function B=threshsea(A,threshold)
% A must be real and not complex!
minvalue=min(min(A)); % find minimum value
maxvalue=max(max(A)); % find max value
sealevel=(maxvalue-minvalue)*threshold+minvalue; % 0 < threshold < 1
C=A-sealevel;         % shift value down by sealevel
maxCvalue=max(max(abs(C))); % find max value
constant=1;
if maxCvalue>0         % avoid divide by 0
   C=0.9*(C/maxCvalue);% scale down between -0.9 and 0.9
   constant=maxCvalue/0.9;
end
D=ceil(C);   %form indicator of non-sea locations
B=(D.*C)*constant;            % scale from +/- 1 to 0,1