I am trying to use Landsat to identify small-scale water bodies in Eastern Africa. I first pansharpened a composite image of bands 2-7 to improve the resolution. Bands 2-4 are now at a resolution of 15m, and bands 5-7 remain at 30m resolution but with a cell size of 15m. Visually, bands 5-7 appear to be 30m resolution, but they are really just four 15mX15m cells with the same value.
I used the USGS Landsat 8 website (http://landsat.usgs.gov/Landsat8_Using_Product.php) to calculate TOA reflectance values for each band. As an example, my TOA results for band 4 range from -0.1217 to 1.420. The negative values are either along the dead space outside the image itself or in densely vegetated areas.
I then used the formula suggested by Xu (2006) for the modified NDWI: MNDWI=(Green-MIR)/(Green+MIR), where Green is band 3, and MIR is band 6 in Landsat 8. This step results in NDWI ranging from 2.85e7 to -1.63e7. According to what I have read, NDWI should range from -1 to 1. I tried the MNDWI formula using just the pansharpened DN images and got NDWI values ranging from -1 to 1.
I understand that you can directly download surface reflectance values from EarthExplorer, but this Landsat data does not include a panchromatic layer. I really need to pansharpen my data since I am looking at small-scale features. Most do not show up at a 30m resolution.
Any insight into why the NDWI calculated from TOA values is so wrong? Am I missing a big step or understanding the data wrong? Has anyone else had similar issues?
The scene I am working with is:
LANDSAT_SCENE_ID = "LC81680612015005LGN00"
WRS_PATH = 168
WRS_ROW = 61
DATE_ACQUIRED = 2015-01-05