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9-7. Add a spectral chart

So far you have looked at the Bristol scene as pictures: composites and indices. A spectral chart lets you look at the numbers behind those pictures. You will add a Pixel Values chart to the Bristol natural colour layer. In the Preview, clicking any point on the map shows the spectral signature of that pixel: its value in every band of the image.

Why spectral signatures are useful

Different surfaces reflect light differently at different wavelengths. Healthy vegetation reflects little red light but a lot of near infrared (NIR). Water absorbs almost all NIR and SWIR light. Bare soil and buildings tend to get brighter steadily from visible to SWIR. Comparing the signatures of a few points shows why a pixel looks the way it does in a composite or an index. You may even want to choose different bands as a result.

Steps

  1. Open the Bristol Sentinel 2 natural colour layer card you styled in 9-3.
  2. Open the layer's Charts tab and click + Add Chart.
  3. Choose Pixel Values as the chart source. This option is only available when the layer has a COG dataset.
  4. The X axis fills in automatically with one entry per band. If the file has no band descriptions, the entries are named Band 1, Band 2, and so on.
  5. Save the chart settings, then Preview. Click on the Charts tab then on some pixels on the image to see the spectral chart.

Optional: Customise the chart

  1. Give the chart a title, e.g. Bristol spectral signature, and optionally a subtitle, e.g. Sentinel 2 surface reflectance.
  2. Replace the generic band labels with meaningful names. For a typical Sentinel 2 band order these are noted in the table below. Note the first band (B02) would be band 1 in the COG and so on.
Band Suggested label Wavelength (approx.)
B02 Blue 490 nm
B03 Green 560 nm
B04 Red 665 nm
B05 Red edge 1 705 nm
B06 Red edge 2 740 nm
B07 Red edge 3 783 nm
B08 NIR 842 nm
B8A NIR narrow 865 nm
B11 SWIR 1 1610 nm
B12 SWIR 2 2190 nm

!!! warning "Check the band order" Use the (i) metadata button on the dataset to confirm which bands your file contains, and in which order, before typing labels. If the labels are in the wrong order, the chart will be misleading.

  1. Set the axis titles under Chart configuration, e.g. Band for the X axis and Reflectance for the Y axis.
  2. Optionally, adjust the trace styling: choose lines+markers mode, a line colour and width.
  3. Save the chart, then Save the layer card.

Try it in the Preview

  1. Open Preview, switch on the Bristol natural colour layer and open its chart.
  2. Click on some contrasting places, for example:
    • water in the Floating Harbour or the Severn Estuary
    • woodland such as Leigh Woods
    • the city centre or Avonmouth docks
  3. Compare the traces. Look for the sharp rise from Red to NIR over vegetation, low values across all bands over water, and the gradual rise towards SWIR over built-up areas.

Link back to your indices

The indices in 9-5 and 9-6 are built from these differences. NDVI compares NIR and Red, and NDWI compares Green and NIR. Seeing the signatures shows why the indices separate vegetation, water and built-up land.

Did you remember to export?

Export your configuration now so you don't lose your work.