The normalized difference water index (NDWI) is uses similar principles as Normalized Difference Vegetation Index (NDVI) to enhance the reflectance values of surface water bodies. NDVI was introduced in 1973 by Rouse et al. Differences of two bands, red and near-infra-red (NIR) is carried out suing the formula. NDVI= ((NIR-RED))/((NIR+RED)), As a result the Spectral Reflectance values of the image will be highlighted. Using similar principles in 1996 GAO derived NDWI technique. The primary idea was to assess the liquid water content of vegetation canopy. Formula used is NDWI= ((NIR-SWIR))/((NIR+SWIR)) . Where SWIR is Short Wave Infrared. This was further modified by McFeeters in 1996 to enhance the spectral reflectance values of surface waterbodies in an image. The modified formula is NDWI=((Green-NIR))/((Green+NIR)) .
There is readymade model available in ERDAS on NDVI. All we need to do is just replace the relevant bands as required. Primary concept involved is the arithmetic operation as per the formula on each pixel of one band with the corresponding pixel in the bother band.
Here is the video on calculating NDWI in the simplest way using ERDAS Imagine.
Notes and Tips:
- There will be slight changes in carrying out the process depending on the version of the software.
- Logically and Theoretically the pixel values of NDWI and NDVI result shall be varying from -1 to +1. In ERDAS, it is stretched to 255 values for easy comprehension. So don't be under doubt on the accuracy of the result seeing the values beyond the range of -1 to +1.
- NDVI is explained in another post.
- You can try these only with proper satellite images or UAV images having Infra Red band. This can not be done without IR band as you see in the formula, so do not try with Google Earth images.
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