如圖,已知地面水準(zhǔn)點(diǎn)A的高程為HA=1966.5m,若在基坑內(nèi)引測一高程點(diǎn)B,測設(shè)時(shí)α=0.986m,b=30.255m,a1=0.489m,b1=1.205m,求B點(diǎn)高程HB?