Calculate the difference of max image and min image in GEE

Question

I am preparing parameters for the RF classification model. I calculated the NDVI,NDWI,EVI,NDBI from landsat8 time series. Then I calculated the minimum image colletcion (var min) and maximum image colletion (var max1) of the time series. Both of the two image collections include NDVI,NDWI,EVI,NDBI.

I define a function (DIFF) to further calculate the difference between the maximum and minimum image collection (including the difference between maximum value and minimum value of NDVI,NDWI,NDBI,EVI), but made mistakes in the codes (telling maxmin.map is not a function).

I think there should be easier way to calculate the difference, but I am still not practiced in GEE...

Link: https://code.earthengine.google.com/1fe1753f49d5ebbbb7aa4d86040e839c

var s2col = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
.filter(ee.Filter.calendarRange(2018, 2019, 'year'))
  .filter(ee.Filter.calendarRange(1, 3, 'month')).filterBounds(Linzhi);
  
//NDVI
function NDVI(image) {
  return image.addBands(image.normalizedDifference(["B5", "B4"]).rename("NDVI"));
}

//NDWI
function NDWI(image) {
  return image.addBands(
    image.normalizedDifference(["B3", "B5"])
         .rename("NDWI"));
}
  
//NDBI
function NDBI(image) {
  return image.addBands(
    image.normalizedDifference(["B6", "B5"])
         .rename("NDBI"));
}

//difference
function DIFF(image){
  return image.addBands(ee.Image.cat([image.select("NDVI_max").subtract(image.select("NDVI_min")).rename("NDVI_diff"),
        image.select("NDWI_max").subtract(image.select("NDWI_min")).rename("NDWI_diff"),
        image.select("NDBI_max").subtract(image.select("NDBI_min")).rename("NDBI_diff"),
        image.select("EVI_max").subtract(image.select("EVI_min")).rename("EVI_diff")]));
   }
   
//EVI
function EVI(image){
  return image.addBands(image.expression(
              '2.5*(NIR-RED)/(NIR+6*RED-7.5*BLUE+10000)',{
              NIR:image.select('B5'),
              RED:image.select('B4'),
              BLUE:image.select('B2'),
            }).float().rename('EVI')); 
}

var l8Col = ee.ImageCollection("LANDSAT/LC08/C01/T1_SR")
              .filterBounds(Linzhi)
              .filterDate("2018-1-1", "2020-10-13")
              .filter(ee.Filter.lte("CLOUD_COVER", 50))
              .map(NDVI)
              .map(NDWI)
              .map(NDBI)
              .map(EVI);


var min = l8Col.reduce(ee.Reducer.intervalMean(0,5)).select(["NDVI_mean","NDWI_mean","NDBI_mean","EVI_mean"],["NDVI_min","NDWI_min","NDBI_min","EVI_min"]);
print(min,"min");
var max1 = l8Col.reduce(ee.Reducer.intervalMean(95,100)).select(["NDVI_mean","NDWI_mean","NDBI_mean","EVI_mean"],["NDVI_max","NDWI_max","NDBI_max","EVI_max"]);
print(max1,"max");

var maxmin = min.addBands(max1);
var maxmin1 = maxmin.map(DIFF);
print(maxmin,"maxmin");

Answer 1

If you want the difference you can subtract the two images (max and min) right away. This would work:

var diff = max1.subtract(min);
print(diff,"diff, max min");

However be aware that the bands won't be renamed like this.