DispFieldSTall: Diplacement fields for spatiotemporal data when velocity...
In ICvectorfields: Vector Fields from Spatial Time Series of Population Abundance

DispFieldSTall

R Documentation

Diplacement fields for spatiotemporal data when velocity varies spatially

Description

This is an implementation of a novel algorithm that differs from more traditional digital image correlation implementations that are applied in the DispField and DispFieldbb functions. This version is similar to the DispFieldST function except that it does not require a specific time lag. Instead the user specifies a maximum time lag and the function computes displacement vectors using the time lag that produces the maximum speed (magnitude of displacement divided by time lag). The function calculates a displacement field representing persistent movement based on the cross-covariance in a raster stack (in this case a sequential series of rasters) presumably representing spatial population abundance or density at more than two different instances of time. If analysis is restricted to only two time instances, DispField is more appropriate.

Usage

DispFieldSTall(inputstack1, lagmax, factv1, facth1, restricted = FALSE)

Arguments

`inputstack1`	a raster stack with each raster layer representing an instance of time. The raster stack should be organized such that the first raster in the stack is the first observed spatial dataset and time progresses forward with the third dimension index of the raster stack. The raster stack should contain only numeric values. Any NA value will be converted to a zero
`lagmax`	an integer representing the maximum time lag
`factv1`	an odd integer for the vertical dimension of subgrids
`facth1`	an odd integer for the horizontal dimension of subgrids
`restricted`	logical (TRUE or FALSE)

Details

The DispFieldSTall function has the same inner workings as the DispFieldST function except that instead of specifying a specific time lag, the user specifies a maximum time lag. The function then cycles through all lags up to the maximum time lag and chooses the for each location the maximum speed. The DispFieldSTall function is more appropriate than DispFieldST when velocity is variable in space.

Caution is warranted when defining the sub-grid dimensions because the function can produce erroneous results when sub-grids are too small.

Value

A data frame is returned with the following column names: rowcent, colcent, frowmin, frowmax, fcolmin, fcolmax, centx, centy, dispx, and dispy. The rowcent and colcent column names are the row and column indices for the center of each sub-grid; frowmin and frowmax are the sub-grid minimum and maximum row indices; fcolmin and fcolmax are the sub-grid minimum and maximum column indices; centx and centy are the projected coordinates of the centre of the subgrid derived from the raster input files; dispx and dispy are the orthoganal velocity vectors in units of space per timestep in the horizontal and vertical directions in the same spatial units as the projected coordinates of the raster input files.

Examples

(Mat1 <- matrix(rep(c(1:5, 0, 0, 0, 0), 9), nrow = 9, byrow = TRUE))
(Mat2 <- matrix(rep(c(0, 1:5, 0, 0, 0), 9), nrow = 9, byrow = TRUE))
(Mat3 <- matrix(rep(c(0, 0, 1:5, 0, 0), 9), nrow = 9, byrow = TRUE))
(Mat4 <- matrix(rep(c(0, 0, 0, 1:5, 0), 9), nrow = 9, byrow = TRUE))

# rasterizing
rast1 <- terra::rast(Mat1)
terra::plot(rast1)
rast2 <- terra::rast(Mat2)
terra::plot(rast2)
rast3 <- terra::rast(Mat3)
terra::plot(rast3)
rast4 <- terra::rast(Mat4)
terra::plot(rast4)

teststack1 <- c(rast1, rast2, rast3, rast4)
(VFdf4 <- DispFieldSTall(teststack1, lagmax = 2, factv1 = 9, facth1 = 9))
# block is moving rightward at a speed of 1 unit of space per unit of time
# dispx = 1

ICvectorfields documentation built on March 18, 2022, 7:34 p.m.