Inpolygon returning a NAN

data=netcdf.open('gisst_full.nc','NC_NOWRITE')
lat=netcdf.getVar(data,0);
lon=netcdf.getVar(data,1);
time=netcdf.getVar(data,2);
sst=netcdf.getVar(data,3);
sst(sst==-32768)=NaN;
ti=datetime(1800,1,1)+days(time);
n=length(ti);

sst1=[sst(181:360,:,:);sst(1:180,:,:)];
lon1=[lon(181:360);lon(1:180)]
sst2=permute(sst1,[2,1,3]);
lonn=wrapTo360(lon1);

[X,Y]=meshgrid(lonn,lat);
idx=inpolygon(X,Y,[160 170],[-35 -25]);
data=sst2(:,:,1333:12:1584);
data1=mean(data,3),'omitnan';
Jan=mean(nonzeros(data1(idx))),'omitnan';

When I run the above script I get as far as data1 but the last line to calculate Jan value I am getting 'NaN' for Jan instead of the mean value of my inpolygon. The gisst data is monthly SST values and I just want the mean for an area 160-170E and 25-35S as specified in my idx. What am I missing?

Answer 1

[Steven Lord]

The inpolygon function can't return a NaN value. The problem is later on in your code. If you ask for the mean of an empty array the answer is NaN.

mean([])

ans = NaN

That implies that neither of the points in your inpolygon call are inside your polygon. You can check this by calling any. If it returns false, neither of the points are inside.

any([false false])

ans = logical
   0

any([false true])

ans = logical
   1

I can't offer any guidance for how to correct this problem since it's not clear to me from your code what your goal is. If you describe in words not code what the variables in your code represent (some of the names seem suggestive of their purposes, but suggestions can be deceiving) and describe what you intend this code to do (again in words not code) we may be able to offer some suggestions.

[Simbarashe Chidzambwa]

I am trying to extract a January average value from GISST data of SST from an area 160-170E and 25-35S represented by the line:

"idx=inpolygon(X,Y,[160 170],[-35 -25]);"

over a period of 21 years between 1982-2002 which is represented by the line:

"data=sst2(:,:,1333:12:1584);".

The scripts runs OK and even produces "data1" which contains the average of all the grid values but its only the extraction of the defined area in that last line which is not working.