水准测量中，采用前后视距相等可以消除哪些误差？

<span>（</span> <span>1</span> <span>）仪器校正后的残余误差（水准管不平行与视准轴）；<span>（</span><span>2</span><span>）</span><span>地球曲率的影响<span>（</span><span>3</span><span>）大气折光的影响</span></span></span>