import pandas as pd
import gpxpy

def to_linestring(gpx_val):
    gpx_parsed = gpxpy.parse(gpx_val)
    pts = ((point.latitude, point.longitude)
            for track in gpx_parsed.tracks
            for segment in track.segments
            for point in segment.points)
    pt_strs = [" ".join(map(str, pt)) for pt in pts]
    if len(pt_strs) == 0:
        return 'LINESTRING EMPTY'
    return "LINESTRING (" + ", ".join(pt_strs) + ")"


df = pd.read_csv('gpx-tracks-from-hikr.org.csv')
df['wkt'] = df.gpx.apply(to_linestring)