Chapter 11: Image Collection Iteration¶
This chapter provides a workflow to iterate through an image collection and calculate the cumulative NDVI difference for imagery in Rocky Mountain National Park, Colorado, United States. The full GEE code can be found here.
Functions¶
/**
* Calculate and add NDVI band to Landsat 8 image
* @param {ee.Image} image - Landsat 8 image
* @return {ee.Image} - Landsat 8 image with NDVI band added
*/
var add_ndvi = function(image) {
var ndvi = image.normalizedDifference(['B5', 'B4']).rename('NDVI');
return image.addBands(ndvi);
};
/**
* Mask Landsat 8 image with cloud and shadow masks
* @param {ee.Image} image - Landsat 8 image
* @param {ee.List} list - List with initialize image
* @return {ee.Image} - Masked Landsat 8 image
*/
var accumulate = function(image, list) {
// Get the latest cumulative anomaly image from the end of the list with
// get(-1). Since the type of the list argument to the function is unknown,
// it needs to be cast to a List. Since the return type of get() is unknown,
// cast it to Image
var previous = ee.Image(ee.List(list).get(-1));
// Add the current anomaly to make a new cumulative anomaly image
var added = image.add(previous)
// Propagate metadata to the new image
.set('system:time_start', image.get('system:time_start'));
// Return the list with the cumulative anomaly inserted
return ee.List(list).add(added);
};
Data Acquisition & Preprocessing¶
// Define boundary for Rocky Mountain National Park, Colorado (from GEE Asset)
var rmnp_boundary = ee.FeatureCollection("users/calekochenour/Rocky_Mountain_National_Park__Boundary_Polygon");
// print(rmnp_boundary);
// Define Landsat 8 collection
var landsat8_t1_sr = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR');
// Add NDVI band to each image in the collection (with mapping)
var landsat8_t1_sr_ndvi = landsat8_t1_sr.map(add_ndvi).select('NDVI');
// Filter Landsat 8 NDVI collection for June-September for 2013-2018 (baseline/reference data)
var rmnp_summer_ndvi = landsat8_t1_sr_ndvi
.filter(ee.Filter.calendarRange(2013, 2018, 'year'))
.filter(ee.Filter.calendarRange(6, 9, 'month'))
.filterBounds(rmnp_boundary)
.sort('system:time_start', false);
print(rmnp_summer_ndvi, 'NDVI');
Data Processing¶
// Compute mean for the entire NDVI collection; clip to RMNP
var rmnp_summer_ndvi_mean = rmnp_summer_ndvi.mean().clip(rmnp_boundary);
// Compute NDVI difference (image - reference mean) series for all images post-baseline (2019-2020)
var rmnp_ndvi_diff_series = landsat8_t1_sr_ndvi
.filter(ee.Filter.calendarRange(2019, 2020, 'year'))
.filter(ee.Filter.calendarRange(6, 9, 'month'))
.filterBounds(rmnp_boundary)
.map(function(image) {
return image.clip(rmnp_boundary)})
.map(function(image) {
return image.subtract(rmnp_summer_ndvi_mean).set('system:time_start', image.get('system:time_start'));
});
print(rmnp_ndvi_diff_series, "NDVI Difference Series (collection)");
// Compute min and max accumulated NDVI difference (for plotting min/max)
var ndvi_diff_min = ee.Number(rmnp_ndvi_diff_series.sum().reduceRegion({
reducer: ee.Reducer.min(),
geometry: rmnp_boundary.geometry(),
scale: 30,
maxPixels: 1e9
}).get('NDVI'));
var ndvi_diff_max = ee.Number(rmnp_ndvi_diff_series.sum().reduceRegion({
reducer: ee.Reducer.max(),
geometry: rmnp_boundary.geometry(),
scale: 30,
maxPixels: 1e9
}).get('NDVI'));
// Print min/max to the Console
print('Accumulated NDVI Difference Min: ', ndvi_diff_min);
