What is NDVI?
Normalised Difference Vegetation Index (NDVI) represents a spectral signature, that is, a measurement of photosynthetic capacity based on the relative absorption of different light wavelengths by a plant. In layman’s terms, it is a measure of vegetation “greenness”, and as such, represents a standardised way to measure a vegetations health. Healthier vegetation reflects more near infrared (NIR) and green light, but absorbs more red and blue light, meaning a plant with higher reflectance in the NIR and green wavelengths will yield a higher NDVI value. And vice versa. With this in mind, the consideration of NDVI can be useful across a range of environmental applications, and is currently utilised successfully in agricultural, forestry and invasive species surveys to name but a few. Here, its application is considered with regards to new land development and site appraisal practices, where the identification of areas of healthy vegetation combined with accurate assessments of its distribution may prove useful as a factor for assessing sites for potential development.
Pictured below is a side by side comparison of NDVI mapped from the European Space Agency’s Sentinel-2 satellite using Google Earth Engine’s (GEE) code editor (left) with google earth’s DigitalGlobe base layer imagery (right). The improved contrast between highly vegetated and non-vegetated to urban areas is immediately noticeable, with green areas representing high NDVI values and red areas representing low NDVI values. When zoomed in further, the NDVI layer allows for highly urbanised areas to be more easily identified, and highlights areas where vegetation is lacking, or is in poor health. As a tool for land assessments, its advantages are clear over that of standard satellite imagery and offers potential as a preliminary stage for site identification before more thorough ground level observations are made.
But NDVI’s potential doesn’t end there! Nasa’s MODIS sensor allows for long term mean vegetation changes to be acquired and manipulated in GEE to show temporal trends of increasing or decreasing NDVI. Shown below, areas in blue represent increasing NDVI values, green represents no change and red represents decreasing values over a 10-year period. However, MODIS datasets are only currently available at a coarse 500m spatial resolution, meaning its application for specific site appraisals is limited for now.