Wind fetch

Wind fetch is an important measurement in coastal applications. It provides a measurement for the unobstructed length of water over which wind from a certain direction can blow over. The higher the wind fetch from a certain direction, the more energy is imparted onto the surface of the water resulting in a larger sea state. Therefore, the larger the fetch, the larger the exposure to wind and the more likely the site experiences larger sea states.

Why fetchR?

Averaging the wind fetch for numerous directions at the same location is a reasonable measure of the overall wind exposure. This process of calculating wind fetch can be extremely time-consuming and tedious, particularly if a large number of fetch vectors are required at many locations. The fetchR package calculates wind fetch for any marine location on Earth. There are also plot methods to help visualise the wind exposure at the various locations, and methods to output the fetch vectors to a KML file for further investigation.

Installation

You can install and load the latest version of fetchR from CRAN.

# Install fetchR
install.packages("fetchR")

# Load the fetchR package
library(fetchR)

Calculating wind fetch with fetchR

If you already have a SpatialPolygons object representing the coastline and surrounding islands, and a SpatialPoints object representing the locations, then calculating wind fetch with fetchR is easy. You can just pass these two arguments into the fetch function.

Map showing the projected SpatialPolygons (filled grey polygons) and
SpatialPoints (black crosses) objects.

Note

The data for the polygon layer was sourced from Land Information New Zealand[1].

# Calculate wind fetch by passing in the projected SpatialPolygons object (nz_poly_proj)
# and the projected SpatialPoints object (fetch_locs_proj) to the fetch function.
my_fetch_proj = fetch(nz_poly_proj, fetch_locs_proj)

my_fetch_proj

#> Is projected : TRUE
#> Max distance : 300 km
#> Directions   : 36
#> Sites        : 3
#> 
#>                  North East South West Average
#> Kawau Bay          2.3  8.6   6.7  4.8     5.6
#> Chetwode Islands 158.9 75.4  12.1 17.1    65.9
#> Foveaux Strait    27.6 89.5 112.4 96.8    81.6

The my_fetch_proj provides a summary of the fetch for all the four quadrants, along with an overall average of the fetch length at all the sites.

Visualise the fetch vectors

# Plot the fetch vectors, along with the coastline and surrounding islands
plot(my_fetch_proj, nz_poly_proj)

Fetch vectors at all the sites

Fetch vectors at Kawau Bay

Fetch vectors at the Chetwode
Islands

Fetch vectors at Foveaux Strait

Export to a KML file

# Export the fetch vectors to a KML file for further investigation
kml(my_fetch_proj, colour = "white")

Note that the distances calculated in Google Earth are (almost) the same as the distances calculated with fetchR. This can be seen in the KML output as the fetch vector at 90 degrees for the Foveaux Strait site is 300km (the maximum distance by default) in both fetchR and Google Earth, although these algorithms differ.

Output to KML

Get started with fetchR

Read the short introductory vignette to get you started with fetchR, and have a look at the simple, reproducible example in the fetch function.

# Read the short vignette
vignette("introduction-to-fetchR")

# Reproduce a simple example
example(fetch)

Citation

citation("fetchR")
#> 
#> To cite package 'fetchR' in publications use:
#> 
#>   Blake Seers (2020). fetchR: Calculate Wind Fetch. R package version
#>   2.1-2. https://cran.r-project.org/package=fetchR
#> 
#> A BibTeX entry for LaTeX users is
#> 
#>   @Manual{,
#>     title = {fetchR: Calculate Wind Fetch},
#>     author = {Blake Seers},
#>     year = {2020},
#>     note = {R package version 2.1-2},
#>     url = {https://cran.r-project.org/package=fetchR},
#>   }

[1] This README contains data that was sourced from Land Information New Zealand without adaptation that is protected under CC-By Land Information New Zealand.

blasee / fetchR

readme