The determination of the gravimetric geoid is based on the magnitude of the 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 using unclassified Digital Terrain Models (DTM) usually presume that negative elevations are reserved for ocean stations. In case of Qattara Depression, the elevations are negative, i.e., below sea level. This leads to an obvious error in the topographic-isostatic reduction using, for example, TC-program employing unclassified DTM by assuming water masses filling the depression instead of air, besides computing at the non-existing sea level instead of computing at the actual negative topography. The aim of this paper is to determine the effect of Qattara Depression on gravity reduction and geoid computation, as a prototype of the effect of the unclassified land depressions on gravity reduction and geoid determination. The results show that the effect of Qattara Depression on the gravity reduction reaches 20 mGal and is restricted only to the depression area, while its effect on the geoid exceeds 1 m and has a regional effect which extends over a distance of about 1000 km.

重力大地水准面的确定是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，必须去除大地水准面内的地球表面和大地水准面之间的质量。然后必须将重力值减小到大地水准面，在边界面上形成边界值。使用未分类的数字地形模型（DTM）进行的重力折减技术通常假定负海拔保留给海洋站。如果是卡塔尔De陷，则海拔为负，即低于海平面。这会导致地形等静线缩小的明显错误，例如，使用TC程序采用未分类的DTM，方法是假设水团填充了凹陷而不是空气，除了在不存在的海平面上进行计算外，而不是在实际负地形上进行计算。本文的目的是确定卡塔拉De陷对重力减小和大地水准面计算的影响，作为未分类的陆面s陷对重力减小和大地水准面影响的原型。结果表明，卡塔拉De陷对重力减小的影响达到20 mGal，仅限于restricted陷区域，而对大地水准面的影响超过1 m，并具有延伸到约1000 km距离的区域效应。作为未分类的土地depression陷对重力减小和大地水准面确定影响的原型。结果表明，卡塔拉De陷对重力减小的影响达到20 mGal，仅限于restricted陷区域，而对大地水准面的影响超过1 m，并具有延伸到约1000 km距离的区域效应。作为未分类的土地depression陷对重力减小和大地水准面确定影响的原型。结果表明，卡塔拉De陷对重力减小的影响达到20 mGal，仅限于restricted陷区域，而对大地水准面的影响超过1 m，并具有延伸到约1000 km距离的区域效应。