Elevation | EPFL Graph Search

The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vertical datum). The term elevation is mainly used when referring to points on the Earth's surface, while altitude or geopotential height is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit, and depth is used for points below the surface. Elevation is not to be confused with the distance from the center of the Earth. Due to the equatorial bulge, the summits of Mount Everest and Chimborazo have, respectively, the largest elevation and the largest geocentric distance. Aviation In aviation, the term elevation or aerodrome elevation is defined by the ICAO as the highest point of the landing area. It is often measured in feet and can be found in approach charts of the aerodrome. It is not to be confuse

Very high resolution digital elevation models: are multi-scale derived variables ecologically relevant?

Stéphane Joost, Kevin Leempoel, Pascal Vittoz

Digital Elevation Models (DEMs) are often used in landscape ecology to retrieve elevation or first derivative terrain attributes such as slope or aspect in the context of species distribution modelling. However, DEM-derived variables are scale-dependent and, given the increasing availability of very high resolution (VHR) DEMs, their ecological relevance must be assessed for different spatial resolutions. In a study area located in the Swiss Western Alps, we computed VHR DEMs-derived variables related to morphometry, hydrology and solar radiation. Based on an original spatial resolution of 0.5 meters, we generated DEM-derived variables at 1m, 2m and 4m spatial resolutions, applying a Gaussian Pyramid. Their associations with local climatic factors, measured by sensors (direct and ambient air temperature, air humidity and soil moisture) as well as ecological indicators derived from species composition, were assessed with multivariate Generalized Linear Models (GLM) and Mixed Models (GLMM). Specific VHR DEM-derived variables showed significant associations with climatic factors. In addition to slope, aspect and curvature, the underused wetness and ruggedness indices modeled measured ambient humidity and soil moisture, respectively. Remarkably, spatial resolution of VHR DEM-derived variables had a significant influence on models’ strength, with coefficients of determination decreasing with coarser resolutions or showing a local optimum with a 2m resolution, depending on the variable considered. These results support the relevance of using multi-scale DEM variables to provide surrogates for important climatic variables such as humidity, moisture and temperature, offering suitable alternatives to direct measurements for evolutionary ecology studies at a local scale.

Wiley-Blackwell2015

Reliable Averages and Risky Extremes

Annelen Kahl, Michael Lehning, Viet Anh Nguyen, Karine Maria Sarrasin

With the perspective of Switzerland’s phase-out from nuclear energy, solar energy potential may take a leading role for the country’s future in renewable energy. Unlike nuclear power stations, photovoltaic (PV) production is prone to intermittency as it depends on the immediate solar irradiance, which fluctuates in space and time. If a large percentage of Switzerland’s electricity was to be derived from solar radiation, stochastic fluctuations pose a risk to the robust supply and healthy function of the electricity network. For most efficient PV planning and siting, it is hence imperative to understand and quantify this variability in solar radiation, in order to anticipate average production as well as worst-case scenarios. Based on 12 years of satellite derived, spatially distributed data of daily average surface incoming shortwave radiation (SIS) this work analyses standard statistics, spatial correlation patterns and extreme conditions of cloud cover over Switzerland. Having compared different irradiance products, we decided to use the SIS product captured on the Meteosat Second Generation satellites, because it provides the most reliable snow/cloud discrimination, which is essential for spatial analysis over alpine terrain. Particularly in regions with high elevation differences, correlation between cloud cover and elevation undergo an annual cycle. In winter more clouds are found in the valleys, while in summer convective clouds dominate at higher elevations. The highest average irradiance values occur in the southern parts of the country, and make the cantons of Vallais, Tessin and Grison ideal candidate locations for PV installations. Simultaneously the Tessin shows a higher risk of periods with long lasting cloud cover, which would discourage from relying too much on solar power in that area. However looking at the question of suitability by studying spatial and temporal correlations of extremes, we see that the Tessin appears to be comparably decoupled from the rest of the country, and leads us to believe that this zone plays a valuable role in compensating temporary deficits in solar production from other regions of the country. This is just one of many example that highlight the complexity of spatio-temporal variability in solar irradiance and its implications for a reliable electricity supply in a future renewable Switzerland.

2016

Development of a Semi-distributed Hydrological Model for Glaciated Punatshangchu Basin in Bhutan

Pedro Filipe De Almeida Manso, Giovanni De Cesare, Vasker Sharma, Jessica Zordan

Hydropower plays a pivotal role in the socio-economic development of Bhutan where water resource is abundantly available and therefore several hydropower plants are being planned and a few under construction. However, with the presence of several potentially dangerous glacier lakes within the higher elevations, the country is always at the risk of Glacial Lake Outburst Flood (GLOF) and climate change which poses higher uncertainty regarding the sustainability of hydropower reservoirs in the long run. To understand the hydrological response of the basin, where new hydropower plants are going to be installed soon, a complex semi-distributed hydrological model has been prepared for the Punatshangchu basin using RS MINERVE. After calibration and validation of the model, it is observed that the model reflects low relative volume bias (-0.196 - 0.050) and high Nash efficiency (0.540 - 0.990) which is an important aspect to be considered for any hydropower dam and its operational schemes. Such a model is a viable tool well adapted to an operational flood forecasting system and management. With the built-in scheme for hydropower, reservoir, planner, and turbines within RS Minerve, it could be used to understand the array of scenarios for planning and operations.

2021

Reliable Averages and Risky Extremes

Annelen Kahl, Michael Lehning, Viet Anh Nguyen, Karine Maria Sarrasin

2016