coord: Simple Threshold Geolocation Estimates
In GeoLight: Analysis of Light Based Geolocator Data

Description Usage Arguments Details Value Author(s) Examples

Estimate location from consecutive twilights

1 2	coord(tFirst, tSecond, type, twl, degElevation = -6, tol = 0, method = "NOAA", note = TRUE)

`tFirst`	vector of sunrise/sunset times (e.g. 2008-12-01 08:30).
`tSecond`	vector of of sunrise/sunset times (e.g. 2008-12-01 17:30).
`type`	vector of either 1 or 2, defining `tFirst` as sunrise or sunset respectively.
`twl`	data.frame containing twilights and at least `tFirst`, `tSecond` and `type` (alternatively give each parameter separately).
`degElevation`	the sun elevation angle (in degrees) that defines twilight (e.g. -6 for "civil twilight"). Either a single value, a `vector` with the same length as `tFirst` or `nrow(x)`.
`tol`	tolerance on the sine of the solar declination (only implemented in method 'NOAA').
`method`	Defines the method for the location estimates. 'NOAA' is based on code and the excel spreadsheet from the NOAA site (http://www.esrl.noaa.gov/gmd/grad/solcalc/), 'Montenbruck' is based on Montenbruck, O. & Pfleger, T. (2000) Astronomy on the Personal Computer. Springer, Berlin.
`note`	`logical`, if TRUE a notation of how many positions could be calculated in proportion to the number of failures will be printed at the end.

This function estimates the location given the times at which the observer sees two successive twilights.

Longitude is estimated by computing apparent time of local noon from sunrise and sunset, and determining the longitude for which this is noon. Latitude is estimated from the required zenith and the sun's hour angle for both sunrise and sunset, and averaged.

When the solar declination is near zero (at the equinoxes) latitude estimates are extremely sensitive to errors. Where the sine of the solar declination is less than tol, the latitude estimates are returned as NA.

The format (date and time) of tFirst and tSecond has to be "yyyy-mm-dd hh:mm" corresponding to Universal Time Zone UTC (see: as.POSIXct, time zones)

A matrix of coordinates in decimal degrees. First column are longitudes, expressed in degrees east of Greenwich. Second column contains the latitudes in degrees north the equator.

Simeon Lisovski, Simon Wotherspoon, Michael Sumner

data(hoopoe2)
  hoopoe2$tFirst <- as.POSIXct(hoopoe2$tFirst, tz = "GMT")
  hoopoe2$tSecond <- as.POSIXct(hoopoe2$tSecond, tz = "GMT")
crds <- coord(hoopoe2, degElevation=-6, tol = 0.2)
## tripMap(crds, xlim=c(-20,20), ylim=c(5,50), main="hoopoe2")