deltalon.per.km: Convert kilometers (East-West) to degrees longitude.

In ejanalysis/proxistat: Create a Proximity Score in Each Census Unit, Based on Distances to Specified Points (as in EPA's EJScreen tool, for example)

Convert kilometers (East-West) to degrees longitude.

Description

deltalon.per.km returns change in decimal degrees longitude per kilometer (East-West), given latitude (Northern Hemisphere).

Usage

deltalon.per.km(lat)

Arguments

lat	The decimal degrees of latitude of the Northern Hemisphere location(s) of interest, as number or vector of numbers.

Details

This function returns the change in decimal degrees longitude per kilometer moving East-West, at a given latitude (N. Hemisphere). This is an approximation and is less accurate further from the given latitude. The degrees of longitude per km traveled East-West is larger closer to the poles, and smallest at the equator. See http://en.wikipedia.org/wiki/Longitude and http://en.wikipedia.org/wiki/Decimal_degrees Not using meters.per.degree.lon the results would be roughly ( 1 / ((pi/180) * 6378.137 * cos(lat * 0.01745329)) ) where 0.01745329 is pi/180 to convert to radians. see http://en.wikipedia.org/wiki/Decimal_degrees The equator is divided into 360 degrees of longitude, so each degree at the equator represents 111,319.9 metres or approximately 111.32 km. As one gets further from the equator, one degree of longitude must be multiplied by the cosine of the latitude, decreasing distance per degree lon, approaching zero at the pole. For most northern point of USA http://en.wikipedia.org/wiki/Extreme_points_of_the_United_States, where one finds maximum degrees lon per mile, latitude is just under 72, and cos of 72 degrees= cos(72 * pi/180) = 0.309 , so miles/degrees is about 111.32 km * 0.309 = 34.4 km = 21.4 miles roughly.

Value

Returns decimal degrees longitude per km traveled E-W, as a number or vector of numbers the same length as the input lat.

See Also

meters.per.degree.lon for the inverse of this function (other than a factor of 1000), and get.distances which allows you to specify a search radius and get distances only within that radius, and related functions.

Examples

deltalon.per.km(0) 
  # Roughly 111 km/degree moved East-West at the equator roughly, or around 21 miles.
deltalon.per.km(c(0,45,72))

ejanalysis/proxistat documentation built on Jan. 1, 2025, 10:02 a.m.