ndvi_extraction: ndvi_extraction

In hmcdevitt/spExtract: Extract values at spatial locations from Raster* objects as matrices

Description

This function matches NDVI values to spatial point data. It will return:

1. iNDVI: interpolated NDVI, giving the estimated value of NDVI if the date of the spatial data falls in between the dates of NDVI layers (which are produced at 16 day intervals)

2. dNDVI: delta NDVI describes the change in NDVI between the closest date layer and the layer previous

3. aNDVI: anomally NDVI is (iNDVI-mNDVI)/mNDVI where mNDVI is the long-term mean NDVI value for the given cell

4. cNDVI: cummulative NDVI is the sum of the raw NDVI value for a given number of NDVI files backwards from the observation date

The function is most accurate using projected data, i.e. both the NDVI files and point data should be in UTM (or similar). The function will still work if the data are in latlong, however there may be some cases where the values returned are associated with a neighbouring NDVI cell. This is due to rounding issues.

The code used in this function is based off code written by Jon Lindsay and Thomas Morrison.

Usage

ndvi_extraction(
  files_desc,
  array,
  extent,
  resolution,
  avg_matrix = NULL,
  cndvi_range = NA,
  date,
  coords
)

Arguments

files_desc	object returned from build_ndvi_desc
array	an array with dimensions 1 and 2 being the y and x values (respectively) of the rasters layers identified in files_desc, the 3rd dimension is multiple dates for same spatial extent. Build using build_ndvi_array
extent	an extent object. contains xmin, xmax, ymin, ymax of the raster layers used to make the array
resolution	a vector with 2 values indicating the x and y resolution of each raster cell
avg_matrix	(default: NULL) if calculating anomally NDVI this is a matrix with the long-term mean NDVI values
cndvi_range	(default: NA) if calculating cummulative NDVI, this is the number of scenes back to include
date	a vector of the dates
coords	a 2 column matrix/dataframe with the x and y coordinates in same projection as extent

hmcdevitt/spExtract documentation built on July 13, 2020, 2:11 p.m.