Create a box around points and use in inpolygon

Dear all,

I used this code below in order to select the points that are placed in/on the polygon which is correct and gave me the right answer.

% lat and lon are the coordinates of my 96 points that I want to select between them
% and polygon1_x and polygon1_y are coordinates of the shape file

[in,on] = inpolygon(lat,lon,polygon1_x,polygon1_y);         % Logical Matrix
inon = in | on;                                             % Combine ‘in’ And ‘on’
idx = find(inon(:));                                        % Linear Indices Of ‘inon’ Points
latcoord = lat(idx);                                        % X-Coordinates Of ‘inon’ Points
loncoord = lon(idx);                                        % Y-Coordinates Of ‘inon’ Points
clf
figure(1)
plot(lon, lat, '.')                                        % Plot All Points
hold on
plot(polygon1_y, polygon1_x, '.')                          % Plot Polygon
plot(loncoord, latcoord, 'gp')                             % Overplot ‘inon’ Points
hold off
idx = idx.';

But I want to edit this code in order to consider each point as the center of 0.5 x 0.5 grid-box and then start checking. So if any part of this box placed in/on polygon I want to select (index) the former point (center).

I attached my data. I tried to edit this by myself but unfortunately it not accomplished well. So any suggestion is highly appreciated.

Thank you all.

Answer 1

[KSSV]

load("polygon1_x.mat") ; 
load("polygon1_y.mat") ; 
load("lon.mat") ; 
load("lat.mat") ; 
% remove Nan's from the data 
xv = polygon1_y ; yv = polygon1_x ; 
xv(isnan(xv)) = [] ;
yv(isnan(yv)) = [] ;
%
loncoord = zeros([],1) ; 
latcoord = zeros([],1) ;
count = 0 ; 
for i = 1:length(lon)
  i
    % make grid around (lon,lat)
  x = lon(i)-0.5:0.5:lon(i)+0.5 ; 
  y = lat(i)-0.5:0.5:lat(i)+0.5 ; 
  [X,Y] = meshgrid(x,y) ; 
  [in,on] = inpolygon(X(:),Y(:),xv,yv);         % Logical Matrix
  inon = in | on;                                             % Combine ‘in’ And ‘on’
  idx = find(inon(:));                                        % Linear Indices Of ‘inon’ Points
  
  if any(idx)
    count = count+1 ; 
    loncoord(count) = lon(i);                                        % Y-Coordinates Of ‘inon’ Point
    latcoord(count) = lat(i) ;   
  end
end

plot(polygon1_y,polygon1_x,'b')
hold on
plot(loncoord,latcoord,'*r')

[BN]

Dear KSSV

Thank you for your answer, But after I run this code I saw nothing has been indexed. I mean after the run I don't know where is my output, idx is empty!

[KSSV]

X is lon, check whether you have to use:

[in,on] = inpolygon(X(:),Y(:),polygon1_x,polygon1_y);

Or

[in,on] = inpolygon(Y(:),X(:),polygon1_x,polygon1_y);

Points are stored in loncoord, latcoord.

[BN]

Thank you, I am so sorry Even after replace it by:

[in,on] = inpolygon(Y(:),X(:),polygon1_x,polygon1_y);

it gave me this error:

Unable to perform assignment because the left and right sides have a
different number of elements.

I tried to solve it using debugger but it generates this error every time I tried:

Unable to perform assignment because the left and right sides have a
different number of elements.
Error in Untitled3 (line 15)
    loncoord(count) = lon;                                        %
    Y-Coordinates Of ‘inon’ Point

[KSSV]

Edited the code....check the answer...

[BN]

Thank you so much