Clasificación geomorfométrica del relieve mexicano: una aproximación morfográfica por densidad de curvas de nivel y la energía del relieve

At present the geomorphometric studies of relief and the digital mapping are an essential geomorphological platform for delimitation of natural synthetic units, so indispensable to the territorial organization, and others applied researches as the optimization of agricultural land use, microlocalization of settlement human, designation of forest programs and even for military strategic plans, among many social and productive utilities. Thus, this article aims to provide detailed information on analytical geomorphological methods and interpretations of morphography and energy for Mexico’s relief, considering two calculations according to unit area: density contour (km/km²) and maximum difference of heights (m/km²) - both relief índices -, with the support of GIS ArcView, ArcGIS, and ArcInfo software. A review of existing national atlases revealed that not all countries have morphometric data on their reliefs. Indeed, mapping of this type was only found, in chronological order, in the National Atlas of Cuba (1970), the Atlas of the Slovak Socialist Republic (Slovak Cartography (1983), the National Atlas of Hungary (Geographic Research Institute of the Hungarian Academy of Sciences (1989), the New National Atlas of Cuba (1989), and the National Atlas of Mexico (1990), all on a small scale. In other national atlases, there are only hypsometric and slope steepness maps. The stages for mapping the geomorphometric units of Mexico’s relief were: (1) Construction of the morphographic map: a) Calculation of density contours, b) Data analysis and reclassification, c) Spatial generalization by minimum mappable area, and d) Smoothing of geographic limits; (2) Obtaining a relief ’s energy map: a) Construction of the Digital Elevation Model (DEM), b) Calculations for the relief ’s energy map, c) Data reclassification, and d) Spatial generalization; (3) Production of a map of the relief ’s geomorphometric units: a) Overlaying of the morphographic and relief ’s energy maps, b) Spatial statistics, and c) Map of geomorphometric relief units: and 4) Definition, according to morphographic contour, of the two predominant classes of the relief ’s energy. The morphographic map (density of contours in km/km²) shows nine range classes: less than 1.0, from 1.0 to 2.0, from 2.1 to 3, from 3.1 to 4, from 4.1 to 5, from 5.1 to 6, from 6.1 to 7, from 7.1 to 8.0, and more than 8.0. On the other hand, the map of the relief ’s energy has 9 ranges (m/km²): less than 10.0, from 10.1 to 20.0, from 20.1 to 50.0, from 50.1 to 100.0, from 100.1 to 200.0, from 200.1 to 300.0, from 300.1 to 400.0, from 400.1 to 500.0, and over 500.0. An analysis of this country’s territory reveals that very highly dissected mountainous relief, with dissection values ranging from 501 to 1 300 m/km², evidences a monolithic location pattern associated with major breakups due to erosion in the central portion of the Sierra Madre Occidental and, in a more isolated way, in its southern portion; in the central sector of the Sierra Madre del Sur in the State of Guerrero; in the sierras of Miahuatlán and Juárez, in the State of Oaxaca; and in very scattered sectors of the southwestern part of the Sierra Madre de Chiapas and the Sierra Madre Oriental. These orogenic regions have land areas characterized by greater ascending widths and intensities in terms of neotectonic movements responsible for the formation of Mexico’s modern relief. In contrast, morphometric units belonging to the vertical dissection category from 21 to 50 m/km² exhibit moderately dissected, hilly relief typical of high and mid plains formed by a weaker neotectonic process. Lastly, morphometric units with dissection values between 0 and 20 m/km² stretch along the plains of the coastal states on the Gulf of Mexico, including karstified plains in the Yucatán Peninsula. Units in this category also stretch along the coastal plains of the Baja California Peninsula, the states of Sonora and Sinaloa, and the Pacific Coast in the State of Chiapas. They can also be found in isolated patches on the coastal plains of the Sierra Madre del Sur, on the plains of the Central Mexican Plateau, and along depressed areas of the Trans-Mexican Neovolcanic Belt, where they occupy countless graben depressions, as well as among volcanic edifices throughout the region. The present methodology for determining geomorphometric unities in Mexico’s relief at a scale of 1:250 000, supported by a GIS platform, using density contours as the morphographic parameter for the country’s relief, provided rather precise results consistent with the relief ’s mathematical-statistical and geomorphological realities. The cartographic expression at a 1:250 000 scale included 122 map sheets of Mexico representing several different levels of data, as morphographical, morphometric (relief ’s energy) and topographic, all of great significance for the preparation of other issues geomorphological mapping.