How does Google Earth Engine handle scale?

Question

I read the scaling documentation for Google Earth Engine, but I didn't understand, so I need to make an example. I have two collections: one at 4 km resolution (MODIS) and one at 10 meters resolution (Sentinel). I want compare two bands, say the NDVI. My point is I don't want to compare a 10 meters pixel with a 4 km pixel, but I want to have enough pixels from Sentinel to cover the same area of MODIS and maybe use a ee.Reducer.Mean() to get a single value. Does this code do what I want?

point=ee.geometry.Point(11.05,37.58)

var chart_sentinel=ui.Chart.image.series({
    imageCollection: sentinel_ndvi,
    region: ee.Feature(point).geometry(),
    reducer:ee.Reducer.mean(),
    scale:4000
    });

print(chart_sentinel)
var chart=ui.Chart.image.series({
    imageCollection: modis_ndvi,
    region: ee.Feature(point).geometry(),
    reducer:ee.Reducer.mean(),
    scale:1
    });

Am I actually comparing approximately the same area?

Answer 1

You're probably best off using the same scale for both charts. Pick the largest scale you have - MODIS. EE will use its image pyramids to get pixel values at the specified scale. So, the Sentinel values charted will be aggregates of 10 m pixels, and will be comperable to the charted MODIS values.

var point = ee.Geometry.Point([37.58, 11.05])

Map.setOptions('HYBRID')
Map.centerObject(point, 12)
Map.addLayer(point)

var modis = ee.ImageCollection('MODIS/006/MOD13A1')
  .filterBounds(point)
  .select('NDVI')
  
var s2 = ee.ImageCollection('COPERNICUS/S2')
  .filterBounds(point)
  .map(function (image) {
    return image
      .normalizedDifference(['B8', 'B4'])
      .multiply(10000) // Match MODIS scale (where NDVI has been multiplied by 10,000)
      .int16()
      .updateMask(image.select('QA60'))
      .rename('NDVI')
      .copyProperties(image, image.propertyNames())
  })

var modisChart = ui.Chart.image.series({
  imageCollection: modis, 
  region: point, 
  reducer: ee.Reducer.mean(), 
  scale: 500 // The MODIS image collection has a 500m scale
})
print(modisChart)

var s2Chart = ui.Chart.image.series({
  imageCollection: s2, 
  region: point, 
  reducer: ee.Reducer.mean(), 
  scale: 500 // Use same scale as MODIS imagery
})
print(s2Chart)

https://code.earthengine.google.com/80b2274c229ed030c0a644b2516c6240