例子 help convexHull 

x = rand(10,1)
y = rand(10,1)
dt = delaunayTriangulation(x,y)
k = convexHull(dt)
plot(dt.Points(:,1),dt.Points(:,2), '.', 'markersize',10); hold on;
plot(dt.Points(k,1),dt.Points(k,2), 'r'); hold off;

x = [STA(:,1);STA(:,3)];
y =[STA(:,2);STA(:,4)];
dt = delaunayTriangulation(x,y);
k = convexHull(dt)
plot(dt.Points(:,1),dt.Points(:,2), '.', 'markersize',10); hold on;
plot(dt.Points(k,1),dt.Points(k,2), 'r'); hold off;

dt.Points(k,1),dt.Points(k,2) 即多边形边界

%% 多边形边界外的不画图，掩膜处理

isin=inpolygon(point_p_lon,point_p_lat,dt.Points(k,1),dt.Points(k,2));
result1(~isin)=NaN;
% pcolor(point_p_lon,point_p_lat,result1);
% load MyColormaps;colormap(mymap);colorbar;shading interp;
contourf(point_p_lon,point_p_lat,result1,10);shading flat
colorbar;
% colormap(flipud(jet));%色标反转
hold on;
load('shengjielat.mat')
load('shengjielon.mat')
plot(shengjielat,shengjielon)
set(gcf, 'PaperPositionMode', 'auto')