Masking FeatureCollection using image in Google Earth Engine

Question

I have a FeatureCollection with my study area, and an image with urban areas. I want to remove the areas in my FeatureCollection where the image has urban areas. How can this be done?

var studyArea = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017')
  .filter(ee.Filter.eq('country_na', 'Japan'));
Map.addLayer(studyArea, {}, "studyArea");

var mask = ee.Image("Tsinghua/FROM-GLC/GAIA/v10").clip(studyArea);
Map.addLayer(mask, {}, "mask");

https://code.earthengine.google.com/8cce1074c07cea956fe9aee77a98c336

Answer 1

This is possible in theory, but not necessarily a good idea. EE is great at working with large amount of pixels but less good at working with complicated features. To do this, you turn your image mask into a FeatureCollection, using reduceToVectors(). This works for simple and small images, but will be problematic for more complicated ones, resulting in a large amount of features.

I would suggest that you try to find a different way to attack your use-case, find away where you don't need to turn your image into features.

That said, here's how you could go about if your image was simpler - it will almost certainly not work with yours:

var mask = ee.Image(1)
  .clip(ee.Geometry.Point([139.8647, 35.9642]).buffer(10000))

var scale = 1000 // Tweak this
var vectors = mask.reduceToVectors({
  geometry: studyArea.geometry(),
  scale: scale,
  // Will increase the scale of processing to fit within maxPixels. 
  // You might want to drop this if you need to know the exact scale you've worked with
  bestEffort: true, 
  maxPixels: 1e6 // Tweak this
})

var filtered = studyArea.geometry()
  .difference(vectors, scale)
  
Map.addLayer(vectors, null, 'vectors', false)
Map.addLayer(filtered, null, 'filtered')

https://code.earthengine.google.com/d9f69229ca8f88586f731de8c65d72db