Do you want to extract the latitude and longitude values, or the projected X/Y coordinates?
If the former, the coordinates are in the SC matrix in your example. Its format is a bit obscure, so I use a slightly-modified version of this contourcs function to extract it.
Below is an example that I wrote a while ago that I think does the exact thing you're attempting (masking a contour plot to show within a polygon only).
If you really want x/y coordinates, then you'll need to add
for ii = 1:length(Cout)
[Cout(ii).X2, Cout(ii).Y2] = mfwdtran(Cout(ii).Y, Cout(ii).X);
end
after you extract the contours.
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The full example:
% Load coastlines (replace with your polygon)
latlim = [23 50];
lonlim = [-127 -65];
Usa = shaperead('landareas', 'usegeo', true, 'bounding', [lonlim' latlim']);
[latusa, lonusa] = maptrimp(Usa(1).Lat, Usa(1).Lon, latlim, lonlim);
% Create contours (replace with your gridded data)
n = 50;
xdata = linspace(lonlim(1), lonlim(2), n);
ydata = linspace(latlim(1), latlim(2), n);
zdata = peaks(n);
figure;
usamap('conus');
plotm(latusa, lonusa, 'k');
[C,h] = contourm(ydata, xdata, zdata);
% contourcs-stolen code
K = 0;
n0 = 1;
while n0<=size(C,2)
K = K + 1;
n0 = n0 + C(2,n0) + 1;
end
el = cell(K,1);
Cout = struct('Level',el,'Length',el,'X',el,'Y',el);
n0 = 1;
for k = 1:K
Cout(k).Level = C(1,n0);
idx = (n0+1):(n0+C(2,n0));
Cout(k).Length = C(2,n0);
Cout(k).X = C(1,idx);
Cout(k).Y = C(2,idx);
n0 = idx(end) + 1; % next starting index
end
% Trim to coastlines
[xc, yc] = deal(cell(length(Cout),1));
for ii = 1:length(Cout)
[xc{ii}, yc{ii}] = polybool('&', Cout(ii).X, Cout(ii).Y, lonusa, latusa);
end
% Plot
figure;
usamap('conus');
plotm(latusa, lonusa, 'k');
cellfun(@(x,y) plotm(y,x,'r'), xc, yc);
