getWind: Connect points to closest wind data in space and time

In mpio-be/windR: Analysis of tracking data in relation to wind

Description

This function takes given points (and their datetime) and connects them with the closest wind data in space.

Usage

getWind(x, y, w, PROJ)

Arguments

x	Longitude of the point
y	Latitude of the point
w	Data table with wind data containing u & v wind component and the datetime
PROJ	Projection of the point and wind data (has to be the same)

Value

A list containing the u and v component for the point

Examples

# Load example wind data
PROJ = '+proj=laea +lat_0=90 +lon_0=-156.653428 +x_0=0 +y_0=0 +datum=WGS84 +units=m
        +no_defs +ellps=WGS84 +towgs84=0,0,0 '
wind_data = system.file('ERA_Interrim', 'ERA_Interrim_850mb_4_7_June_2014_10km.RDS',
                         package = 'windR')
w = readRDS(wind_data)
setkey(w, datetime_)

# Create example points within the boarders of the wind data (space and time)
library(raster)
library(rgeos)
# Get extent of the wind data, defining the extent of the map
w1 = w[datetime_ == unique(w$datetime_[1])]
wp = SpatialPoints(cbind(w1$x, w1$y), proj4string = CRS(PROJ))
datBdry = gEnvelope(wp)

ext   = extent(datBdry)
x_ext = seq(ext[1], ext[2])
y_ext = seq(ext[3], ext[4])

n_points = 100

d = data.table(point_id  = 1:n_points,
               x         = sample(x_ext, n_points, replace=TRUE),
               y         = sample(y_ext, n_points, replace=TRUE),
               datetime_ = c(sample(w$datetime_, n_points)) + 60*60) # existing wind data + 1 h

# Assign closest datetime of wind data
setkey(d, point_id)
d[, w_date := closestDatetime(datetime_, unique(w$datetime_)), by = point_id]

# Assign closest wind data
d[, c('u', 'v') := getWind(x = x, y = y, w = w[J(w_date), nomatch=0L], PROJ), by = point_id]

mpio-be/windR documentation built on Feb. 2, 2020, 10:04 a.m.