CODEWITHSUNDEEP
pythongoogle-earth-engine
Ayaz49
Ayaz49

Reputation: 355

Why normalizedDifference and simple band operation produces different results on Google Earth Engine?

I am exploring Google Earth Engine and trying some band operation. I have calculated NDVI using two methods,

  1. Calculating using normalizedDifference function
  2. Calculating using normal band operation.

But I have checked both returns different results, why is so? Is not it the same operation that runs in the normalized difference function.

Here is my code.

#Import earth engine
import ee
 
# Trigger the authentication flow.
ee.Authenticate()
 
# Initialize the library.
ee.Initialize()

# Load two 5 year Landsat 7 composite
landsat2008 = ee.Image('LANDSAT/LE7_TOA_5YEAR/2008_2012')






# Compute NDVI using normalizedDifference
ndvi2008_m1 = landsat2008.normalizedDifference(["B4", 'B3'])

##NDVI using method2 

diff=landsat2008.select('B4').subtract(landsat2008.select('B3'))
added=landsat2008.select('B4').add(landsat2008.select('B3'))

ndvi2008_m2 = diff.divide(added)

ndvi2008_m2==ndvi2008_m1

ndviParams = {'palette': ["#d73027", "#f46d43", "#fdae61", "#fee08b", "#d9ef8b", '#a6d96a', '#66bd63', '#1a9850']}



# Import the Folium library.
import folium

# Define a method for displaying Earth Engine image tiles to folium map.
def add_ee_layer(self, ee_image_object, vis_params, name):
  map_id_dict = ee.Image(ee_image_object).getMapId(vis_params)
  folium.raster_layers.TileLayer(
    tiles = map_id_dict['tile_fetcher'].url_format,
    attr = 'Map Data &copy; <a href="https://earthengine.google.com/">Google Earth Engine</a>',
    name = name,
    overlay = True,
    control = True
  ).add_to(self)

# Add EE drawing method to folium.
folium.Map.add_ee_layer = add_ee_layer

# Create a folium map object.
my_map = folium.Map(location=[9, 39], zoom_start = 2)

# Add the layer to the map object.
my_map.add_ee_layer(ndvi2008_m1, ndviParams, 'ndvi2008_m1')
my_map.add_ee_layer(ndvi2008_m2, ndviParams, 'ndvi2008_m2')
# Add a layer control panel to the map.
my_map.add_child(folium.LayerControl())

# Display the map.
display(my_map)

Upvotes: 1

Views: 185

Answers (1)

Ron Drori
Ron Drori

Reputation: 11

Casting the image to float solve this (in Javascript). Anyway, the difference is very (very) small.

Upvotes: 1

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