The curvature of the earth and the convergence of the meridians are ignored. The distances measured during the survey are reduced to horizontal distances at a local datum. No adjustments are made to compensate for differences between distances between points on this level surface and distances between the same points on the flat plane of the projection. In short, the tangent plane projection system accounts for the curvature of the earth by ignoring it. Indeed, so little attention is given to the shape of the earth in performing tangent plane projection surveys that many people, even Land Surveyors, forget that every map or plat derived from measured angles and distances is the projection of a curved surface onto a flat plane. Stephen V. Estopinal - A Guide to Understanding Land Surveys