Spectral Index Maps

1. Description

Aether provides a wide range of basic to advanced spectral index maps for field monitoring and analysis. It is also possible to generate custom spectral index maps based on custom formulas.

This article provides details about spectral index maps and explains how to analyze the maps.

2. Generating spectral index maps?

There are different ways to generate spectral index maps:

  • The Photogrammetry‍ process: in the settings of the photogrammetry, it is possible to select basic and advanced spectral index maps that will be generated within the process (for multispectral only).
  • The Spectral index map analytic allows the generation of basic and advanced spectral index maps from an orthomosaic (multispectral or RGB)
  • The Custom Spectral Index Map‍ analytic allows the generation of custom spectral index maps from an orthomosaic (multispectral or RGB) based on a specific formula.

3. Generic spectral index maps provided by Aether

3.1 Basic spectral index maps

RGB Bird View Bird View Composition: red, green, blue
NDVI Normalized Difference Vegetation Index Crop Vigor: Map to analyze vigor, which is a mix of biomass development and crop health. The basic tool to scout for field anomalies.

NDVI gives an indication of the photosynthetic activity, and crop vigor and helps spot anomalies.

  • NDVI of dense vegetation canopy will tend to positive values (0.3 to 0.8).
  • Soils generally exhibit a near-infrared spectral reflectance somewhat larger than the red, and thus tend to also generate rather small positive NDVI values (0.1 to 0.2).
  • Moderate values represent shrub and grassland (0.2 to 0.3)
  • High values indicate temperate and tropical rainforests (0.6 to 0.8)
NDVI = (nir-red)/(nir+red)
NDRE Normalized Difference Red Edge Chlorophyll Content: Map to analyze chlorophyll content. Correlates well with chlorophyll. The basic tool for nitrogen recommendation is based on chlorophyll analysis.

NDRE is used to assess field chlorophyll content, and thus nitrogen content or stress detection (at the earliest stage possible, before NDVI).

  • Soil typically has the lowest values
  • Unhealthy plants have intermediate values
  • Healthy plants have the highest values

NDRE = (nir-red_edge)/(nir+red_edge)
VARI Visible Atmospherically Resistant Index Greenness: Map to discriminate green plants from the remaining, soil as an example. VARI = (green-red)/(green+red-blue)
MCARI2 Modified Chlorophyll Absorption Index Ratio 2 Green Biomass: Map that is a good predictor of green leaf area index to analyze biomass development. MCARI2 = 1.5*(((2.5*(nir-red))-1.3*(nir-green))/sqrt(((2*nir+1)^2)-(6*nir-5*sqrt(red))-0.5))
PRI Photochemical Reflectance Index The map that values spectral bands where photosynthetic pigments might be affected by water stress conditions such as chlorophyll and carotenoids. PRI = (green_b531-green) / (green_b531+green)

3.2 Advanced spectral index maps


MSAVI2 Modified Soil-Adjusted Vegetation Index 2 Soil-Ajusted Crop Vigor: Map to analyze crop vigor, that is mix of biomass development and crop health to scout for field anomalies. The main difference with NDVI is that it removes partly the effect of soils. Thus, this index is more suitable to use at an early stage of crop development or where the canopy is not closed. Combination of red and NIR infrared.

MSAVI2 = (2*nir+1-sqrt(((2*nir+1)^2)-8*(nir-red)))/2

CIR Colored Infra-Red The false color photograph highlights vegetation. Composition: nir, red, green
CCCI Canopy Chlorophyll Content Index The map that is correlated to N Status / Chlorophyll concentration and can be valued to benchmark chlorophyll status or feed nitrogen variable rate maps.

CCCI = (nir+red)*(nir-red_edge)/((nir-red)*(nir+red_edge))

4. Exploring and analyzing maps

Open your project and survey, and from the left panel, go to the "BASE LAYERS" section. Rearrange this layer organization if needed (read Browsing and Arranging Layers‍).

Click on the layer name, an information panel appears on the right side with different tools to analyze the map.

4.1 Value at point

This tool allows checking spectral index map value at any point of the map.

  • Click on "Value at point" and click on the map view
  • Press "ESC" to cancel.

4.2 Changing opacity and style

The opacity and color gradient of the map can be customized.

The color chart options are the following:

  • Spectral
  • Green-Yellow-Red (i.e. lower values are in green)
  • Red-Yellow-Green
  • Grayscale

4.3 Spectral index map pixel distribution and display

Visualize the distribution of the pixel values (ex NDVI value) in the graph below and interact with this graph to adjust the display on the map.

The spectral index map display can be customized with the options described in the table below. Values out of the selected range will be displayed with the edge color (for example red and green if the scale "Red-Yellow-Green" is selected).

Mean and standard deviation (by default) The color scale is centered on the mean and 1 standard deviation is discarded at each extremity
Absolute All pixels are included in the color scale.
Cumulative % cut Lower and upper values are discarded. You can set the % of value to discard (default is +/- 3%).
Custom Select the range of values on which you want to display the color scale
  • manually on the graph with the mouse pointer
  • or inputting the min and max values

4.4 Hide out-of-range data

  • The button "Hide out of range data" is empty, pixels outside the range will be displayed with the edge color of the graph.

  • Tick the button "Hide out of range data", the pixels outside the range won't be displayed on the map and the other maps selected in the background will be visible.

4.5 Data Extraction from a point annotation

Draw a point annotation (1) & (2) and press "Extract data" (3) from the information panel. All available indices values at the point will be displayed.

5. Exporting spectral index maps & statistics

All generated Maps can be exported from the Downloads section. Maps are available in tif format.

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