Boundary/polygon around coastline of Greenland

I have a big dataset of the topography of Greenland. x and y are in stereographic coordinates, with x being a vector of length 10218x1, y being a vector of length 18346x1 and z being a matrix of size 18346x10218 containing values of topography, negative values in the sea and postive inland.

I have a meshgrid of the x and y coordinates and I wish to calculate the distance from the closest point of the coast at every point in this meshgrid. My problem is to create a boundary or a border around the coast and islands surrounding Greenland, ie. where z goes from being negative to positive. I have tried to create a polygon, but the difference in length of the x and y arrays is giving me problems.

The dataset is too large (2.5 gb) to attach, but can be downloaded here.

filename = 'BedMachineGreenland-2021-04-20.nc';
x = ncread(filename,'x');
y = ncread(filename,'y');
x = double(nc_x);
y = double(nc_y);
dx = 5000;
x_ax=min(x):dx:max(x);
y_ax=min(y):dx:max(y);
[xx,yy]=meshgrid(x_ax,y_ax);

z = ncread(filename,'bed')'; 
figure
imagesc(nc_x,nc_y,z); axis xy equal; caxis([-500 1800]);
colorbar

Any help is appreciated. Thanks.

Answer 1

[KSSV]

You can get the coastline using the function contour. Read about it.

[M,c] = contour(x,y,Z,[0 0]) ; % M has the coordinates you want 

[Christian Mathiesen]

Cheers mate!

Answer 2

[Chad Greene]

I've just added a 'gl' option to plot Greenland's grounding line using BedMachine's mask. https://github.com/chadagreene/BedMachine, so

bedmachine('gl','greenland')  

produces this:

or

bedmachine('gl','color','red','linewidth',2,'greenland') 

produces this: