sgo_area {sgo}

Calculate area from an ordered set of points

Description

Calculates the planar area for a set of points defined in the OS BNG or ETRS89-LAEA. An accurate approximation of the geodetic area is calculated when points are expressed in angular coordinates.

Usage

sgo_area(x, interpolate = NULL, ...)

Arguments

x

A sgo_points object describing an ordered set of points.

interpolate

Numeric variable. If not NULL, defines the maximum distance in metres between adjacent coordinates. It is only used with angular coordinates.

...

Currently ignored

Details

Calculate areas using the Gauss's area formula (https://en.wikipedia.org/wiki/Shoelace_formula).

When using angular coordinates the function performs an approximation of the geodetic area following the methodology discussed in Berk & Ferlan (2018) where the area on the ellipsoid is determined by using a region-adapted equal-area projection (Albers Equal-Area Conic) with one standard parallel. The standard parallel and the projection origin are tied to the moment centroid of the polygon.

Boundary segments can be divided by interpolating vertices on the projected geodesic to reduce the error introduced by boundary simplification and to provide an even more accurate area computation for angular coordinates. For instance, if interpolate = 500 then any segment between adjacent coordinates whose length is greater than interpolate will be split in parts no greater than 500 m and new vertices will be added.

The area calculation in this package is best suited for features that would be represented in a large or medium scale (like plots or council boundaries). It will provide much less accurate results for features usually represented at smalle scale (countries, continents, etc.).

Value

Value of the area in squared metres rounded up to the first decimal.

References

Sandi Berk & Miran Ferlan, 2018. Accurate area determination in the cadaster: case study of Slovenia. Cartography and Geographic Information Science, 45:1, 1-17. DOI: 10.1080/15230406.2016.1217789

Snyder, J.P. 1987. Map Projections — A Working Manual. US Geological Survey Professional Paper, no. 1395. Washington, DC: US Government Printing Office. DOI: 10.3133/pp1395

Examples

lon <- c(-6.43698696, -6.43166843, -6.42706831, -6.42102546,
-6.42248238, -6.42639092, -6.42998435, -6.43321409)
lat <- c(58.21740316, 58.21930597, 58.22014035, 58.22034112,
58.21849188, 58.21853606, 58.21824033, 58.21748949)
A <- sgo_area(sgo_points(list(lon, lat), epsg=4326))