The following article describes how to generate vegetation indices using the default index options and how to generate a customized index using the index calculator.
The complete workflow on how to import and process drone images to generate a vegetation index is described in My first vegetation index map - PIX4Dfields. 🌱
Index generator
To generate predefined indices based on the orthomosaic after processing the images.
1. Click Index above the Layers Menu. The INDEX GENERATOR tool opens.
2. Select the Source layer.
3. Select the vegetation index that you want to generate by checking the corresponding boxes.
4. Click Generate.
Available indices
Index to assess chlorophyll content in areas of complete leaf coverage.
Value range clamp between -1 and 1.
Index used in areas with high variability in canopy structure (e.g. forestry).
Index calculator
To generate a custom index using the index calculator.
1. Open the INDEX GENERATOR by clicking the icon at the top right of the window.
2. Choose the source layer.
3. Click Create custom index below the list of available indices.
4. Name the index.
5. Enter the formula either by clicking the Operations and the Available bands buttons or by using your keyboard.
When a formula is valid, the Preview button turns green.
6. Click the Preview button to preview your index.
7. When you are satisfied with the preview, click Apply
8. The generated custom index now appears in the list of layers and in the map interface.
Custom indices can be created depending on the available bands from your imported images, these are listed as buttons under Available bands.
The Operations buttons are described below:
Understanding the NDVI
The NDVI (Normalized Difference Vegetation Index), the so-called "Green Index", is sensitive to chlorophyll content.
Healthy vegetation absorbs most of the visible light that hits it and reflects a large portion of the near-infrared light. As a result, healthy vegetation will have a high NDVI value.
The NDVI scale ranges from -1 to +1. This scale is designed to represent different types of land cover and vegetation health or density. Here's a brief overview of what values within this range typically indicate:
- Values close to +1: High NDVI values (close to +1) indicate dense green leaves and are typically associated with healthy, vigorous vegetation. This is because healthy vegetation reflects more near-infrared (NIR) light and absorbs more visible light.
- Values close to 0: NDVI values around 0 indicate barren areas of rock, sand, or snow, where there is little difference in the reflectance between the red and NIR wavelengths, suggesting minimal vegetation.
- Values close to -1: Negative NDVI values (close to -1) are possible but less common and can occur when the reflectance in the red wavelength is greater than the reflectance in the NIR wavelength. This situation might be observed in water bodies, where the absorption of NIR is higher than the absorption of visible light, resulting in negative NDVI values.
It's important to note that NDVI values are influenced by the specific characteristics of the area being observed, including the type of vegetation, the stage of growth, and the presence of any stress factors (like water availability or diseases). Here's a more detailed breakdown of the NDVI value obtained for a wheat field data provided in My first vegetation index map - PIX4Dfields:
- -1.0 to 0.0: There are no pixels with these values, as it's mainly related to non-vegetated surfaces, (water, snow, clouds, etc.)
- 0.0 to 0.5: Bare soil, as evident from the machine tire marks.
- 0.5 to 0.7: Non-healthy vegetation, low plant density.
- 0.5 to 0.7: Healthier vegetation.
- 0.9 to 0.973: Dense vegetation, healthy crops at peak vitality.