Python-对点列表进行下采样，在列表中两个相邻点之间具有给定的坐标和距离

Question

I really hope someone can help me with this! I'm trying to make this script run as I want, but I cannot seem to get the hang of it.

The file with the data to be handled is input from a GPS and looks like this:

Line 20081002-1119.nmea
$GPGGA,094406.00,5849.40174,N,01738.15828,E,2,08,00.9,00003.26,M,0024.93,M,005,0734*62
$GPGGA,094407.00,5849.40177,N,01738.15827,E,2,08,00.9,00003.22,M,0024.93,M,005,0734*6B
$GPGGA,094408.00,5849.40174,N,01738.15826,E,2,08,00.9,00003.00,M,0024.93,M,006,0734*65
$GPGGA,094409.00,5849.40171,N,01738.15831,E,2,08,00.9,00003.24,M,0024.93,M,005,0734*62
$GPGGA,094410.00,5849.40176,N,01738.15833,E,2,08,00.9,00003.29,M,0024.93,M,006,0734*61
$GPGGA,094411.00,5849.40172,N,01738.15831,E,2,08,00.9,00003.31,M,0024.93,M,004,0734*6D
$GPGGA,094412.00,5849.40172,N,01738.15830,E,2,08,00.9,00003.15,M,0024.93,M,005,0734*68
$GPGGA,094413.00,5849.40175,N,01738.15834,E,2,08,00.9,00003.18,M,0024.93,M,005,0734*67
$GPGGA,094414.00,5849.40173,N,01738.15835,E,2,08,00.9,00003.16,M,0024.93,M,006,0734*6A
EOL

My output file should look like this (now with made up distances just to show what I want):

Line 20081002-1119.nmea
58.853952   17.643113   102.15 
58.853946   17.643243   101.63 
58.853939   17.643372   105.93 
58.853933   17.643503   104.01 
58.853927   17.643633   104.25 
...
EOL

The columns are: longitude, latitude, distance to the point above.

How do I do to downsample this to a given interval between two points (100 meters in my case)?

The script I've manged so far:`

indata=open('C:/nav.nmea', 'r')
outdata=open('C:/nav_out.txt', 'w')

from math import *

coords_list=[]
coords=[]

def distance(coords_list):
    for (longi2,lati2) in coords_list:
        for (longi1,lati1) in coords_list:
            a = sin(lati1) * sin(lati2)+cos(longi1-longi2)*cos(lati1) * cos(lati2)
            c= 2* asin(sqrt(a))

            s= (6367* c)/100000 # For results in meters

        if s<100:
            # Here I want to discard current line if not s<100 and jump to the next line
        else:
            "Return the valid lines"
    return s


for line in indata:

    if line.startswith('$GPGGA'):

        data=line.split(",")
        # Import only coordinates from input file

        LON=float(data[2])/100

        LAT=float(data[4])/100

        # Convert coordinates from DDMM.MMMM to DD.DDDDDD

        lon=((LON-int(LON))/60)*100+int(LON)

        lat=((LAT-int(LAT))/60)*100+int(LAT)

        coords_list.append((lon,lat))

        outdata.writelines("%0.6f\t" %lon)

        outdata.writelines("%0.6f\t" %lat)
        outdata.writelines("%s \n" %distance(coords_list))


    elif line.startswith('EOL'):

        outdata.writelines("EOL")

    elif line.startswith('Line'):

        LineID=line

        outdata.writelines('\n%s' %LineID)


indata.close()     

outdata.close() 

`

Answer 1

For point reduction in a curve, you probably want to use the Ramer–Douglas–Peucker algorithm . This maintains the overall shape of the curve, but removes the detail, the amount removed can be controlled by a parameter.

Note that the code on the linked Wikipedia page is pseudo code, and not python.

---

I've found a python implementation of the DP line-simplication algorthim , I haven't tested it and can't vouch for its correctness.

Answer 2

Here is a fairly simple way to downsample the data: coord1 is used to store the previous coordinate. Each time through the loop, the distance between coord1 and the new coordinate, coord2 , is computed. When the distance is <100, the rest of the loop is skipped.

import math
import itertools

def distance(coord1,coord2):
    longi1,lati1=coord1
    longi2,lati2=coord2
    a = (math.sin(lati1)*math.sin(lati2)
         +math.cos(longi1-longi2)*math.cos(lati1)*math.cos(lati2))
    c = 2*math.asin(math.sqrt(a))
    s = (6367*c)/100000 # For results in meters
    return s

with open('nav.nmea', 'r') as indata:
    with open('nav_out.txt', 'w') as outdata:
        coords=[]
        coord1=None
        for line in indata:
            if line.startswith('$GPGGA'):
                data=line.split(",")
                # Import only coordinates from input file
                LON=float(data[2])/100
                LAT=float(data[4])/100
                # Convert coordinates from DDMM.MMMM to DD.DDDDDD
                lon=((LON-int(LON))/60)*100+int(LON)
                lat=((LAT-int(LAT))/60)*100+int(LAT)
                coords.append((lon,lat))
                if coord1 is None:
                    # The first time through the loop `coord1` is None
                    outdata.write('%0.6f\t%0.6f\t%s \n'%(lon,lat,0))
                    coord1=(lon,lat)
                else:
                    coord2=(lon,lat)
                    dist=distance(coord1,coord2)
                    if dist<100:
                        # go back to the beginning of the loop
                        continue
                    outdata.write('%0.6f\t%0.6f\t%s \n'%(lon,lat,dist))
                    coord1=coord2
            elif line.startswith('EOL'):
                outdata.writelines("EOL")
            elif line.startswith('Line'):
                LineID=line
                outdata.writelines('\n%s' %LineID)