The number of revisits to a polygon is calculated as the number of segments of the trajectory passing through the polygon.
# S3 method for Move getRecursionsInPolygon(trajectory, polygon, threshold = 0, timeunits = c("hours", "secs", "mins", "days"), verbose = TRUE) getRecursionsInPolygon(trajectory, polygon, threshold = 0, timeunits = c("hours", "secs", "mins", "days"), verbose = TRUE) # S3 method for data.frame getRecursionsInPolygon(trajectory, polygon, threshold = 0, timeunits = c("hours", "secs", "mins", "days"), verbose = TRUE) # S3 method for MoveStack getRecursionsInPolygon(trajectory, polygon, threshold = 0, timeunits = c("hours", "secs", "mins", "days"), verbose = TRUE)
| trajectory | A data frame with four columns (the x-coordinate, the y-coordinate, the datetime, and the animal id). |
|---|---|
| polygon | A SpatialPolygons object with a single convex polygon. |
| threshold | A time difference (in units |
| timeunits | Character string specifying units to calculate time differences in for the time spans inside the radius and since the
visit in |
| verbose |
|
A list with several components, revisits and residenceTime
are vectors of the same length as the trajectory dataframe. revisits is the number of revisits for each
location, where 1 means that there were
no revisits, only the initial visit. residenceTime is the total time spent withing the radius. radius
is the specified radius used for all the calculations. timeunits is the specified time units used to specify
timespans.
When verbose = TRUE, additional information
is also returned, dists and revisitStats. Next, dists gives the distance matrix between
all locations. Finally, revisitStats gives further statistics on each visit. These are calculated
per location (i.e., no aggregation of nearby points is performed), and give the index and location
of the point of the track at the center of the radius, the radius entrance and exit time of the track for that
visit, how much time was spent inside the radius, and how long since the last visit (NA for the first visit).
The number of segments of the trajectory passing through the polygon is counted as the number of revisits. For each revisit, the time spent inside the polygon is calculated, as well as the time since the last visit (NA for the first visit). In order to calculate the time values, the crossing time of the polygon is calculated by assuming linear movement at a constant speed between the points inside and outside the polygon. Note the polygon must be convex as described in further detail below. Projection. Consider the projection used. Since segments are counted passing through the polygon, an equal area projection would ensure similar size comparisons (e.g., spTransform). A geographic projection is not appropriate. The projection for the polygon and the trajectory must be the same. Polygon. The polygon must be specified as a SpatialPolygons object. It should consist of a single polygon (rather than a list of multiple polygons). It should further be convex, though this requirement is not enforced, calculations for non-convex polygons will not necessarily be accurate. It may be advantageous to simplify complex geometry in order to shorten the time to run. If it is necessary to use a non-convex polygon, one approach would be to split it into convex pieces that can be run one-by-one. However, some visits would then be double-counted and would need to be combined back together based on the entrance/exit times and sequence of trajectory locations.
Either single or multiple individuals are supported, but be aware that this function will be slow with
large amounts of data (e.g. millions of points). Multiple individuals are handled via the id column of the
data.frame.
Move: Get recursions in polygon for a Move-class trajectory
data.frame: Get recursions inside a polygon for a trajectory data.frame object consisting of columns x, y, datetime, and id
MoveStack: Get recursions in polygon for a MoveStack trajectory
data(track) poly = sp::SpatialPolygons( list( sp::Polygons( list(sp::Polygon(cbind(c(4,6,6,3,4),c(1,2,4,3,1)))), ID = 1 ) )) revisits = getRecursionsInPolygon(track, poly)