Whether an area belongs to mountains or not is a matter of definition and mountains have always been and remain difficult to define. This is not because of their ridges and tops, which are relatively easy to identify. It is because of the difficulty to define where exactly mountainous terrain grades into surrounding hills or flatland. What is regarded as a mountain by some people may appear to others as a hill (Smith and Mark 2003). Over the years, different definitions have been proposed to capture the spatial extent of mountainous areas (Meybeck et al. 2001; Sayre et al. 2018; Price et al. 2019). All have been extensively used for various applications, including the calculations of food insecurity (Romeo et al. 2020) or more recently the estimation of mountain populations (Thornton et al. 2022).
Regardless of the context of use, defining a mountain requires to make a distinction between elevation (and its implications such as reduced barometric pressure and atmospheric temperature), and steepness as a measure of inclination and gravitational forcing.
See Körner et al. (2017), Sayre et al. (2018), and Körner et al. (2021) for an overview of existing definitions and their differences.
In 2011, GMBA proposed a definition of mountains that constrains mountains by ruggedness of terrain only, irrespective of elevation. Ruggedness is defined as the maximal elevational difference among 3 x 3 = 9 grid points of 30" in 2.5' pixels and is calculated based on the digital elevation model used by WorldClim (Hijmans et al. 2005). For a 2.5' pixel to be defined as rugged (i.e., 'mountainous'), the difference between the lowest and highest of the 9 points must exceed 200 m.
The GMBA definition differs from the widely-used definition proposed by Kapos et al. (2000), which constrains mountains by a combination of elevation and ruggedness. Accordingly, based on the GMBA definition mountains cover 12.3 % of the Earth's surface outside of Antarctica and are home to 511 million "mountain people", whereas based on the alternative definition by Kapos et al. mountains cover approximately 22% of the Earth's surface and host 1.27 billion people.
Detailed information about the mountain definition is available in Körner et al (2011) (PDF, 2.0 MB). The ruggedness data are available in various format for unrestricted download and usage. The ruggedness layer displayed in the GMBA Mountain Portal and used as baseline for the GMBA mountain inventory is based on the GMBA mountain definition.
1When using the data please cite Körner et al., 2011 and the dataset DOI: 10.7892/boris.83486