Hello everyone
I tried to download some indices and ETA using ESPA. however, the raster values are (-3800- 8700) for NDVI. how can I convert these numbers to true ones? how to get the mult. factors and offset?
I found the scale factor for ETA
Here:"Landsat 4-9 Collection 2 Level-3 Provisional Actual Evapotranspiration Product Guide "
and for NDVI
"Landsat Normalized Difference Vegetation Index | U.S. Geological Survey "
the question is:
when I applied the scale factor of NDVI--> I got negative values, is there a reason for that?
Hadeel Abdelhamid You can use the SREM method for surface reflectance estimation and then can perform NDVI calculation. If you need any help regarding NDVI calculation using SREM, feel free to contact me.
Article A Simplified and Robust Surface Reflectance Estimation Metho...
Dear Hadeel,
I see you already found the scale and offset values to convert the integer NDVI values into the floating point values you need. Regarding the negative values you're getting, in general you should not be worried about that: NDVI can hold values between -1 and 1, so you're bound to find both negative and positive values. In particular, negative values occur when the reflectance of the pixel in the red channel is higher than that in the NIR channel. This usually happens for water, while snow is usually associated to negative values closer to zero. More rarely, some types of soils may exhibit NDVI values very close to zero or slightly negative.
Basically, if you are getting negative NDVI values where you should expect to get them, everything is normal. Otherwise, there could be a problem somewhere in the data processing chain.
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