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Precise Determination of the Orthometric Height of Mt . Kilimanjaro

SUMMARY Mount Kilimanjaro, located in Tanzania, is the highest mountain in Africa. Furthermore, this dormant volcano is the highest " stand-alone " (it has an almost perfectly shaped volcano form) and " walkable " (it is not necessary to use special climbing equipment) mountain in the world. Located on the plate boundary between Somalia and Victoria tectonic blocks, Mount Kilimanjaro started to be formed about 750000 years ago being currently constituted by three major volcanic cones, Kibo, Mawenzi, and Shira. The first reaches approximately 5900m. In the past, several attempts were carried out to observe the precise height of this mountain. In 1952, the British Ordnance Survey did the first determination of the height of Uhuru peak (the top of Kibo volcano) by triangulation points over distances of more than 55Km. The computed value was 5895m. In 1999, a first attempt to apply spatial techniques (GPS) has been done and a new value of approximately 5892m was obtained. However, the observations were obtained using single-frequency receivers and the EGM96 global model was used to convert from ellipsoid height to orthometric height. This was a major issue since the uncertainties associated with this model for this region were of several meters. In order to reduce the uncertainties in the determination of the height of Mount Kilimanjaro, a new mission was carried out in October 2008. was combined with gravimetric observations. The gravimetric observations were necessary in order to construct a local geoid with sufficient accuracy. Three teams carried out observations during 10 days, with two teams carrying gravimetric around and in the mountain, whereas a third team was doing GPS observations on the top of the mountain. This presentation will focus on the description of the adopted methodologies that were used to accurately observe the ellipsoidal heights and to compute the local geoid. In order to validate the results, different groups processed the GPS observations using different software packages and different strategies. Similar procedures were applied to the geoid computation. The computed surface is also compared with the one obtained using global models, in particular the recently released EGM2008 model.