here exists change of gradient for earth magnetic intensity in directions of altitude and latitude, and hence it is needed to carry out altitude correction and altitude correction during the performance of high-precision magnetic survey. The common method for calculating correction values is based on the thirteen-stage approximate formula of the spherical harmonics expression, and there also exist the ten-stage one and the one-stage one. There commonly exists a small error in using the ten-stage one. In comparison with the thirteen-stage approximate formula, there is a small error in using the ten-stage approximate formula: when there is 30 km between the base station and the measuring points, the error for latitude correction is -0.21 nT; nevertheless, there is a large error in using the one-stage approximate formula: when there is 4 km between base station and the measuring points, the error for latitude correction is about 1 nT, and when there is 300 m in height between the base station and the measuring points, the error for altitude correction is about 1nT.