Band Names in Google Earth Engine - Modis LST data

Question

I am working with some modis LST data, and am running into the following problem:

I am running two functions on the data, which I will need to combine later. For that reason, the band names need to be the same. When I run the code below however, one output has band [LST_DAY_1km], and one has band [constant]. This obviously doesnt add up, and I am confused where the problem is coming from, given I'm using the same input data.

See code below:

    // Use the expression function to generate scaled LST.

var scaledlst1 = lst.map(function (lst) {
  return lst.expression(
    '(lst/30)',{
      'lst': lst.select('LST_Day_1km')
    });

});

Map.addLayer(scaledlst1, {band:'LST_Day_1km' ,max: 1 , min: 0}, 'scaledlst1');

//

var scaledlst2 = lst.map(function (lst) {
  return lst.expression(
    '(2.5-(0.05*lst))',{
      'lst': lst.select('LST_Day_1km')
    });

});

Map.addLayer(scaledlst2, {band:'LST_Day_1km', max: 1 , min: 0}, 'scaledlst2');

Does anyone have any suggestions as to why this might be?

My next step is to take the minimum value per pixel between the two products:

//select images

var ListOfImages1 = scaledlst1.toList(scaledlst1.size())
var ListOfImages2 = scaledlst2.toList(scaledlst2.size())


var LST = ee.Image(ListOfImages1.get(0))
var LST2 = ee.Image(ListOfImages2.get(0))


//Take min value
var scaledLST = ee.ImageCollection.fromImages([LST,LST2]).min()

Map.addLayer(scaledLST, {max: 1 , min: 0}, 'Scaled LST');

Which yields the following error:

Scaled LST: Tile error: Expected a homogeneous image collection, but an image with incompatible bands was encountered: First image type: 1 bands ([LST_Day_1km]). Current image type: 1 bands ([constant]). Image ID: 1 Some bands might require explicit casts.

Answer 1

It seems that the "constant" band name arises because the second expression begins with a constant, which is treated as a single-band constant image. Note that with a small adjustment to your code, the resulting band names in scaledls2 is the expected "LST_Day_1km":

// Select a geometry for reproducibility
var geometry = ee.FeatureCollection('TIGER/2016/Counties')
  .filter(ee.Filter.eq('NAME', 'Waldo'));
print(geometry);
// Explicitly call dataset
var MOD = ee.ImageCollection("MODIS/006/MOD11A1");
var lst = MOD.filterDate("2019-01-01", "2019-02-01").filterBounds(geometry);
// First scaling operation
var scaledlst1 = lst.map(function (lst) {
  return lst.expression(
    '(lst / 30)',{
      'lst': lst.select('LST_Day_1km')
    });

});
print("scaledlst1",scaledlst1);
Map.addLayer(scaledlst1, {band:'LST_Day_1km' ,max: 1 , min: 0}, 'scaledlst1');

// Second scaling operation
var scaledlst2 = lst.map(function (lst) {
  return lst.expression(
    '(lst * (-0.05)) + 2.5',{
      'lst': lst.select('LST_Day_1km')
    });

});
print("scaledlst2",scaledlst2);
Map.addLayer(scaledlst2, {band:'LST_Day_1km', max: 1 , min: 0}, 'scaledlst2');

Also note Kevin Reid's discussion here on the tile error issue and how to work around that.