Im fiddling around with a GPS i bought a while back, a "GlobalSat GPS Receiver" model BU-S353S4. And it seems to be working well!
I have been able to read the GPS signal from the device with the help from "Cody Wilsons" excellent explanation! And convert the GPGGA output to a longitude and latitude coordinate with the help of the excellent python package "Pynmea2".
But how can I calculate my current speed, from two positions? I have found this thread that refers to this thread on how to calculate the distance with the Haversine formula.
My code looks like this:
from math import radians, cos, sin, asin, sqrt
import serial
import pynmea2
ser = serial.Serial('/dev/ttyUSB0', 4800, timeout = 5)
def haversine(lon1, lat1, lon2, lat2):
"""
Calculate the great circle distance in kilometers between two points
on the earth (specified in decimal degrees)
"""
# convert decimal degrees to radians
lon1, lat1, lon2, lat2 = map(radians, [lon1, lat1, lon2, lat2])
# haversine formula
dlon = lon2 - lon1
dlat = lat2 - lat1
a = sin(dlat/2)**2 + cos(lat1) * cos(lat2) * sin(dlon/2)**2
c = 2 * asin(sqrt(a))
r = 6371 # Radius of earth in kilometers. Use 3956 for miles. Determines return value units.
return c * r
counter = 0
while 1:
line = ser.readline().decode('UTF-8')
splitline = line.split(',')
if splitline[0] == '$GPGGA':
counter += 1
msg = line
data = pynmea2.parse(msg)
lat1 = data.latitude
lon1 = data.longitude
if counter % 2:
distance = haversine(lon1, lat1, data.longitude, data.latitude)
print(distance)
This outputs approx every the distance i have traveled. But heres the problem, it always returns 0.0. I maybe havent traveled far enough?
And how should i proceed to calculate the speed?
I know the formula speed = distance/time. Pynmea2 have a time property (msg.timestamp). But frankly, i dont know how to do this.
Final result that seems to be working
from math import radians, cos, sin, asin, sqrt, atan2, degrees
import serial
import pynmea2
ser = serial.Serial('/dev/ttyUSB0', 4800, timeout = 5)
def haversine(lon1, lat1, lon2, lat2):
"""
Calculate the great circle distance in kilometers between two points
on the earth (specified in decimal degrees)
"""
# convert decimal degrees to radians
lon1, lat1, lon2, lat2 = map(radians, [lon1, lat1, lon2, lat2])
# haversine formula
dlon = lon2 - lon1
dlat = lat2 - lat1
a = sin(dlat/2)**2 + cos(lat1) * cos(lat2) * sin(dlon/2)**2
c = 2 * asin(sqrt(a))
r = 6371 # Radius of earth in kilometers. Use 3956 for miles. Determines return value units.
return c * r
prev_data = None
while 1:
line = ser.readline().decode('UTF-8')
splitline = line.split(',')
if splitline[0] == '$GPGGA':
msg = line
data = pynmea2.parse(msg)
if prev_data is not None:
distance = haversine(data.longitude, data.latitude, prev_data.longitude, prev_data.latitude)
print('distance', distance)
print('speed', round(distance*3600, 2))
prev_data = data
The interface updates once every second hence the speed formula.