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Vertical accuracy evaluation of freely available latest high-resolution(30 m)global digital elevation models over Cameroon(Central Africa)with GPS/leveling ground control points.

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International Journal of Digital Earth 2019年第5期12卷 500-524页

作者： Loudi Yap ludovic houetchak kande Robert Nouayou Joseph Kamguia Nasser Abdou Ngouh Marie Brigitte Makuate Research Laboratory in Geodesy National Institute of CartographyYaound3Cameroon Department of Physics Faculty of SciencesUniversity of Yaoundé1YaoundéCameroon

Digital Elevation Models(DEMs)contain topographic relief data that are vital for many geoscience *** study relies on the vertical accuracy of publicly available latest high-resolution(30 m)global DEMs over *** models are(1)the ALOS World 3D-30 m(AW3D30),(2)the Shuttle Radar Topography Mission 1 Arc-Second CBand Global DEM(SRTM 1)and(3)the Advanced Spaceborne Thermal Emission and Reflection Global DEM Version 2(ASTER GDEM 2).After matching their coordinate systems and datums,the horizontal positional accuracy evaluation was carried out and it shows that geolocation errors significantly influence the vertical accuracy of global *** this,the three models are compared among them,in order to access random and systematic effects in the elevation data each of them ***,heights from 555 GPS/leveling points distributed all over Cameroon are compared to each DEM,for their vertical accuracy *** and robust statistical measures,normality test,outlier detection and removal were used to describe the vertical quality of the *** test of the normality rejected the hypothesis of normal distribution for all tested global *** vertical accuracies obtained for the three models after georeferencing and gross error removal in terms of Root Mean Square(RMS)and Normalized Median Absolute Deviation(NMAD)are:AW3D30(13.06 m and 7.75 m),SRTM 1(13.25 m and 7.41 m)and ASTER GDEM 2(18.87 m and 13.30 m).Other accuracy measures(MED,68.3% quantile,95% quantile)supply some evidence of the good quality of AW3D30 over ***,the effect of land cover and slope on DEM vertical accuracy was also *** models have proved to be worse in the areas dominated by forests and shrubs *** 1 and AW3D30 are more resilient to the effects of the scattering objects respectively in forests and cultivated *** dependency of DEMs accuracy on the terrain roughness is *** all slope intervals,AW3D30 is performing

关键词： DEM AW3D30 SRTM 1 ASTER GDEM 2 Cameroon GPS/leveling orthometric heights vertical accuracy

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Reference Ellipsoid Parameters of Cameroon from GPS Data

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International Journal of Geosciences 2016年第8期7卷 1049-1056页

作者： ludovic houetchak kande Joseph Kamguia Loudi Yap Louis Foyang National Institute of Cartography Yaounde Cameroon

The standard spheroidal reference surface also referred as the reference ellipsoid is a part of mapping basic infrastructures of a country. In Cameroon, the ellipsoid Clarke 1880 with unknown parameters used for a long time as reference system in basic mapping, has lead to the use of World Geodetic System 1984 (WGS 84) whose parameters are well known. Meanwhile, the latter is not accurate locally due to the fact that the best ellipsoid is the one that fits very well with the local geoid. In order to look for the parameters of the local ellipsoid that fits best to the local geoid (Cameroon Geoid Model 2011 (CGM11)), the Global Positionning System (GPS) data made of 525 geodetic ground control points of the new geodetic network of Cameroon set up in 2011 were used. These GPS measures provide for each point the values of the ellipsoid height and the elevation which are used to determine the parameters of the local ellipsoid model for Cameroon through the least square form of the Molodensky analytical method. The results are given as the difference in ellipsoidal height relative to the parameters of the WGS84 ellipsoid assuming the two ellipsoids in their parallel. These results show that the obtained ellipsoid fits better to the local geoid with 1.072 m as the standard deviation value, is improved considerably in comparison to the previous studies conducted in Cameroon whose standard deviation was fairly equal to 1.679 m.

关键词： Ellipsoid Parameters Geoid GPS Molodensky Analytical Method Cameroon

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Geoelectric Structure of the Crust along the Mbere Trough (South Adamawa) from Audio-Magnetotelluric Data

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International Journal of Geosciences 2013年第8期4卷 1215-1227页

作者： ludovic houetchak kande Joseph Kamguia Jean Marie Tadjou Robert Nouayou Department of Physics Faculty of Science University of Yaounde I Yaounde Cameroon National Institute of cartography Yaounde Cameroon

The South Adamawa trough represents a major structure associated to the tectonic activity in the Adamawa region. The Mbere and the Djerem troughs represent the eastern and the western parts of the South Adamawa trough respectively. The audio-magnetotelluric (AMT) method is used to study the Mbere trough and to bring out tectonic features that affect the basement of this region during the Cretaceous. From analysis and interpretation of AMT data, based on pseudo-section and geoelectric section, two geologic models are constructed across the trough at Belel and Nyambaka. Those models bring more information on the sediments thickness and the borders faults of the trough compared to previous studies. The results shows that the Mbere trough is dissymmetric, and bounded on its northern side by an intra-gneissic fault and on its southern side by two faults with at least one considered as a faulted contact between the granitic and gneissic rocks. The sediments filling of the trough are consist mainly of conglomerates on which lay sandstones. The thickness of these sediments is about 2060 m at Belel and decreases to about 1800 m at Nyambaka, with a low sandstones thickness, which does not exceed 360 m. The basement consists of granite in the South of the trough and gneiss in the North, with resistivity more than 10,000 Ω.m. The Moho is located between 25 and 35 km in the South Adamawa region.

关键词： Adamawa Trough Fault Resistivity Pseudo-Section Geoelectric Section

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Subsurface Structural Mapping Using Combined Terrestrial and Grace Gravity Data of the Adamawa Plateau (North-Cameroon)

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International Journal of Geosciences 2017年第7期8卷 869-887页

作者： Bouba Apollinaire Kamguia Joseph Tabod Charles Tabod Yap Loudi Nouayou Robert kande houetchak ludovic Oyoa Valentin Department of Physics Higher Teachers’ Training College University of Maroua Maroua Cameroon Department of Physics Faculty of Science University of Yaounde I Yaounde Cameroon National Institute of Cartography Yaounde Cameroon

In order to analyze and determine the geological structures of the Adamawa plateau, the terrestrial gravity data were combined to data computed from GGM02C gravity model. The dense gravity net obtained were further introduced into qualitative and quantitative interpretations. The resulting Bouguer anomaly map obtained from combined data shows NE-SW direction which nearly coincides with the main direction of the fractures affecting the basement in the region and indicates strong gradients marking the presence of discontinuities between heavy and negative gravity anomaly. In order to conduct the quantitative interpretation of the combined gravity data, three profiles were drawn on the residual Bouguer anomaly map and therefore were interpreted using spectral analysis method and 3D density inversion. The knowledge of the depth and density of the geological structures show an uplift of dense rocks under the granite-gneiss substratum. This dense material found in the ENE-WSW direction of the Adamawa Plateau is interpreted as basaltic intrusion probably resulting from tectonic processes. According to this study, the depths of 3.83 km and 9.62 km are the new values of depths obtained for futures investigations in the Adamawa plateau.

关键词： GGM02C Adamawa Plateau Bouguer Anomaly 3D Density Model Spectral Analysis

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