How it works

The Sentinel-1 satellites pass over the same spot again and again and record the radar echo. By comparing the images with a technique called radar interferometry (InSAR), the service works out how much the ground has moved towards or away from the satellite between passes.

The service has three product levels: Basic, Calibrated and Ortho. Ortho combines ascending and descending orbits to separate vertical movement from east-west movement and resamples it to a 100 by 100 metre grid, which makes it easier for non-specialists to read.

Why it matters when buying

The ground moves for natural reasons, such as tectonic activity, and human ones, such as groundwater extraction or mining, and that movement can affect buildings. Knowing whether the area is moving, and whether it moves more than its surroundings, helps you decide what to ask the owners' association and whether a professional visit is worth it before you sign.

What the HOUSINGFAX report tells you

Check 13, «Ground movement», uses the vertical Ortho product from the 2019-2023 release. It sums up the points measured within 150 metres of the building as a median vertical velocity, in mm a year, and compares it with a reference ring from 200 to 500 metres away. It also shows the spread, the gradient and the number of points, and plots them on aerial imagery coloured by velocity.

Look through the sample report and check your home for free.

What the satellite data does not tell you

It measures surface movement, not its cause, and does not by itself describe the foundations or the state of the building. No points nearby means the radar has no stable signal there, not that the ground is still. The report sums up what the official sources say and does not replace an on-site technical inspection of the building.

Short answers

What does EGMS stand for?

European Ground Motion Service, the ground motion service of the European Union's Copernicus programme.

How precisely does it measure ground movement?

Copernicus speaks of millimetre precision. The data is expressed as an average velocity in millimetres a year at each measured point.

How often is it updated?

Every year. The HOUSINGFAX report uses the 2019-2023 release of the vertical Ortho product.

If the ground is sinking, is the building in danger?

Not necessarily. What matters is how much it moves, whether the building moves differently from its surroundings and whether there are cracks. If the movement stands out, ask a professional to look at it before you sign.

Official sources you can consult

The links let you review the primary source. Their inclusion does not widen their purpose or turn a piece of context into a diagnosis.