Saturday, October 9, 2021

Maps: India Contours

Once in a while I point readers to interesting and useful web mapping applications. Last week, Raj Bhagat Palanichamy, a specialist in GIS and Remote Sensing,  showcased a contour map on his Twitter feed. Intrigued, I found out that the contour map was from a web based application developed by Axis Maps. A detailed blog post explains how they developed the application and the elevation data sources they used to generate the contours. It is quite easy to use, with some controls to set contour intervals, line color, and to depict relief in color shades. 

I am putting a few examples from different Indian terrains to showcase this utility. It is not meant to be a critical review of the app (map scale is missing!), but a fun exploration of its capabilities, and the insights one can get about topography and other aspects of geomorphology and geology. Each example has a contour map on top and a satellite image of roughly the same area in the lower panel. All contours maps have been created using Contours- Axis Maps.

Western Ghats- Edge of the Deccan Plateau: Ghangad, Tail Baila Mesas

The edge of the Deccan Plateau has been deeply dissected to form some stunning relief. Steep sided ridges, mesas, and pinnacles poke upwards from more gently sloping and flattish surfaces. This step like appearance occurs due to the differing styles in which lava flows of varying hardness weather and erode away.  Towards the left-center of the contour map, where the brown meets the darker green, is what appears to be a sinuous thick brown line. It is really an amalgamation of contours, spaced closely, due to the extremely steep slopes and cliffs that make up the Western Ghat escarpment. There is a sudden fall there from the plateau to the coastal plain. Contour interval is 100 feet. 

Himalaya-Tibetan Plateau

Notice how the closely spaced contours in the lower left of the contour map give way to more openly spaced contours towards the top right of the map. This is the transition from the Gharwal Himalaya in to the Tibetan Plateau. The Himalaya, because it receives more rainfall, has more prominent relief, formed by rivers carving deep valleys, and glaciers gouging out rock faces into steep sided, sharp edged mountains. The Tibetan Plateau, although also standing high at around 4500 meters, is in the rain shadow region. It shows less relief. 

This has impacted the geology too. In the Himalaya, the rapid stripping of rock cover over millions of years has exhumed rocks which were once buried 25 kilometers below the surface. In Tibet, lesser erosion has meant that the surface geology is still dominated by 'supracrustal rocks', either volcanic or sedimentary rocks formed at shallow levels of the crust. Erosion has not dug deep down. Contour interval is 500 feet.

Nallamalai Fold Belt- Andhra Pradesh

This map shows the folded ridges of sedimentary rocks formed in the Cuddapah Basin around 1500-1600 million years ago. A fine example of topographic expression giving away the geologic structure of the rocks. Contour interval is 200 feet. 

I would urge those readers who are on Twitter to follow Raj Bhagat Palanichamy's excellent account (@rajbhagatt). He is a mapper par excellence.

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