The exact value of the upper bound of the approximation error by the construction of the type "angle" built using the Fourier-Haar partial sums is obtained on the function class of two variables in the uniform metric.
International Journal of Advanced Research in Mathematics (Volume 5)
A. N. Shchitov "On Approximation of the Continuous Functions of Two Variables by the Fourier-Haar ”Angle”", International Journal of Advanced Research in Mathematics, Vol. 5, pp. 23-31, 2016