Printing max and min values of Landsat NDVI time series to GEE console

Question

I am trying to extract the min and max values of an NDVI time series and print those values to the console, so I could then subtract those values to determine the seasonal stability of an area. Most of the questions and answers in this forum explain how to reduce a collection to a Max NDVI composite image. I just want the values. I know I can download the CSV and get the values from there, but I am wishing to do this programmatically in Google Earth Engine.

Note: in the code below, the bounds are filtered by a polygon which I couldn't figure out how to attach. Creating any sample polygon should work to run the code.

//This approach starts with a collection of L8 TOA images and filters them by cloud cover 
  // Note: - CC is calculated only over the ROI (ie. not whole image CC score).
  //       - the simpleCloudScore algorithm only works with TOA images.

// --------------Set Location and Filter by Date and Cloud-------------------//

var MAX_LIST_SIZE = 100;

var L8_TOA = ee.ImageCollection("LANDSAT/LC08/C01/T1_RT_TOA")

    .filterDate('2013-12-25', '2016-12-25')
    .filter(ee.Filter.eq('WRS_PATH', 41))
    .filter(ee.Filter.eq('WRS_ROW', 36)); 
var count = L8_TOA.size();

var c = L8_TOA.filterBounds(geometry);        

var withCloudiness = c.map(function(image) {
  var cloud = ee.Algorithms.Landsat.simpleCloudScore(image).select('cloud');
  var cloudiness = cloud.reduceRegion({
    reducer: 'mean', 
    geometry: geometry,                        
    scale: 30,  });
  return image.set(cloudiness);});

// Here is where the computed cloud score threshold is set!
var filteredCollection = withCloudiness.filter(ee.Filter.lt('cloud', 5));

// print('L8 TOA Filtered Collection, by date and cloud',filteredCollection);

//---------------------------Add NDVI Band--------------------------------//

// Create an NDVI band  
var addNDVI = function(filteredCollection) {
var nir = filteredCollection.select('B5');
var red = filteredCollection.select('B4');
var ndvi = filteredCollection.normalizedDifference(['B5', 'B4']).rename('NDVI');
return filteredCollection.addBands(ndvi);};

// Add the NDVI band to each image in the collection

var withNDVI = filteredCollection.map(addNDVI);

// print('L8 Filtered Collection with NDVI band',withNDVI)

// Filter collection to only include NDVI Band

var NDVI_bands = withNDVI.select('NDVI');

print('L8 Collection with cloud filtering added', NDVI_bands)

//--------------------------Create Map and Chart-----------------------------//

// Create an image time series chart, average it over the ROI

var chart = ui.Chart.image.series({
  imageCollection: NDVI_bands,
  region: geometry,
  reducer: ee.Reducer.mean(),
  scale: 200});

// Add the chart to the map.

chart.style().set({
  position: 'bottom-right',
  width: '400px',
  height: '200px'});
Map.add(chart);

// Outline and center the map around coordinates.

var BakersfieldLayer = ui.Map.Layer(boundary, {color: 'FF0000'}, 'Bakersfield');
Map.layers().add(BakersfieldLayer);
Map.setCenter(-118.72, 35.4, 9);

// Create a label on the map.
var label = ui.Label('Click a point on the chart to show the image for that date.');
Map.add(label);

// When the chart is clicked, update the map and label.

chart.onClick(function(xValue, yValue, seriesName) {
  if (!xValue) return;

// Show the image for the clicked date.

  var equalDate = ee.Filter.equals('system:time_start', xValue);
  var image = ee.Image(L8_TOA.filter(equalDate).first());
  var l8Layer = ui.Map.Layer(image, {gamma: 1.3, min: 0, max: 0.3,});
  Map.layers().reset([l8Layer, BakersfieldLayer]);
  
// Show a label with the date on the map.

  label.setValue((new Date(xValue)).toUTCString());});

  // Predefine the chart titles.

var title = {
  title: 'Bakersfield site',
  hAxis: {title: 'Date'},
  vAxis: {title: 'NDVI'},};

print(Chart.image.series(NDVI_bands, geometry, ee.Reducer.mean(), 200).setOptions(title));

Answer 1

It seems you actually forgot to calculate the max and min values. Adding:

var NDVI_mean = NDVI_bands.mean();

var stats = NDVI_mean.reduceRegion({
  reducer: ee.Reducer.max().combine({
  reducer2: ee.Reducer.min(),
  sharedInputs: true
}),
  geometry: geometry,
  scale: 200,
  maxPixels: 1e13
});
  
print('Maximum NDVI',stats.getNumber('NDVI_max'));
print('Minimum NDVI',stats.getNumber('NDVI_min'));

Should do the trick.

EDIT :

I was calculating the geographic max of a temporal mean, rather than the temporal max of a geographic mean.

Here's the solution:

var stats = NDVI_bands.map(function(image) {
  
    var imgStats = image.reduceRegion({
      reducer: ee.Reducer.mean(),
      geometry: geometry,
      scale: 200,
      maxPixels: 1e13
    });
    
    return ee.Image.constant(imgStats.getNumber('NDVI')).toFloat();
    
});

print(stats);

var maxImg = stats.max();
var maxValue = maxImg.reduceRegion(ee.Reducer.max(),geometry,200).getNumber('constant');
print('Maximum NDVI:',maxValue);

var minImg = stats.min();
var minValue = minImg.reduceRegion(ee.Reducer.min(),geometry,200).getNumber('constant');
print ('Minimum NDVI:',minValue);

print('NDVI difference:',maxValue.subtract(minValue));

Answer 2

To print the minimum and maximum values of an image to the console, you simply need to use reducer: ee.Reducer.min().combine(ee.Reducer.max(), '', true) .

The following is an example which calculates the minimum and maximum values of an elevation raster for a given box extent.

//  Create box extent
var geometry = ee.Geometry.Rectangle({
  coords: [94.8,18.3,99.1,22.8],
  proj: 'EPSG:4326',
  geodesic: false
  });
  
//  Center map viewer
Map.centerObject(geometry,7);

//  Load elevation data
var elev = ee.Image("USGS/SRTMGL1_003").clip(geometry);

//  Retrieve min/max values
var MinMax = elev.reduceRegion({
  reducer: ee.Reducer.min().combine(ee.Reducer.max(), '', true),
  geometry: geometry,
  maxPixels: 1e13 // is required if geometry extent is large
});

//  Print to console
print(MinMax);
//  Visualize map (manual)
var elevViz = {min: -12, max: 2664, palette: ['000000', 'FFFFFF']};
Map.addLayer(elev, elevViz, 'SRTM elevation');