GeoDistance - Digits

Geodistance Precision

What is the distance between points. How many digits of accuracy to put on my location records.

Let’s look with the GeoDesic library in Python.

The Function

First, the function:

import pandas as pd
from geopy.distance import geodesic

def geodist(lat1, lon1, lat2, lon2):
    coords = pd.Series([lat1, lon1, lat2, lon2])
    if coords.isnull().values.any():
        return None
    return round(geodesic((lat1, lon1), (lat2, lon2)).km, 5)

The Test

Using this python cell, we can generate distance based on the last digit; 5th or 6th decimal place.

lat1, lon1 = [-29.912948,-51.18651]
lat2, lon2 = [-29.912948,-51.18650]
dist5 = geodist(lat1, lon1, lat2, lon2) * 1000

lat1, lon1 = [-29.912948,-51.18651]
lat2, lon2 = [-29.912949,-51.18651]
dist6 = geodist(lat1, lon1, lat2, lon2) * 1000

# print meters
print(round(dist5,2)) # 0.97m
print(round(dist6,2)) # 0.11m

The Results

Approximately:

• a five (5) decimal point value can convey one (1) meter of accuracy
• a six (6) decimal point value can convey ten (10) centimeters of accuracy

The impact of the final digit will vary depending of global location. Your mileage may vary.

The Conclusion

Processing more than 5 of 6 decimal places is a waste of time and effort. Many GPS devices will more limited accuracy as well.