$2\mathrm{.}2$ The slope distance and zenith angle between points A and $B$ were observed with a total station instrument as $65$ and $500m$ respectively. The height of the device, hi and rod reading, $r$ were $1\mathrm{.}15m$ and $1\mathrm{.}36m$ respectively. If the rod leaning from vertical by $2$ minutes. Correct the rod reading against all errors. If the elevation of $A$ is $50m,$ compute the elevation of $B.$