# Understanding the Unit of Measurement for Land Subsidence Map with 6 Months Data

Hello everyone,

I’m currently working on deriving a land subsidence map using the SNAP toolbox based on 6 months of data. While I’ve made progress with the processing, I’m looking to clarify the unit of measurement for the resulting subsidence map.

As far as I understand, the unit of measurement should be in meters due to the vertical nature of land subsidence. However, my specific query pertains to how to express this unit when the map is based on a 6-month time frame. Would it be appropriate to express it as “meters per 6 months” or simply “meters (m)” to represent the vertical change in elevation over this half-year period?

Any insights or guidance on this matter would be greatly appreciated. Thank you!

Hello Karishma,

You did not explain what steps you took to make your land subsidence map. When you first make an interferogram and unwrap it, it is in radians. Then the SNAP convert to displacement converts the unwrapped phase to line-of-sight displacements in meters.

If you make a 6-month interferogram and convert that to displacement, it will be in meters. Again, it is line-of-sight displacement, not purely vertical, over the time interval of your interferogram.

If you process many interferograms covering 6 months or longer and then do time-series analysis with a package such as STaMPS or MintPy, then the cumulative displacements are usually in meters relative to your reference scene. The time-series software may also do linear velocity fit to the set of dates you processed and provide a mean velocity in meters/year or some other units such as cm/y or mm/y. These measurements are still in the radar line-of-sight direction, not purely vertical.

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Thank you so much for the response, sir!

I made a six months interferogram and then converted it to displacement. My idea was to study the land subsidence in a particular region for the last 30 years. For that, I considered the monsoon/flood time frame of each year. So, I will be generating the land subsidence map of each year’s 6 months data for 30 years.

If I use the six months interferogram → displacement step, Should the subsidence measurements be presented in meters per 6 months or meters per year?

As you have replied, do I need to use STaMPS or MintPy to analyse the land subsidence rate for 30 years in that particular region ?

If you only process interferograms during half of the year, then you are only measuring the displacement during that time. You would need a strong geological reason to believe that there is no displacement in the other half of the year. You should try making one-year interferograms from the same season to look at the displacement during the rest of the year if you want to know the long-term rates. Time series analysis cannot predict how the displacement varies over a long time without a connected set of interferograms. In places where there are large variations in precipitation and temperature during the year, the groundwater extraction rate changes from wet to dry season, so the displacements can be very different at different times of the year.