Google Earth Engine – How to Get Global Min and Max Values of Image

google-earth-engine

I want to determine the 'global' minimum and maximum value for a calculated variable in Google Earth Engine. Essentially, my goal is to use a global geometry in reduceRegion(). However, Earth Engine complains when I use an unbounded geometry (image.geometry()) as my region argument. Is there a way to get the range of a calculated image in GEE?

Here is my code:

////////////////////////////////////////////////////////
// Get global min and max of SPEI from TerraClim data///
////////////////////////////////////////////////////////
// Get dataset
var terraClim = ee.ImageCollection('IDAHO_EPSCOR/TERRACLIMATE').filterDate('2017-10-01', '2018-09-30');
terraClim = terraClim.select('pr', 'pet');

// Calculate SPEI from the precipitation and potential evapotranspiration bands
var spei = terraClim.map(function(img){
  img = ee.Image(img);
  var pr = img.select("pr");
  var pet = img.select("pet");
  var spei = img.expression('pr - pet', {pr:pr,pet:pet});
  return img.select()
    //.addBands(pr.rename('pr'))
    //.addBands(pet.rename('pet'))
    .addBands(spei.rename('spei'))
    .set('system:time_start', img.get('system:time_start'));
});

Map.addLayer(spei.first(), 
{min:-2000, max:100, palette:['white','red', 'orange', 'yellow', 'brown', 'green', 'blue', 'purple']}, 'spei');

// Get global min and max values of SPEI using reduceRegion
var spei_min = spei.first().reduceRegion({
  reducer: ee.Reducer.min(), 
  geometry: spei.geometry(), 
  bestEffort: true, 
  });

var spei_max = spei.first().reduceRegion({
  reducer: ee.Reducer.max(), 
  geometry: spei.geometry(), 
  bestEffort: true, 
  });
  
print('Global minimum SPEI:', spei_min);
print('Global maximum SPEI:', spei_max);

Here is the error I get when I try to print the resulting dictionaries:

Image.reduceRegion: Provide 'geometry' parameter when aggregating over an unbounded image.

Link to my code in code editor: https://code.earthengine.google.com/32957edb7eb0b4797b503dc7605a10b5

Best Answer

Use ee.Geometry.BBox(-180, -90, 180, 90) as the geometry argument.

Related Solutions

Google Earth Engine – Image Collection Minimum and Maximum Extraction in Google Earth Engine

img.reduceRegion returns a dictionary with the result of the reduction in each band, so you have to get the result for the band that you are interested in, which I understand is NDVI.

var modis = ee.ImageCollection("MODIS/006/MOD13Q1")
//.filterBounds(geometry) // no need to filter by bounds in MODIS
.filterDate("2000-01-01","2001-01-01")
.select("NDVI");

print(modis);

var mod13 = modis.map(function(img){
  return img.multiply(0.0001)
  .copyProperties(img,['system:time_start','system:time_end']);
});

print(mod13);

var vci = mod13.map(function(img){
 var id = img.id();
 var min =  img.reduceRegion(ee.Reducer.min(), geometry,250).get('NDVI');
 var max = img.reduceRegion(ee.Reducer.max(), geometry,250).get('NDVI');
 return img.expression(
   "(NDVI-min)/(max-min)",{
     "NDVI" : img,
     "max" : ee.Number(max),
     "min" : ee.Number(min)
   }).copyProperties(img,['system:time_start','system:time_end']);
});
print(vci)

Google Earth Engine – Calculate Landsat 7 NDVI Time Series and Get Maximum NDVI Value

To get the maximum ndvi you could use qualityMosaic function. To be able to know which image ID the pixel comes from is more complicated because as it is not a numerical property, you have to encode it some how. This is how I'd do it, but it's just one way, there could be other ways (may be easier)

l7 = ee.ImageCollection("LANDSAT/LE07/C01/T1_SR")
l7 = l7.filterDate("1999-01-01", "2018-04-30").filterBounds(geometry)
       .map(function(image) { 
          var ndvi=image.expression('float(b("B4") - b("B3")) / (b("B4") + b("B3"))').rename('ndvi');
          ndvi = ndvi.multiply(100)
          return image.addBands(ndvi); 
        })
        // add date band
        .map(function(img){
          var millis = img.date().millis()
          var days = millis.divide(1000).divide(60).divide(60).divide(24).toInt() // 86400000
          var dateband = ee.Image.constant(days).rename('date')
          return img.addBands(dateband)
        })
        // add PATH ROW band
        .map(function(img){
          var path = ee.Number(img.get('WRS_PATH')).toInt()
          var row = ee.Number(img.get('WRS_ROW')).toInt()
          var pathi = ee.Image.constant(path).rename('path')
          var rowi = ee.Image.constant(row).rename('row')
          return img.addBands(pathi).addBands(rowi)
        })
        // cast to Int
        .map(function(img){
          return img.toUint16()
        })

// function to get the image ID from date, path and row
var get_id = function(date, path, row) {
  var d = ee.Date(ee.Number(date).multiply(86400000))
  var i = l7.filterDate(d, d.advance(1, 'day'))
            .filterMetadata('WRS_PATH', 'equals', path)
            .filterMetadata('WRS_ROW', 'equals', row)

  return ee.Image(i.first()).id()
}

// apply qualityMosaic
var mosaic = l7.qualityMosaic('ndvi')
Map.addLayer(mosaic, {bands:['ndvi'], min:0, max:80})

// function to be able to print the image ID when clicking on the Map over the image
var click = function(coords) {
  var lon = coords.lon
  var lat = coords.lat
  var p = ee.Geometry.Point([lon, lat])
  var values = mosaic.reduceRegion(ee.Reducer.first(), p, 30)
  var date = get_id(values.get('date'), values.get('path'), values.get('row'))
  print('ID on point', p, date)
}
Map.onClick(click)

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