if the sum of squares of weights has been defined (via Sumw2),
this function returns the sqrt(sum of w2).
otherwise it returns the sqrt(contents) for this bin.

I am not sure, how I can get a h1 from h2 without Projection i.e. h2->h1->GetBinError(xbin)
Projection projects everything. Is there a way to project only a section of x axis ?

I actually tried this, it works, here’s an eg of what I am looking for :
x : [0, 1, 2] // bins
xval: [0, 3, 6] // bin values
xerr: [0, 0.001, 0.3] // bin errors

y : [0, 1, 2] // bins
yval: [0, 2, 4] // bin values
yerr: [0, 0.01, 0.2] // bin errors

when I populate TH2D, I guess I am doing (x, y), so whenever there’s an x, y, I fill it by something i.e.
(3, 5) > 1 // first time
(3, 5) > 1 // second time
and so on.

per se the data structure didn’t seem like taking both data, errors for both axes separately, all together and the error can be retrieved when there’s a coincidence i.e. error related to (3, 5) is what I get through the method you’ve given me. Right ?

Sorry, I’m not sure to fully understand what you describe… If the documentation and the examples are not clear enough, then I’ll ask @moneta to give more precise (and technical) details about TH2, errors and such…