Boundary/polygon around coastline of Greenland

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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.

Accepted Answer

KSSV
KSSV on 22 Jan 2022
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

More Answers (1)

Chad Greene
Chad Greene on 3 Apr 2022
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:

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