Mapping Land Subsidence in Kolkata

You may have seen old buildings which appear distorted, before. Where a part of the building has caved in and hence does not appear flat / leveled to the eye. This could happen due to Land Subsidence i.e. lowering of the ground level due to excessive pumping of water underneath, among other factors. India is much exposed to this phenomenon due to our increasing water needs and it is a risky & costly affair too.

With radar remote sensing, it is possible to map the changes in ground level over a period of time. The process is called Differential Interferometry. Simply put, electromagnetic waves transmitted by the satellite bounce back after hitting the earth's surface. The time it takes for the wave to return, among other factors, helps us to understand and process the properties of the surface underneath. In differential interferometry, two or more images of the same surface over a period of time are analysed to see the time differential of the wave return, i.e. Backscatter, which gives an indication of the changes in the depth of the land below. There are three possible outcomes - subsidence, no change and uplift. To learn about radar remote sensing and interferometry you may opt for this informative online course.

The result of the Land Subsidence - Kolkata exercise is depicted in the map below -

Extracted using two Sentinel-1A SLC images roughly one year apart (January 2020 v/s January 2019). The exercise is only meant to be informative and the results are indicative in nature. The outcome has not been validated - methodology can be shared. Kolkata city extent is approximated - includes outskirts too. True North is towards the North-East of the map. Much thanks to RUS Copernicus for the training material.

To interpret the map - Yes, the city is sinking and at a discomforting rate. And there is a visible pattern to it with supporting historical evidence. The highest subsidence is towards the northern suburbs of the city - max. sinking at 6 cm. The central / main city area has relatively less subsidence at 1-3 cm., nonetheless discomforting too. Recent developments such as New Town and Rajarhat have moderately high subsidence levels at around 3 cm. Certain sections of the southern tips of the city (Maheshtala/Barisha) have an uplift at 0-3 cm.

(What the map doesn't show is the situation further north of Barrackpore - I had not used it as it was technically much outside the city extent - the level of land subsidence there was >10 cms!)

You may wonder why there are blank spaces between the colored areas of the map. These were due to low coherence levels of < 0.4. Simply put, both the images being studied did not demonstrate similar land properties for the areas covered in those blank pixels. Hence the analysis of those land areas wouldn't be an apple to apple comparison and hence were avoided. The trend of subsidence can be easily discerned, in spite of it, I would presume.

(Possible technical reason - SRTM 1sec DEM used may not be of the desired high resolution. Tried to load Cartosat's Level 3 DEM - the highest resolution in the world currently - on the processing software but was unable to load).

Can you spot any additional patterns or have local area knowledge explaining this phenomenon - excessive groundwater extraction? Would you like to study the changes over a larger period of time?

Feel free to share your thoughts as comments or via email at projects@mapmyops.com.

I hope this exercise helped you appreciate the risk of reckless water usage - how we consume and manage water resources would play an important role in determining the sustainability of our planet.

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Regards,

Arpit