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One of the steps in the reduction of seismic data is the correction for variations in surface elevation. This requires the adoption of a common reference surface, usually known as the seismic datum, but also referred to as the floating datum, the undulated or warped datum, or the datum plane. Ideally, the seismic datum should parallel the velocity layers, and in that sense it is an isovelocity surface; in practice, the datum is more often a surface of convenience. In its simplest form, the datum is a horizontal plane, but commonly it is undulated to conform to the generalized topography. While the warping of a datum surface serves to simplify and minimize the elevation corrections, it is usually done in the knowledge that the isovelocity surfaces are often similarly undulated. Such undulations of the velocity layers are generally attributed to the variations in loading caused by the irregularity of the ground surface. In an attempt to develop an objective means of constructing a satisfactory universal seismic datum, a zone chart was designed, similar to the terrain‐correction zone chart employed in gravity reductions. The compartmented transparent disk, superimposed on a topographic map and centered over the shotpoint, permits the calculation of a properly weighted, or “normalized,” elevation which serves as the basis for the undulated seismic datum. Tests of the resulting datum indicate that it will normally compensate adequately for velocity variations due to topographic loading and, consequently, insure a more accurate interpretation of the seismic data.