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# misc.py - miscellaneous geodesy and time functions
#
# This file is Copyright (c) 2010 by the GPSD project
# BSD terms apply: see the file COPYING in the distribution root for details.
import time, calendar, math
# some multipliers for interpreting GPS output
METERS_TO_FEET = 3.2808399 # Meters to U.S./British feet
METERS_TO_MILES = 0.00062137119 # Meters to miles
METERS_TO_FATHOMS = 0.54680665 # Meters to fathoms
KNOTS_TO_MPH = 1.1507794 # Knots to miles per hour
KNOTS_TO_KPH = 1.852 # Knots to kilometers per hour
KNOTS_TO_MPS = 0.51444444 # Knots to meters per second
MPS_TO_KPH = 3.6 # Meters per second to klicks/hr
MPS_TO_MPH = 2.2369363 # Meters/second to miles per hour
MPS_TO_KNOTS = 1.9438445 # Meters per second to knots
# EarthDistance code swiped from Kismet and corrected
def Deg2Rad(x):
"Degrees to radians."
return x * (math.pi/180)
def Rad2Deg(x):
"Radians to degress."
return x * (180/math.pi)
def CalcRad(lat):
"Radius of curvature in meters at specified latitude."
a = 6378.137
e2 = 0.081082 * 0.081082
# the radius of curvature of an ellipsoidal Earth in the plane of a
# meridian of latitude is given by
#
# R' = a * (1 - e^2) / (1 - e^2 * (sin(lat))^2)^(3/2)
#
# where a is the equatorial radius,
# b is the polar radius, and
# e is the eccentricity of the ellipsoid = sqrt(1 - b^2/a^2)
#
# a = 6378 km (3963 mi) Equatorial radius (surface to center distance)
# b = 6356.752 km (3950 mi) Polar radius (surface to center distance)
# e = 0.081082 Eccentricity
sc = math.sin(Deg2Rad(lat))
x = a * (1.0 - e2)
z = 1.0 - e2 * sc * sc
y = pow(z, 1.5)
r = x / y
r = r * 1000.0 # Convert to meters
return r
def EarthDistance((lat1, lon1), (lat2, lon2)):
"Distance in meters between two points specified in degrees."
x1 = CalcRad(lat1) * math.cos(Deg2Rad(lon1)) * math.sin(Deg2Rad(90-lat1))
x2 = CalcRad(lat2) * math.cos(Deg2Rad(lon2)) * math.sin(Deg2Rad(90-lat2))
y1 = CalcRad(lat1) * math.sin(Deg2Rad(lon1)) * math.sin(Deg2Rad(90-lat1))
y2 = CalcRad(lat2) * math.sin(Deg2Rad(lon2)) * math.sin(Deg2Rad(90-lat2))
z1 = CalcRad(lat1) * math.cos(Deg2Rad(90-lat1))
z2 = CalcRad(lat2) * math.cos(Deg2Rad(90-lat2))
a = (x1*x2 + y1*y2 + z1*z2)/pow(CalcRad((lat1+lat2)/2), 2)
# a should be in [1, -1] but can sometimes fall outside it by
# a very small amount due to rounding errors in the preceding
# calculations (this is prone to happen when the argument points
# are very close together). Thus we constrain it here.
if abs(a) > 1: a = 1
elif a < -1: a = -1
return CalcRad((lat1+lat2) / 2) * math.acos(a)
def MeterOffset((lat1, lon1), (lat2, lon2)):
"Return offset in meters of second arg from first."
dx = EarthDistance((lat1, lon1), (lat1, lon2))
dy = EarthDistance((lat1, lon1), (lat2, lon1))
if lat1 < lat2: dy *= -1
if lon1 < lon2: dx *= -1
return (dx, dy)
def isotime(s):
"Convert timestamps in ISO8661 format to and from Unix time."
if type(s) == type(1):
return time.strftime("%Y-%m-%dT%H:%M:%S", time.gmtime(s))
elif type(s) == type(1.0):
date = int(s)
msec = s - date
date = time.strftime("%Y-%m-%dT%H:%M:%S", time.gmtime(s))
return date + "." + repr(msec)[3:]
elif type(s) == type("") or type(s) == type(u""):
if s[-1] == "Z":
s = s[:-1]
if "." in s:
(date, msec) = s.split(".")
else:
date = s
msec = "0"
# Note: no leap-second correction!
return calendar.timegm(time.strptime(date, "%Y-%m-%dT%H:%M:%S")) + float("0." + msec)
else:
raise TypeError
# End
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