and I also do pick up a few spurious spots that are'nt part of the grid
If outliers are present, I would use the approach below, which requries the download of this FEX contribution,
I don't really know the distribution of your outliers, so that may affect choices of certain parameters, as noted below in the code.
load Image_outliers
C=vertcat(regionprops(Image,'Centroid').Centroid);
x=C(:,1);
y=C(:,2);
D=pdist2([x,y],[x,y],'city','Smallest',2);
D(1,:)=[];
d=median(D(:));
Xedges=binEdges(x,d);
Yedges=binEdges(y,d);
[~,~,~,Gx,Gy]=histcounts2(x,y,Xedges,Yedges);
xl=splitapply(@median,x,Gx);
yl=splitapply(@median,y,Gy);
imshow(Image,[]);
xline(xl,'y--'); yline(yl,'y--')
function edges=binEdges(z,d)
Z=-d/2:round(max(z)+d/2);
T=sum(abs(Z-z(:))<=d/5,1);
thresh=0.5*max(T); %depends on outlier rate
[start,stop]=groupLims(groupTrue(T<=thresh),1);
edges=(Z(start)+Z(stop))/2;
end
