Advanced topics on image collections

In this tutorial you will learn some of the more advanced functions of a collection, such as:

  1. count available images and display the result with color codes;
  2. introduction to temporal sliders.

Count available images and display with color codes

From a collection of Sentinel 2 images, we will filter the images that cover the Sardegna island and have been acquired in Summer 2016 (from 2016-7-1 to 2016-9-30).

We first open an interactive map used as basis for the interactive visualisation and processing of a collection:

map = Map()
map

We also filter on cloudcoverpercentage (<=5), ensuring that the attribute “jrc_filepath” is not empty. Finally we count the number of images in the collection using the function ImageCollection.count():

coll = inter.ImageCollection("S2")
coll = coll.filterOnGeoName("Sardegna")
coll = coll.filterOnDate(2016,7,1, 2016,9,30)
coll = coll.filterOn("cloudcoverpercentage", "<=", 5)
coll = coll.filterOn("jrc_filepath", "<>", "")
coll.count()

We will now present a way to visualise the available images for each pixel, using the function ImageProcess.count(). This function takes two arguments:

  1. “bandName”: (e.g., “B11”);
  2. divider x (optional): value x by which the result is divided

The result is an image process that we will convert to a layer and add to the map:

p = coll.process().count("B11",2)
map.clear()
map.zoomToImageExtent(p)
tlayer = map.addLayer(p.toLayer())

The color coding applied uses a predefined color scheme, named “simple”. Alternatively, you can use “Rainbow”:

p.colorScheme("Rainbow")
map.clear()
map.zoomToImageExtent(p)
tlayer = map.addLayer(p.toLayer())

The simple palette is defined as follows with x denoting the value of the divider:

palette for image counts
Decimal Binary Color code
1x 001 Red
2x 010 Green
3x 011 Red+Green=**Yellow**
4x 100 Blue
5x 101 Red+Blue=**Magenta**
6x 110 Green+Blue=**Cyan**
7x 111 Red+Green+Blue=**White**

For example, if the divider is 2, all pixels displayed in white (which is the 7th color of the palette) are zones where 14 (that is 7 multiplied by 2) or more images are available.

Color scheme Simple Color scheme Rainbow

Temporal sliders

Temporal sliders can help in displaying all available acquisition dates for a specific Sentinel 2 images collection. To display the slider (created by using ipyWidgets) a call to the inter.temporalSlider method is used, that requires as arguments:

  1. the collection object;
  2. the map where the images have to be displayed;
  3. the name of a python function to be used as callback. This function will receive as argument a reference to the collection element and has to return a processing element to be displayed on the map. In this example a True Color RGB composition is returned by the getprocessing python function.
map = Map(center=[45.81, 8.628], zoom=14)
map

coll = inter.ImageCollection("S2")
coll = coll.filterOnGeoName("Ispra")
coll = coll.filterOnDate(2016,1,1, 2017,12,31)
coll = coll.filterOn("cloudcoverpercentage", "<=", 5)
coll = coll.filterOn("jrc_filepath", "<>", "")

def getprocessing(collection):
    return collection.process().bands("B04","B03","B02",1.1).opacity(255)

inter.temporalSlider(coll,map,getprocessing)

By operating on the slider a single acquisition day is selected and the corresponding images are displayed on the map.

Temporal slider

Temporal sliders can also help in displaying all available acquisition dates for a specific Sentinel 2 images collection. In this case the slider allows for the selection of a range of acquisition dates:

Temporal slider range to display all available acquisition dates for a specific Sentinel 2 collection