What is NDVI?
Normalized Difference Vegetation Index or NDVI maps are being used in agriculture, forestry, ecology and more. But what is NDVI?
Healthy vegetation (or chlorophyll) reflects more near-infrared (NIR) and green light compared to other wavelengths. It absorbs more red and blue light.
This is why our eyes see vegetation as the color green. This is also why more near-infrared is reflected back into satellite sensors.
The NDVI Formula
Normalized Difference Vegetation Index (NDVI) uses the NIR and red channels to measure healthy vegetation.
The result of this formula generates a value between -1 and +1. If you have low reflectance (or low values) in the red channel and high reflectance in the NIR channel, this will yield a high NDVI value. And vice versa.
It’s really using the difference between these two bands to output a standardized way to measure healthy vegetation. When you have high NDVI values, you have healthier vegetation.
READ MORE: 15 Free Satellite Imagery Data Sources
How to create NDVI maps in ArcMap
You can create NDVI maps with the image analysis toolbar in ArcGIS 10. All you need is imagery with red and NIR bands.
The main objective is to classify high and low vegetation. This NDVI tutorial shows you step-by-step how to create NDVI maps in ArcMap.
In this NDVI example, we use Worldview-2 (WV-2). Worldview-2 is multispectral imagery. We say multispectral because it has red, green and blue – and other bands as well. The band configuration for WV-2 is:
Worldview-2 Band Configuration
Band 1: Coastal Band
Band 2: Blue
Band 3: Green
Band 4: Yellow
Band 5: Red
Band 6: Red Edge
Band 7: Near Infrared 1 (NIR-1)
Band 8: Near Infrared 2 (NIR-2)
To display true (visible) color composite, in layer properties – change the “Red” channel to band 5, “Green” channel to band 3 and the “Blue” channel to band 2. We say true color because it is the same as how our eyes see.
To display near-infrared (NIR) false color composite, you need band 7 (NIR-1) as the “Red” channel, band 5 (red) as the “Green” channel, and 3 (green) as the “Blue” channel. We say near-infrared false color because this configuration uses NIR in the “Red” channel. The vegetation should already be shouting out at you in bright red!
Step 1: Enable Image Analysis Toolbar
Enable the Image Analysis Toolbar (Windows > Image Analysis). The image analysis window will be displayed in ArcMap.
Step 2: Check Scientific Output Properties
Under image analysis options, select the red band and the near infrared band.
For Worldview-2 imagery, under the NDVI tab – the red band is “band 5” and the NIR band is “band 7”.
Optionally select “Scientific Output” so your values range from -1 to 1.
Step 3: Click NDVI Icon
Highlight your layer by clicking it.
Under properties, Select the NDVI icon which looks like a leaf.
This will create temporary layer in the table of contents.
Step 4: Export Raster
Highlight the new NDVI layer that you want to export by selecting it in the image analysis toolbar.
Right click layer, and export raster to save into memory.
High positive NDVI values (green) means high vegetation. While water usually has negative NDVI values (yellow and red), urban features usually are near zero.
Where is NDVI being used?
NDVI maps have numerous uses and applications across different sectors. For example, NASA is using NDVI and EVI to measuring global vegetation.
But there are other fields:
- Agriculture: Precision farming, crop insurance fraud, biomass and food security
- Ecology: Animal migration patterns, environment change and population dynamics
- Forestry: Leaf area index, forest supply and fire danger