I want to extrapolate these contour values to further 20 units range in both directions. I have already extracted these data of huge magnitude and it would take immense time in extrapolating these data. What I want to do is just linearly extrapolate these contours in the range -20 to 140 and -20 to 240 in x and y directions. Is there a way to extrapolate the griddata to those values?

Right now, the graph, when extrapolated gives nan values in the griddata and plots like this

[x1,y1] = meshgrid(-20:10:140, -20:10:240); 
zb = griddata(x,y,contr,x1,y1);
figure
[c2,h2] = contourf(x1,y1,zb,'ShowText','on',FaceAlpha=1,EdgeAlpha=0.5,EdgeColor='k',LevelStep=5);
grid on 

I hope I have explained my problem adequately. Your help in extrapolating griddata will be much appreciated. Thanks in advance and have a great weekend.

Answer 1

[Sulaymon Eshkabilov]

Here you can find an appropriate 3rd party fcn called inpaint_nans() that can be employed to subs NaNs in zb. Note that the MATLAB fcn fillmissing() does the job partially.

clearvars; close all; clc
load('xyref.mat')
load('contr.mat')
[x1,y1] = meshgrid(0:10:120, 0:10:220); 
za = griddata(x,y,contr,x1,y1);
figure
[c,h] = contourf(x1,y1,za,'ShowText','on',FaceAlpha=1,EdgeAlpha=0.5,EdgeColor='k',LevelStep=5);
grid on

[x1,y1] = meshgrid(-20:10:140, -20:10:240); 
zb = griddata(x,y,contr,x1,y1);
zb1=fillmissing(zb, 'nearest');         % fillmissing() substitutes NaNs partially
zb2=inpaint_nans(zb,0);                 % Substitutes all NaNs linearly 
figure
[c2,h2] = contourf(x1,y1,zb2,'ShowText','on',FaceAlpha=1,EdgeAlpha=0.5,EdgeColor='k',LevelStep=5);
grid on 

[UH]

This works. This solves my problem, thanks to you.

[Sulaymon Eshkabilov]

Most welcome. Glad to be of help.