MATLAB: Compute the minimum distance between each element in a vector with another larger array

great circle distanceMATLABminmum distances

I have an array say, MW that contains the latitudes and longitudes of ~1300 pixels. I also have a reference text file, say IR that contains an enormously large number of pixels with latitudes and longitudes and the pixels here are associated with unique IDs. My goal is to attribute each MW pixel to a unique ID found in the IR text file if possible. I have a function called distance_latlon (function d = distance_latlon(lat1,lat2,lon1,lon2)) that computes the distances between any two points given their lat and lon. Essentially, I would like to compute for each MW pixel, the distance to all of the IR pixels and find the IR pixel within the closest distance and attribute the ID. If the MW pixel does not have a single IR pixel within a certain distance threshold, then I can simply assign a 0.
How do I accomplish this task without the extremely expensive way that is, going through each MW pixel in a for loop, running the distance function between each MW pixel and all the IR pixels in the text file in another long for loop and finding the minimum distance? What is the least expensive way of doing this?

Best Answer

I don't think there is a way around doing some calculation for each pair.
Is there a way to vectorize distance_latlon? If not, I would suggest a pre-selection, so you only need to do the expensive calculation a few times:
N_small=5;
[lat2,lon2]=MyFun(IR);
for n_MW=1:1300
    [lat1,lon1]=MyFun(MW(n_MW));
    quickdist=hypot(lat1-lat2,lon1-lon2);
    [~,idx1]=sort(quickdist);
    lat2_small=lat2(idx(1:N_small));
    lon2_small=lon2(idx(1:N_small));
    for n_small=1:N_small
        exactdist(n_small)=distance_latlon(lat1,lat2_small,lon1,lon1_small);
    end
    [min_exactdist,idx2]=min(exactdist);
    rowID=idx1(idx2);
end
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