The determination of the gravimetric geoid is based on the magnitude of gravity observed at the topographic surface of the Earth. In order to satisfy Laplace’s equation, the masses between the surface of the Earth and the geoid must be removed or shifted inside the geoid. Then the gravity values have to be reduced to the geoid, forming the boundary values on the boundary surface. Gravity reduction techniques with unclassified height models usually presume that positive elevations are reserved for positive rock topography. Great lakes, however, are filled with water and may be situated partially or fully above sea level. In case of Lake Victoria, the whole lake including its bed floor is situated above sea level, i. e., having positive elevation (orthometric height). This leads to an obvious error in the topographic-isostatic reduction using, for example, the TC program (Forsberg in A study of terrain reductions, density anomalies and geophysical inversion methods in gravity, 1984; Forsberg and Tscherning in Sansò F, Rummel R (eds) Geodetic boundary value problems in view of the one centimeter geoid, Lecture notes in earth sciences. vol 65, pp 239–272, 1997. 10.1007/BFb0011707) by assuming rock topography filling the lake instead of water. The aim of this paper is to determine the effect of Lake Victoria on gravity reduction and geoid computation in Africa, as a prototype of the effect of great lakes on gravity reduction and geoid determination. The results prove that the masses of Lake Victoria have significant effect both on the reduced anomalies and on the computed geoid, which then have to be considered for precise geoid determination with correct density values.

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大湖对未分类 DTM 引起的重力减少和大地水准面确定的影响：非洲维多利亚湖的案例研究

重力大地水准面的确定基于在地球地形表面观察到的重力大小。为了满足拉普拉斯方程，地球表面和大地水准面之间的质量必须在大地水准面内去除或移动。然后重力值必须减少到大地水准面，在边界面上形成边界值。使用未分类高度模型的重力降低技术通常假定正高程是为正岩石地形保留的。然而，大湖充满了水，可能部分或完全高于海平面。在维多利亚湖的情况下，整个湖包括其床底都位于海平面以上，即具有正高程（正高）。这导致在地形等静压减少中出现明显错误，使用，例如，TC 程序（Forsberg in A study of terrain reduction, density anomalies and geophysical inverse methods in Gravity, 1984; Forsberg and Tscherning in Sansò F, Rummel R (eds) 鉴于一厘米大地水准面的测地边界值问题，地球科学讲义. vol 65, pp 239–272, 1997. 10.1007/BFb0011707) 假设岩石地形充满湖而不是水。本文的目的是确定维多利亚湖对非洲重力减少和大地水准面计算的影响，作为大湖对重力减少和大地水准面确定的影响的原型。结果证明维多利亚湖的质量对减少的异常和计算的大地水准面都有显着影响，然后必须考虑使用正确的密度值进行精确的大地水准面确定。TC 计划（Forsberg in A study of terrain reduction, density anomalies and geophysical inverse methods in Gravity, 1984; Forsberg and Tscherning in Sansò F, Rummel R (eds) 鉴于一厘米大地水准面的大地测量边界值问题，讲座笔记地球科学. vol 65, pp 239–272, 1997. 10.1007/BFb0011707) 假设岩石地形填充湖而不是水。本文的目的是确定维多利亚湖对非洲重力减少和大地水准面计算的影响，作为大湖对重力减少和大地水准面确定的影响的原型。结果证明维多利亚湖的质量对减少的异常和计算的大地水准面都有显着影响，然后必须考虑使用正确的密度值进行精确的大地水准面确定。TC 程序（Forsberg in A study of terrain reduction, density anomalies and geophysical inverse methods in Gravity, 1984; Forsberg and Tscherning in Sansò F, Rummel R (eds) 考虑到一厘米大地水准面的大地测量边界值问题，讲座笔记地球科学. vol 65, pp 239–272, 1997. 10.1007/BFb0011707) 假设岩石地形填充湖而不是水。本文的目的是确定维多利亚湖对非洲重力减少和大地水准面计算的影响，作为大湖对重力减少和大地水准面确定的影响的原型。结果证明维多利亚湖的质量对减少的异常和计算的大地水准面都有显着影响，然后必须考虑使用正确的密度值进行精确的大地水准面确定。