How can I create contour on non-regular shape such as maps?

Hello all,

I have written a code in which the final aim is to provide contour map outlining frost depth on Alaska sate. For each weather station having specific lat. and long., I have estimated the probability that frost depth exceeds 1, 2 ,3, 4, and 5 feet (The first and second columns of "POFDE" are latitude and longitude respectively and columns 3 to 7 are showing the probability that frost depth exceeds 1 to 5 feet respectively) . Hence, I want to draw 5 contour maps, each showing the probabilties exceeding the mentioned depths. To do this, it is required to create a mesh on Alaska map and use scatteredInterpolant function to do interpolation between all existing weather station for which I estimated the probabilities. Can anyone help me in this regard?

In the code below, I created a mesh on a rectangular extending from minimum longitude to maximum longitude in the X direction, and from minimum latitude to maximum latitude in Y direction. However, that would cause to have contour lines in the water which is not acceptable.

Please use 'tl_2018_02_anrc.shp' file located in the shared zip folder for superimposing Alaska map. POFDE is also attached here.

clear
close all
clc
load('POFDE.mat');
S=shaperead('tl_2018_02_anrc.shp','UseGeoCoords',true);
data = cell2mat(POFDE);
%%%%%%generating mesh as query points based on the Alaska bounding box
X=linspace(-179.148909,-130.1,1500);
Y=linspace(51.214183,71.365162,1500);
[lon,lat] = meshgrid(X,Y);
for i = 1:(size(data, 2)-2)
    figure
    geoshow(S,"DisplayType","multipoint")
    xlim([-179.148909,-130.1]);
    ylim([51.214183,71.365162]);
    I = scatteredInterpolant(data(:,[1 2]), data(:,i+2));
    I.Method='linear';
    I.ExtrapolationMethod='linear';
    contourm(lat,lon,min(1,max(0,I(lat,lon))))
    colorbar
    title(['The probability of frost depth exceedance of \Omega_{\delta} = ' num2str(i) ' feet'])
    exportgraphics(gca, sprintf('FrostPlot_%d_feet.png', i))
end

Answer 1

[Kausthub]

I understand that you want the contor lines only on the land and not in water. There are two possible workarounds:

• Consider random coordinates provided in the S which can be accessed using S(i).Lat and S(i).Lon for creating “lon” and “lat” instead of using linspace. This will ensure all the points are inside Alaska.

for i = 1:12 
    idx = randi(size(S(i).Lat),[125,1]); 
    lon = [lon S(i).Lon(idx)]; 
    lat = [lat S(i).Lat(idx)]; 
end 

• Utilize the bounding boxes provided which can be accessed using S(i).BoundingBox and check if the interpolated point lies inside the boxes, which can be done using rectangle (https://www.mathworks.com/help/matlab/ref/rectangle.html) and inpolygon (https://www.mathworks.com/help/matlab/ref/inpolygon.html)then it can be included in the contour plot.

for j = 1:12 
    arr = S(j).BoundingBox; 
    arr = reshape(arr',[1,size(arr,1)*size(arr,2)]); 
    arr(3) = arr(3)-arr(1); 
    arr(4) = arr(4)-arr(2); 
    rectangle('position',arr); 
end 

I hope this helps and addresses your question regarding potting contour lines over non-regular shapes like maps!