# PANDAS nearest site algorithm

I have got CSVs full of property transactions in the UK from 1995 to 2017, separated by year such as "RS2015.csv". I have a 2nd CSV with a list of wind turbines in the UK. Both have coordinates in WGS 1984.

The intention of my project is to find the nearest wind turbine to each property transaction. The output file should look like:

ID, pc_lat, pc_long, Ref. Number, Lat, Lng, NWindFarm_DistKm


Where the first 3 columns are from the Property Transactions Sample above. The 4 columns on the right should correspond to the nearest wind turbine to the property transaction coordinates.

NWindFarm_DistKm is my disorganised term for the distance to the nearest turbine.

How should I speed up my Python code?

import pandas as pd
from proxWindPoI import *

df1['linenumber'] = df1.index
total = len(df1)
df1['completion'] = (df1.linenumber/total)*100

print("Working out nearest Wind Farm site")
df1['Nearest Wind Turbine'] = df1.apply(
lambda row: nearestWindFarm(PoI, row['pc_lat'], row['pc_long'],row['completion']),
axis = 1)

print("Merging with Wind Site Information")
df2 = pd.merge(df1,PoI,left_on='Nearest Wind Turbine', right_on='Ref. Number',how='left')

print("Working Out Nearest Distance")
df2['NWindFarm_DistKm'] = df2.apply(
lambda row: distanceBetweenCm(row['pc_lat'], row['pc_long'],row['Lat'], row['Lng']),
axis = 1)

df2.to_csv(csv2, index=None, encoding="utf-8")


proxWindPoI.py

import math
import pandas as pd
import numpy as np

def distanceBetweenCm(lat1, lon1, lat2, lon2):
"""
https://stackoverflow.com/questions/44910530/
how-to-find-the-distance-between-2-points-in-2-different-dataframes-in-pandas/44910693#44910693
Haversine Formula: https://en.wikipedia.org/wiki/Haversine_formula
"""

a = math.sin(dLat / 2) ** 2 + math.sin(dLon / 2) **2 * math.cos(lat1) * math.cos(lat2)
c = 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a))
return c * 6371 #multiply by 100k to get distance in cm

"""
slightly modified version: of http://stackoverflow.com/a/29546836/2901002
Calculate the great circle distance between two points
on the earth (specified in decimal degrees or in radians)
All (lat, lon) coordinates must have numeric dtypes and be of equal length.
"""
lat1, lon1, lat2, lon2 = pd.np.radians([lat1, lon1, lat2, lon2])

a = pd.np.sin((lat2-lat1)/2.0)**2 + \
pd.np.cos(lat1) * pd.np.cos(lat2) * pd.np.sin((lon2-lon1)/2.0)**2

return earth_radius * 2 * pd.np.arcsin(np.sqrt(a))

def nearestWindFarm(wind, lat, long, pct):
distances = wind.apply(
lambda row: haversine(lat, long, row['Lat'], row['Lng']),
axis = 1)
print("Currently completed: "+str(pct)+"%")
return wind.loc[distances.idxmin(),'Ref. Number']


WindPoIEffect.py

from proxAddDataWindPoI import *
import pandas as pd

def nearestBiomass(startyr, endyr):
"""
This loop goes through csv's by year + adds data from nearest site
NEED TO EDIT CSV NAME
"""
for i in range(startyr, endyr, 1):
year = i
print("Year: "+str(year))
csv1 = "RSRaw/RSPytAnyP2"+str(year)+".csv"
csv2 = "Wind/RS_PoI/RSPoIP2"+str(year)+".csv"
print("Input CSV: "+str(csv1))

def withinXkm(csv1, csv2, buffer):
"""
Function which only keeps rows where the distance between site and
property is less than a specified buffer
"""
df = df[df.NWindFarm_DistKm <= buffer]
print(df.NWindFarm_DistKm)
df.to_csv(csv2,index=None,encoding='utf-8')

def treatmentGroup(year, buffer):
"""
The full process of determining treatment group for a certain year of
Repeat Sales. Need to specify buffer
"""
print("Starting Treatment Group Identification for Year: "+str(year))
nearestBiomass(year, year+1)
csv1 = "Wind/RS_PoI/RSPoIP2"+str(year)+".csv"
csv2 = "Wind/TG_PoI/RSWindPoI_TGP2"+str(year)+".csv"
print("Extracting Repeat Sales within a buffer of "+str(buffer)+" from "+str(csv1) + " to "+str(csv2))
withinXkm(csv1, csv2, buffer)

treatmentGroup(2004, 2)


Brief description of how it is supposed to work:

1. def nearestBiomass has an input CSV "RSRaw/RSPytAnyP2"+str(year)+".csv" which you can see a sample of at the bottom of this question. It enacts the function addWindData from proxAddDataWindPoI.py
2. addWindData creates a pandas dataframe df1 from the input CSV + also reads in a 2nd CSV from PoI = pd.read_csv("Wind/PoI_RUK.csv") as a 2nd dataframe.
3. addWindData creates a linenumber + completion column purely for showing me how far along I am in the function nearestWindFarm
4. addWindData applies a function to create df1['Nearest Wind Turbine']. This function (nearestWindFarm) is imported from proxWindPoI.py
5. nearestWindFarm applies a haversine formula, to find the distances from all wind turbines in the wind dataframe as created in the 2. bullet point. The function returns the Site Name when the distance is at a minimum. This part of my code is the really slow bit - the rest is fine.

There are some things which I might change but I don't know if it's quicker or not:

1. instead of from proxWindPoI import *, have from proxWindPoI import nearestWindFarm since importing all of a module is probably slower than importing specific function
2. I've heard than df.apply is a slow way of applying a function in pandas.

I don't suppose anyone has any advice with regards to speeding up my code. Here are two links of subsets from my original CSVs.

Wind Turbines Sample

Property Transactions Sample

The code in the post computes the distance between every transaction and every wind farm. This means that if there are $n$ transactions and $m$ wind farms, then the overall runtime is $\Omega(nm)$.
The runtime can be reduced to $O(n\log m)$ by using a spatial lookup structure such as a k-d tree. The idea is to insert the locations of the wind farms into the k-d tree, and then for each transaction, to query the tree, taking $O(\log m)$.
For this problem I would try the sklearn.neighbors.KDTree class (from the scikit-learn package) and the 'haversine' distance metric.