The two most commonly used optical sensor are Sentinel 2 and Landsat.
Sentinel 2 is a European imaging system and provides global data every 12 days from 2017.
Landsat is the US imaging system and is the longest-running series of Earth observation satellites. Earth Blox has Landsat data back to 1982 (but not full coverage).
The data are all georectified and atmospherically corrected data. They are analysis ready.
TABLE OF CONTENTS
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.
- 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 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.
- You do not have to choose individual Landsat missions separately. You only have to choose Landsat and Earth Blox will use the data appropriate to the time period you select.
- 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 this data
- Select the Use This Dataset block from the toolbox and from Imagery choose Landsat or Sentinel 2.
- By default it will include the cloud filtering and visualisation blocks with defaults (if you leave Add Recommended Blocks checked), and so 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 data 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 Aggregate Images in Time block 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 Median option will create an image layer that is the median value 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.
- When more than one scene exists in the output, it will automatically generate an animated time sequence on the map.
- 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 with optical sensors
- Within the toolbox section Analayse->Calculate you can choose the Add Index block.
- The list of indices available updates depending on what data you have selected.
- When using optical data, you will be given the choice of 5 categories:
- A full description of the indices is given in this article explaining the Analysis blocks.
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