Reanalyzing the galactic HI survey data with two methods which make use of the geometry of the galactic disk, we explore the outer rotation curve. In addition to the derivation of the outer rotation curve, we investigate their uncertainties, which have not been established previously. Comparing with those obtained previously based on various methods, we find that the rotation curve obtained by the method of Merrifield has the smallest error and the largest coverage of galacto-centric radius among the existing rotation curves. Combining with the inner rotation curve obtained previously by the HI tangential velocity, we present the overall rotation curve of the Galaxy, which ranges from 0.3R0 to 2.5R0. The rotation curve is gradually rising with the galacto-centric radius if the IAU standard galactic constants, R0 = 8.5 kpc and Θ0 = 220 km s−1, are assumed. However, the shape of the rotation curve is fairly sensitive to the adopted values of the galactic constants, and the outer rotation curve could be declining if Θ0 is smaller than 200 km s−1.