Can't visualize ImageCollection using Map.addLayer()

Question

I was trying to retrieving chlorophyll-a levels using Landsat9. After processed it using this function :

 //Clorophyll-a Levels
 // Coefficients
 var a0= 0.341
 var a1= -3.001
 var a2= 2.811
 var a3= -2.041
 var a4= 0.0400

function R(img){
return img.expression(
      'log10(blue/green)',
      {
        'blue': img.select('B2'),
        'green': img.select('B3')
      });
}

var Ratio = citra.map(R)

function chlor(img){
return img.expression(
      '10**(a0+a1*R+a2*R**2+a3*R**3)+a4',
      {
        'a0': a0,
        'a1': a1,
        'a2': a2,
        'a3': a3,
        'a4': a4,
        'R': Ratio
      })
}

var chl = citra.map(chlor)

var ChlVisPar = {min:0.4,max:1.5,palette:['blue','lime','yellow','orange','red']}
Map.addLayer(chl,ChlVisPar,'Chlor-a')

I cant visualize using the Map.addLayer(chla,ChlVisPar,'Chlor-a') and I got messages:

Layer error: Image.pow, argument 'image1': Invalid type.
Expected type: Image.
Actual type: ImageCollection.

How do I solve this?

Answer 1

The error is not because you're trying to add an ee.ImageCollection to the Map. Actually, when you do that, the function adds a mosaic of all images inside the collection, and the inspector shows the values of each image as a series.

So, before adding each individual image into the Map, you need to fix the error.

The method ee.ImageCollection.map returns an ee.ImageCollection, thus the variable Ratio is of that type. In the function chlor you're trying to use Ratio as a variable in the expression, but you can only use ee.Image

This is how I'd solve it:

function R(img){
  return img.expression(
        'log10(blue/green)',
        {
          'blue': img.select('B2'),
          'green': img.select('B3')
        });
}

function chlor(img){
  var r = R(img)
  return ee.Image().expression(
        '10**(a0+a1*R+a2*R**2+a3*R**3)+a4',
        {
          'a0': a0,
          'a1': a1,
          'a2': a2,
          'a3': a3,
          'a4': a4,
          'R': r
        })
}

https://code.earthengine.google.com/eea2a43e47f2f84e8d67d624bbf37cf9

Then, you can add each image on the Map using different techniques. This is what I would do:

var addIC = function(col, property) {
  // choose a property to use as filter and img name
  property = property || "system:index"
  // fetch all values for that property
  var allProp = col.aggregate_array(property).getInfo()
  // loop over each value of the property
  for (var i in allProp) {
    var prop = allProp[i]
    // filter the IC with the property value and get one image
    var i = col.filter(ee.Filter.eq(property, prop)).first()
    // add to map
    Map.addLayer(i, ChlVisPar, prop)
  }
}

addIC(chl)

https://code.earthengine.google.com/28690cc4e4334673d27163b2954d4870