Centre of gravity from survey data

Computes the CPUE-weighted centre of gravity (CoG) from a datras_raw object for each level of the by grouping (typically "Year").

Usage

calc_spatial_indicators(
  x,
  value_var = "HaulN",
  cpue_method = c("per_swept_area", "per_hour", "count"),
  by = "Year",
  lat_var = "lat",
  lon_var = "lon"
)

Arguments

x

A datras_raw object. HH must contain value_var, lat_var, and lon_var.

value_var

Name of the HH column used as CPUE weights. Default "HaulN" (added by add_total_numbers_by_haul()). May be a numeric vector or a matrix; in the latter case one set of indicators is computed per column.

cpue_method

Standardisation applied to value_var before weighting: "per_swept_area" (per km^2; requires SweptArea), "per_hour" (per hour; uses HaulDur), or "count" (raw totals). Default "per_swept_area".

by

Character vector of HH columns used to define groups (e.g. "Year", c("Year","Quarter")). Default "Year".

lat_var, lon_var

Names of the latitude and longitude columns in HH. Defaults "lat" and "lon" (added by clean_datras()).

Value

A data frame with one row per by-group x length-group and columns: the by columns, group, cpue_method, n_hauls, cog_lat, cog_lon.

Details

The centre of gravity is the CPUE-weighted mean position: $$\text{CoG}_\text{lat} = \frac{\sum_i w_i \phi_i}{\sum_i w_i},\quad \text{CoG}_\text{lon} = \frac{\sum_i w_i \lambda_i}{\sum_i w_i}$$ where \(w_i\) is the CPUE at haul \(i\). Hauls with zero or missing CPUE contribute to n_hauls but not to the weighted sums.

See also

add_total_numbers_by_haul(), add_swept_area(), calc_stratified_index(), plot_spatial_indicators()

Examples

if (FALSE) { # \dontrun{
data(dab)
dab <- add_swept_area(dab)
dab <- add_total_numbers_by_haul(dab)
calc_spatial_indicators(dab, by = "Year")

## length-group CoG
dab <- add_total_numbers_by_haul(dab, length_cuts = c(0, 15, Inf))
calc_spatial_indicators(dab, by = "Year")
} # }