¿Cómo obtener la escala correcta y el valor total de un mapa de la población mundial utilizando GEE?

Question

Estoy tratando de entender worldpop datos utilizando GEE.

Mientras que la escala debería ser de 100 m., utilizando la siguiente línea de código obtengo una escala de '111319.49079327357'

var pop_scale = pop2015.select(['population']).projection().nominalScale();

print('Escala pop: ', escala_pop);

Además, cuando intento estimar la población mundial utilizando una escala de 100 m como debería ser, la población mundial total es de unos 6.200 millones en 2015, cuando debería ser en realidad de unos 7.300 millones de personas.

https://code.earthengine.google.com/76f608f35460bd6287173cef2f9e940c

¿Cómo puedo obtener la escala correcta utilizando los datos originales para empezar a trabajar con un valor realista de la población mundial total, que es de unos 7.300 millones de personas?

Answer 1

Sobre el baremo "erróneo" que se obtiene cuando se llama nominalScale() : El error que estás cometiendo es obtener la escala de la ImageCollection con mosaico y no de una Imagen inalterada en la ImageCollection.

El mosaico de la colección devuelve un objeto diferente que es calculado sobre la marcha por Earth Engine utilizando la proyección por defecto WGS84 . Este objeto tiene una escala nominal diferente de 1 grado que corresponde a los 111319,5 m que se obtienen. Si obtienes la escala directamente de una Imagen obtienes los 92,76 m apropiados que corresponden a 3 segundos de arco. Vea también el código de abajo para una comparación.

var worldPop = ee.ImageCollection("WorldPop/GP/100m/pop");

var WP_2015  = worldPop
  .filter(ee.Filter.equals('year', 2015))
  .select('population');

print('No mosaic: ', WP_2015.first().projection().nominalScale())

var pop2015 =  WP_2015.mosaic();
var pop_scale = pop2015.projection().nominalScale();
print('Scale pop: ', pop_scale); // ee.Number

La diferencia en la población mundial que ves puede deberse a que los datos de Worldpop sobre EE se basan en datos censales y la cifra de 7.000 millones que esperas se basa probablemente en una proyección modelada por la ONU.

Los datos del censo pueden estar bastante desactualizados en algunos países, por lo que esto podría ser una razón para las diferencias que se ven. Para más información, también puede consultar el sitio web de worldpop .

Answer 2

No sé cuál es tu fuente pero, calculé la población total preservando la escala y el resultado fue 6308,113,647.54873. Siguiendo script calcula la población total por país y corrobora si se ha conservado la escala. La suma se calculó en el editor de la consola GEE y también se corroboró en una hoja de cálculo utilizando el CSV obtenido.

var worldPop = ee.ImageCollection("WorldPop/GP/100m/pop");

var WP_2015  = worldPop
  .filter(ee.Filter.equals('year', 2015))
  .select('population');

print("WP_2015", WP_2015);

var country = WP_2015.aggregate_array('country');

print(country);

var listOfImages = WP_2015.toList(WP_2015.size());

var popcountry = listOfImages.map(function (img) {

  var idx = ee.List(listOfImages).indexOf(ee.Image(img));

  var geom = ee.Image(img).geometry();

  var pop = ee.Image(img).reduceRegions({
    collection: geom,
    reducer: ee.Reducer.sum(),
    scale: 100,
    crs: 'EPSG:4326', // world wide crs 
  });

  pop = pop.aggregate_array('sum').get(0);

  return [ee.Image(img).projection().nominalScale(), pop, country.get(idx)];

});

var sumPop2015 = popcountry.map(function(ele) {

  return ee.List(ele).get(1);

}).reduce(ee.Reducer.sum());

print("sumPop2015", sumPop2015);

//print(popcountry;

var pop_country_list = ee.FeatureCollection(popcountry 
  .map(function(element){ 
    return ee.Feature(null,{
      'scale':ee.List(element).get(0),
      'population':ee.List(element).get(1),
      'country':ee.List(element).get(2)
    }
    )}));

//Export.table.toAsset(popcountry, 'worldPopTest', 'worldPopTest')
Export.table.toDrive({
  collection:pop_country_list, 
  folder: "GEE_Folder",
  description:'Population_World_2015',
  fileFormat: 'csv'
  });

Después de ejecutar arriba script, tengo en mi Google Drive siguiente archivo CSV. Se puede observar que la escala se conservó en cada cálculo y es la misma para todos los casos (92,7662420315015 metros).

system:index,country,population,scale,.geo
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