Plotting a grid of points with inpolygon check

I have the latitude and longitude coordinates for the points of a polygon (is_15_ok_ks.txt).

I want to create a grid with ranges x = -99.5 : 0.1 : -95.6, y = 33.6 : 0.1 : 37.6

I then need to plot a series of nodes/points. Each node is at the centre of each 0.1x0.1 cell (such that the nodes are the ranges x = -99.45 : 0.1 : -95.65, y = 33.65 : 0.1 : 37.55)

I then will plot the polygon.

Then using inpolygon, I want to be able to determine which of those nodes are inside the polygon (and potentially retrieve each of their coordinates).

Any help on how I would do this?

Thanks Gareth

[Gareth Maver]

I have got this so far. But 0s and 1s created by inpolygon are not correct. Any thoughts?

filename='is_15_ok_ks.txt' ;
data = load(filename) ;
  lon = data(:,1) ;
  lat = data(:,2) ;
  %plot polygon 
plot(lon,lat,'k') ;
hold on
%plot grid lines
g_y=[33.6:0.1:37.6]; 
g_x=[-99.5:0.1:-95.6]; 
for i=1:length(g_x)
 plot([g_x(i) g_x(i)],[g_y(1) g_y(end)],'r:') %y grid lines
 hold on    
end
for i=1:length(g_y)
 plot([g_x(1) g_x(end)],[g_y(i) g_y(i)],'r:') %x grid lines
 hold on    
end
%plot nodes
[X,Y] = meshgrid(-99.45:0.1:-95.65,33.65:0.1:37.55);
scatter(X(:), Y(:), 1);
%find nodes that lie within polygon 
IN = inpolygon(X,Y,lon,lat) ;

Answer 1

[Adam]

Replacing

[X,Y] = meshgrid(-99.45:0.1:-95.65,33.65:0.1:37.55);

with

[X,Y] = meshgrid(-99.45:0.1:-95.65,37.55:-0.1:33.65);

looks to give the correct answer as the y coordinates were just flipped. I don't have time to figure out exactly why they are, but you may spot why yourself now.

[Gareth Maver]

Thanks. Any idea on how I can retrieve the coordinates of each node that is in the polygon?

[Adam]

X(IN)

and

Y(IN)

should give you this if you pair them up, e.g.

[X(IN) Y(IN)]

[Gareth Maver]

Thanks again. One last question. And how do I save those pairs?

I have this - but its not keeping the pairs correct.

fid = fopen('polygon_nodes.txt','wt');
fprintf(fid,'%f%f\n',[X(IN);Y(IN)]); 
fclose(fid);

[Adam]

fprintf(fid,'%f %f\n',X(IN), Y(IN));

should do the job. You need a space between the numbers and you have to pass the X and Y arguments in separately to fill the two %f place-holders.