With this block you can access Sentinel 2 and Landsat data.

Sentinel 2 is a European imaging system and provides global data every 12 days.

Landsat is the US imaging system and is the longest-running series of Earth observation satellites.

The data are all georectified and atmospherically corrected data. They are analysis ready. 

TABLE OF CONTENTS

• When should you use optical imagery?
• Data Properties
  • Landsat
  • Sentinel 2 (S2) 
• How to use the satellite block
• Using indices

When should you use optical imagery?

• The Near Infra-Red (NIR) bands of these sensors make them ideal for identifying healthy vegetation.
• However, note that images may include clouds, so you will need to choose a cloud filter. This is described in the article called How to create a cloud-free image of your area of interest. 
• Optical imagers have multiple spectral bands that make them effective at doing land cover classification, especially for agricultural crops.

Data Properties

Landsat

• When you choose the optical satellite block it will looks like this:

• Landsat is an imaging system provided by the USGS.
• It is the longest-running Earth observation satellite series.  Landsat 1 was launched in 1972.  Landsat 9, the latest addition, was launched in 2021 (the data from which will be in Earth Blox soon).
• The earliest Landsat data you can access on Earth Blox is from 22nd August 1982.
• The coverage is not global for all Landsat satellites.  Some areas will have better coverage than others.  Later satellites have global coverage, but with repeat coverage no better than monthly.
• At the moment, you have to choose individual Landsat missions separately. You cannot look at a sequence from 1982 to 2021. This may change in forthcoming versions of Earth Blox.
• Landsats 4-7 offer 6 bands. In addition to the R, G and B channels, there is also a Near Infra-red (NIR) and two Shortwave IR (SWIR) bands.
• The later Landsat missions have two additional bands that are designed to detect aerosols. 
• These bands all have a spatial resolution of 30m. 

Sentinel 2 (S2) 

• Sentinel 2 is a European imaging system that provides global data every 10 days at the equator with one satellite, and 5 days with 2 satellites.
• At mid-latitudes this corresponds to repeat images every 2-3 days.
• The first Sentinel 2 data is from 28th March 2017 until the present day.
• There are 12 bands to choose from in S2 data.  They are all at 10m resolution except for the 3 red edge bands (5, 6, 7) which are 20m resolution (but these are more sensitive to vegetation parameters).

How to use the satellite block

• The block is available under SATELLITE IMAGERY in the toolbox.
• By default it will include the cloud filtering and optical visualisation blocks, and will look like this:

• The cloud filter block is described in detail in the article called How to create a cloud-free image of your area of interest. Otherwise, leaving it as the default parameters is a great starting point.
• In the visualisation block, you can specify which bands are allocated to which colour channel: red, green and blue.  The drop-down menu will only show the options available for the particular mission you have chosen. 
• The default visualisation is a true colour image (i.e. the red, green and blue bands are allocated to R, G, and B output). 
• The Composite option allows you to combine multiple dates together to create a new image. If you select a time period that covers multiple image acquisitions, then using the Mean option create an image layer that is the average of those scenes.  The other options would allocate to the final pixel value the minimum or maximum value, or the median, over all the images that collected data over that pixel.  The mosaic option takes the most recently available value. 
• Remember to add a Layer name, otherwise, a default name will be used.
• You are now ready to go. Click RUN WORKFLOW and view the results on the map.
  

Using indices

• Within the toolbox section 2.ANALYSE you have a choice of indices.  
• There are four of these that work on the optical data sets. 
  • NDVI: The Normalised Difference Vegetation Index. It exploits the very bright NIR reflectance of healthy vegetation and so is useful for mapping agricultural trends, or forest gain/loss. 
  • NDWI: The Normalised Difference Water Index. This identifies areas of open water. 
  • NDSI: The Normalised Difference Snow Index. This identifies areas of snow cover. 
  • NBI: The Normalised Burn Index. This is used to identify burn scars. 
• You can find out more about these in the Explanation of the Analytics guide.
• To use an index, click it into the workflow in place of the visualisation block. 
• Then add an Other data visualisation block beneath it, so it looks like this:

• You can choose the Minimum, Mean, Median, or Maximum value in the index block.  The Maximum value is a good way to avoid the impact of cloud cover that has got through the cloud filtering block. 
• Alternatively, choose the 95th percentile, which averages over the highest 5% of values.
• Remember to add a Layer name.
• You are now ready to go. Click RUN WORKFLOW and view the results on the map.
  

You can use the Time series block to view a sequence of optical images, or a sequence of images of the indices. Connect the time series block within the optical satellite data block, as shown below. Then within the time series block, place the optical visualisation block, or the combination of the index block and other visualisation block.

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