How to make a spatial plot

Question

Elliot Jenkins 2023-1-10

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https://ww2.mathworks.cn/matlabcentral/answers/1891635-how-to-make-a-spatial-plot

评论： Walter Roberson 2023-1-11

Hi,

I have latitude, longtitude and the data corrosponding to each location. How do I make a simple spatial plot of the data on a map with a gradient scale? I have done all the information in excel and imported it into MATLAB so they are in tables.

I do not have data for every location for the country so also I would like to grey out the areas that I do not have the data for (so locations with 0 are not confused with areas with no data).

Thanks.

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Answer 1

Walter Roberson 2023-1-10

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https://ww2.mathworks.cn/matlabcentral/answers/1891635-how-to-make-a-spatial-plot#answer_1145755

在 MATLAB Online 中打开

You can discretize the coordinates, and accumarray, something like

dxlat = 0.1;  %degree
dylon = 0.25; %degree
latidx = floor((lat-min(lat))/dxlat) + 1;
lonidx = floor((lat-min(long))/dxlon) + 1;
mean_data = accumarray([latidx(:), lonidx(:)], data(:), [], @mean, nan );

Now you can do gradient processing. (Locations with no data in the dxlat x dylon box will have nan in them, which is going to cause problems for the gradients.)

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Elliot Jenkins 2023-1-11

在 MATLAB Online 中打开

Hi,

Thank you for the reply. I do not understand what is happening with the code but I have tried to run it and have found some errors. I replaced the parameters in your code with my parameters so the code looks like this. latitude70, longtitude70 and severity70 are all arrays converted from tables.

A= readtable("1970s.xlsx")
S= table2array(A)
latitude70= S(:,1); 
longitude70= S(:,2);
severity70= S(:,4);  
% data
%assuming vector lat, vector lon, vector data
dxlat = 0.1;  %degree
dxlon = 0.25; %degree
minlat = min(latitude70);
minlon = min(longitude70);
latidx = floor((latitude70-minlat)/dxlat) + 1;
lonidx = floor((latitude70-minlon)/dxlon) + 1;
mean_data = accumarray([lonidx(:), latidx(:)], severity70(:), [], @mean, nan );
scaled_lat = ((1:size(mean_data,2))-1/2) * dxlat + minlat; 
scaled_lon = ((1:size(mean_data,1))-1/2) * dxlon + minlon;
alphadata = double(~isnan(mean_data));
imagesc(mean_data, 'XData', scaled_lon, 'YData', scaled_lat, 'Severity', alphadata);
colorbar();
set(gca, 'color', [0.5 0.5 0.5]);  %grey behind no data

I am getting two errors:

Error in imagesc (line 52)
    hh = image(varargin{:}, 'CDataMapping', 'scaled');
Error in Maptests (line 35)
imagesc(mean_data, 'XData', scaled_lon, 'YData', scaled_lat, 'Severity', alphadata);

Walter Roberson 2023-1-11

'Severity' is not a valid property name for imagesc . The property name is the one I gave in my code, 'AlphaData'

Walter Roberson 2023-1-11

在 MATLAB Online 中打开

I create random data for demonstration purposes. If you just happen to be running Linux or MacOS then this code will operate in demonstration mode, but if you are using Windows then this code will read your file.

if ~isunix()

A = readtable("1970s.xlsx")

S = table2array(A)

else

%random data for illustration purposes

rng(12345);

N = 5000;

S = zeros(N,4);

S(:,1) = (rand(N,1) * 2 - 1) * 20;

S(:,2) = (rand(N,1) * 2 - 1) * 30;

S(:,4) = randn(N,1) / 2 + 0.25;

S(randi(N, floor(N/20), 1), 4) = nan;

end

%the code

S = rmmissing(S(:,[1 2 4])); %get rid of any nan input

latitude70 = S(:,1);

longitude70 = S(:,2);

severity70 = S(:,3);

% data

%assuming vector lat, vector lon, vector data

dxlat = 1; %degree

dxlon = 1.5; %degree

minlat = min(latitude70);

minlon = min(longitude70);

latidx = floor((latitude70-minlat)/dxlat) + 1;

lonidx = floor((longitude70-minlon)/dxlon) + 1;

mean_data = accumarray([lonidx(:), latidx(:)], severity70(:), [], @mean, nan );

scaled_lat = ((1:size(mean_data,2))-1/2) * dxlat + minlat;

scaled_lon = ((1:size(mean_data,1))-1/2) * dxlon + minlon;

alphadata = double(~isnan(mean_data));

imagesc(mean_data, 'XData', scaled_lon, 'YData', scaled_lat, 'AlphaData', alphadata);

colorbar();

set(gca, 'color', [0.5 0.5 0.5]); %grey behind no data

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How to make a spatial plot

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