4

I would like to use speckle lee refined filter on Sentinel-1 collection. But in every document I found, it says that the input should be linear not in dB. Normally S-1 data in GEE is in dB.

How do I convert it to linear?

Couldn't find any code for that.

var filter = ee.Filter.and(
  ee.Filter.bounds(Map.getBounds(true)),
  ee.Filter.date('2022-01-01', '2022-02-01')
)

var db = ee.ImageCollection('COPERNICUS/S1_GRD')
  .filter(filter)
  .mosaic()

// How do I covert db to linear?

Map.addLayer(db, {bands: 'VV,VH,VV', min: -25, max: 0})  

https://code.earthengine.google.com/e9c1de10f5b32f56f8a3857ebbadc4c5

1 Answer 1

10

You can covert an S1 image in db to linear with this:

ee.Image().expression('pow(10, db / 10)', {
  db: db.select(['VV', 'VH'])
})

However, there are two image collections with S1 imagery, and one of them (COPERNICUS/S1_GRD_FLOAT) already comes in linear. Here's a complete example showing conversion back and forth from db to linear:

var filter = ee.Filter.and(
  ee.Filter.bounds(Map.getBounds(true)),
  ee.Filter.date('2022-01-01', '2022-02-01')
)

var db = ee.ImageCollection('COPERNICUS/S1_GRD')
  .filter(filter)
  .mosaic()

var linear = ee.ImageCollection('COPERNICUS/S1_GRD_FLOAT')
  .filter(filter)
  .mosaic()
    
Map.addLayer(db, {bands: 'VV,VH,VV', min: -25, max: 0})  
Map.addLayer(toDb(linear), {bands: 'VV,VH,VV', min: -25, max: 0})
Map.addLayer(toDb(toLinear(db)), {bands: 'VV,VH,VV', min: -25, max: 0})  

  
function toLinear(db) {
  return db.addBands(
    ee.Image().expression('pow(10, db / 10)', {
      db: db.select(['VV', 'VH'])
    }),
    null, true // Replace the bands to keep image properties
  )
}
  
function toDb(linear) {
  return linear.addBands(
    ee.Image().expression('10 * log10(linear)', {
      linear: linear.select(['VV', 'VH'])
    }),
    null, true // Replace the bands to keep image properties
  )
}

https://code.earthengine.google.com/a57f1bef88f429abe73529844d219ae8

1

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.