[GIS] Changing the the range for MODIS NDVI

modisndvi

I need to get MODIS NDVI (MOD13A3), but output from EarthExplorer or Daac2Disk HDF format is a numerical range is -2000 to 9996.

But I need for futher work basic range -1 to 1.

Is there any conversion method?

Best Answer

The product description page says the following:

Science Data Sets

(HDF Layers) (11): 1km monthly NDVI
UNITS: NDVI
BIT: 16-bit
TYPE: signed integer
FILL: -3000
VALID RANGE: -2000, 10000
MULTIPLY BY SCALE FACTOR: 0.0001

My first guess would be to multiply your data with 0.0001 to get to the correct NDVI values. You could use an additional conditional statement to set -3000 values to nan/nodata

UPDATE: I guess in your case the fill value might not be set

UPDATE2: Here is the description (p. 5)

"MOD13A3 product
As presented in Table 2, the NDVI value given in MOD13A3 products is a integer (16 bits) and range between -2000 and 10 000. In order to convert these values in NDVI “true” values was necessary to apply a scale factor. Therefore, all the original data were multiplied by 0.0001 and the final values were in the range between -0.2 and 1.0. "

Related Solutions

[GIS] How to process MODIS ndvi data using QGIS/GRASS/R

You can use the "stack" or "brick" function(s) in the raster package to create a single raster object of your NDVI time-series. The names of each NDVI file will be retained in the object and the rasters will be held out-of-memory, making it memory safe.

You can then retrieve the raster values, associated with a point feature, across the series using the "extract" function. Here is a mock example:

require(raster)
require(rgdal)
require(sp)

setwd("C:/data")

# Read points
  pts <- readOGR(getwd(), "MyPoints")

# Create raster stack using wildcard in list.files
ndvi.files <- list.files(getwd, pattern="img$", full.names=FALSE)
  ndvi <- stack(ndvi.files)

# Extract point values and merge with sp point object 
pts@data <- data.frame(pts@data, extract(ndvi, pts) )
  str(pts@data)

After you have the ndvi values extracted, you could apply the date functionality in R to the names of the ndvi file names (retained in extract) associated with each column. In this way you can write functions to analyze the time-series. Also, keep in mind that you can apply functions to the raster stack itself using the "overlay" function.

[GIS] How to remove cloudy pixel from MODIS NDVI (MOD13Q1)

Although it requires signing up for an account, you can do this through Google Earth Engine's API quite easily. They give comprehensive tutorials and scripts for how to do so, as well as the data products you are asking for. You can remove the cloudy/bad pixels from the NDVI product by adjusting the script of this tutorial for the data layers you'd like to use: https://developers.google.com/earth-engine/ic_composite_mosaic

Best Answer

Related Solutions

[GIS] How to process MODIS ndvi data using QGIS/GRASS/R

[GIS] How to remove cloudy pixel from MODIS NDVI (MOD13Q1)

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