print('Accumulated NDVI Difference Max: ', ndvi_diff_max);
// Type of object
print('Type: ', ndvi_diff_min.name());
// Get the timestamp from the most recent image in the reference collection
var time_0 = rmnp_summer_ndvi.first().get('system:time_start');
print('Time Zero: ', time_0);
// Initialize difference (anomaly) accumulation to 0 (image with all 0s and timestamp time_0)
var initialized_accumulation = ee.List([
// Rename the first band from 'constant' (default) to 'NDVI'
ee.Image(0).set('system:time_start', time_0).select([0], ['NDVI']).clip(rmnp_boundary)
]); // This is a list
print('Initialzed accumulation: ', initialized_accumulation);
print('Type: ', initialized_accumulation.name());
// Create an ImageCollection of cumulative anomaly images (NDVI differences)
// Cast return object of .iterate() to type List, as it is by default unknown (ComputedObject)
var ndvi_diff_cumulative = ee.ImageCollection(
// .iterate() takes a function as param 1 and the initial state as param 2
ee.List(rmnp_ndvi_diff_series.iterate(accumulate, initialized_accumulation))
);
print('Accumulated collection via iteration: ', ndvi_diff_cumulative);
print('Accumulated collection via iteration type: ', ndvi_diff_cumulative.name());
Data Postprocessing¶
// No data postprocessing in this lab.
Data Visualization¶
// Define NDVI visualization parameters
var vis_params_ndvi = {
'min': -1,
'max': 1,
// palette: ['blue', 'white', 'green']
'palette': ['red', 'yellow', 'green']
};
// Define RMNP boundary visualization parameters
var empty = ee.Image().byte();
var vt_boundary_vis = empty.paint({
featureCollection: rmnp_boundary,
color: 1,
width: 3
});
// Center map to Rocky Mountain National Park, Colorado
Map.setCenter(-105.6836, 40.3428, 10);
// Add layers to map
Map.addLayer(
rmnp_summer_ndvi_mean,
vis_params_ndvi,
'Landsat 8 - NDVI Mean - Jun/Sep 2013/2020');
// Display accumulated NDVI differences (anomalies from 6-year mean)
Map.addLayer(
rmnp_ndvi_diff_series.sum(),
{
min: -2.5899382242647437,
max: 2.548139614675886,
palette: ['FF0000', '000000', '00FF00']
},
'Cumulative NDVI Difference'
);
// Add RMNP boundary to map
Map.addLayer(
vt_boundary_vis,
{'palette': 'FF0000'},
'RMNP Boundary');
// Define chart titles
var chart_titles = {
title: 'Cumulative NDVI Difference Over Time',
hAxis: {title: 'Time'},
vAxis: {title: 'Cumulative NDVI Difference'},
};
// Define test points (pixels) for charting
var ute_trail_west_tree = ee.Geometry.Point([-105.77114730224609, 40.44397929249563]);
var ute_trail_west_rock = ee.Geometry.Point([-105.76848655090332, 40.43672831058668]);
var ute_trail_east_tree = ee.Geometry.Point([-105.6988694765701, 40.39268322603617]);
var ute_trail_east_rock = ee.Geometry.Point([-105.69552207971951, 40.3929447073708]);
// Add charts to Console
print('Ute Trail West - Trees',
ui.Chart.image.series(
ndvi_diff_cumulative,
ute_trail_west_tree,
ee.Reducer.first(),
500)
.setOptions(chart_titles)
);
print('Ute Trail West - Rocks',
ui.Chart.image.series(
ndvi_diff_cumulative,
ute_trail_west_rock,
ee.Reducer.first(),
500)
.setOptions(chart_titles)
);
print('Ute Trail East - Trees',
ui.Chart.image.series(
ndvi_diff_cumulative,
ute_trail_east_tree,
ee.Reducer.first(),
500)
.setOptions(chart_titles)
);
print('Ute Trail East - Rocks',
ui.Chart.image.series(
ndvi_diff_cumulative,
ute_trail_east_rock,
ee.Reducer.first(),
500)
.setOptions(chart_titles)
);
Data Export¶
// No data export in this lab.