Frequently Asked Questions (FAQ)

Last updated: 2024-11-02 16:00:18

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datetime

Get current time (ISO)

Example

import datetime
x = datetime.datetime.now().isoformat().replace('T', ' ').split('.')[0]
x

'2024-11-02 16:00:18'

matplotlib

Create ‘steps’ plot

Sample data

import pandas as pd
dat = pd.read_csv('data/ZonAnn.Ts+dSST.csv')
cols = [
    '90S-64S', 
    '64S-44S', 
    '44S-24S', 
    '24S-EQU', 
    'EQU-24N', 
    '24N-44N', 
    '44N-64N', 
    '64N-90N'
]
regions = dat[cols]
regions = regions.mean()
regions

90S-64S   -0.074028
64S-44S   -0.053681
44S-24S    0.047917
24S-EQU    0.083056
EQU-24N    0.066181
24N-44N    0.052500
44N-64N    0.142500
64N-90N    0.273333
dtype: float64

Example

import matplotlib.pyplot as plt
plt.step(regions.index.to_list(), regions.to_numpy(), where='mid');

pandas

Append to CSV if exists

Sample data

import pandas as pd
dat = pd.read_csv('output/stations.csv')
dat

	name	city	lines	piano	lon	lat
0	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
1	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
2	Dimona	Dimona	1	False	35.011635	31.068616

Example

# Delete output file if exists
import pandas as pd
import os
path_out = 'output/pandas_append_example.csv'
if os.path.exists(path_out):
    os.remove(path_out)

# Write
for i in range(3):
    if not os.path.exists(path_out):
        dat.to_csv(path_out, index=False)
    else:
        dat.to_csv(path_out, mode='a', header=False, index=False)

# Print output file contents
pd.read_csv(path_out)

	name	city	lines	piano	lon	lat
0	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
1	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
2	Dimona	Dimona	1	False	35.011635	31.068616
3	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
4	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
5	Dimona	Dimona	1	False	35.011635	31.068616
6	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
7	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
8	Dimona	Dimona	1	False	35.011635	31.068616

# Delete output file
if os.path.exists(path_out):
    os.remove(path_out)

Split DataFrame to parts

Sample data

import pandas as pd
dat = pd.read_csv('output/stations.csv')
dat

	name	city	lines	piano	lon	lat
0	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
1	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
2	Dimona	Dimona	1	False	35.011635	31.068616

Function definition

import math
def split_dataframe(df, chunk_size = 1_000_000): 
    chunks = list()
    num_chunks = math.ceil(len(df) / chunk_size)
    for i in range(num_chunks):
        chunks.append(df[i*chunk_size:(i+1)*chunk_size])
    return chunks

Example

split_dataframe(dat, 2)

[                    name        city  lines  piano        lon        lat
 0      Beer-Sheva Center  Beer-Sheva      4  False  34.798443  31.243288
 1  Beer-Sheva University  Beer-Sheva      5   True  34.812831  31.260284,
      name    city  lines  piano        lon        lat
 2  Dimona  Dimona      1  False  35.011635  31.068616]

References:

• https://stackoverflow.com/questions/17315737/split-a-large-pandas-dataframe

Shift column(s) to beginning

Sample data

import pandas as pd
dat = pd.read_csv('output/stations.csv')
dat

	name	city	lines	piano	lon	lat
0	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
1	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
2	Dimona	Dimona	1	False	35.011635	31.068616

Example

cols = ['lon', 'lat']
dat = dat[cols  + [col for col in dat.columns if col not in cols]]
dat

	lon	lat	name	city	lines	piano
0	34.798443	31.243288	Beer-Sheva Center	Beer-Sheva	4	False
1	34.812831	31.260284	Beer-Sheva University	Beer-Sheva	5	True
2	35.011635	31.068616	Dimona	Dimona	1	False

References:

• https://stackoverflow.com/questions/25122099/move-column-by-name-to-front-of-table-in-pandas

Combine CSV files

Question

How can we combine multiple CSV files (with same columns) into one long table?

Sample data

import pandas as pd
dat = pd.read_csv('output/stations.csv')
dat.to_csv('output/stations_table_1.csv', index=False)
dat.to_csv('output/stations_table_2.csv', index=False)
dat.to_csv('output/stations_table_3.csv', index=False)

Example

import pandas
import glob
files = glob.glob('output/stations_table_*.csv')
dat = []
for i in files:
    tmp = pd.read_csv(i)
    dat.append(tmp)
dat = pd.concat(dat)
dat

	name	city	lines	piano	lon	lat
0	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
1	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
2	Dimona	Dimona	1	False	35.011635	31.068616
0	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
1	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
2	Dimona	Dimona	1	False	35.011635	31.068616
0	Beer-Sheva Center	Beer-Sheva	4	False	34.798443	31.243288
1	Beer-Sheva University	Beer-Sheva	5	True	34.812831	31.260284
2	Dimona	Dimona	1	False	35.011635	31.068616

# Delete sample data
import os
for i in files:
    os.remove(i)

geopandas

Calculating distances in WGS84

Question

How can we calculate distances over large regions given lon/lat points?

Sample data

Two (lon,lat) points:

pnt1 = (0, 0)
pnt2 = (1, 0)

True distance according to Wikipedia:

dist = 111320

Using the Harvesine formula (less accurate)

See Example: distance function:

import math
def distance(origin, destination):
    lon1, lat1 = origin
    lon2, lat2 = destination
    radius = 6371000
    dlat = math.radians(lat2 - lat1)
    dlon = math.radians(lon2 - lon1)
    a = (math.sin(dlat / 2) * math.sin(dlat / 2) +
         math.cos(math.radians(lat1)) * math.cos(math.radians(lat2)) *
         math.sin(dlon / 2) * math.sin(dlon / 2))
    c = 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a))
    d = radius * c
    return d

result = distance(pnt1, pnt2)
result = round(result)
result

111195

dist-result  ## Error of 125 meters

125

Using geodesic distance function from geopy (most accurate)

See geopy documentation:

import geopy.distance
result = geopy.distance.distance(tuple(reversed(pnt1)), tuple(reversed(pnt2))).meters
result = round(result)
result

111319

dist - result  ## Error of 1 meter

1

Beyond distance: The S2 Geometry Library

The S2 Geometry Library by Google can be used for more complicated calculations in WGS84, such as polygon area. It has a Python interface called s2sphere.

rasterio

Splitting a raster

Question

How can we split a raster into two equal halves, such as east-west or north-south?

Sample data

import numpy as np
import rasterio
import rasterio.plot

src = rasterio.open('output/carmel2.tif')
rasterio.plot.show(src);

Example

# Column where we split to east-west
x = round(src.shape[1] / 2)
x

266

# Western part
w1 = rasterio.windows.Window(0, 0, x, src.shape[0])
r1 = src.read(1, window=w1)
w1_transform = src.window_transform(w1)
rasterio.plot.show(r1, transform=w1_transform);

# Eastern part
w2 = rasterio.windows.Window(x, 0, src.shape[1]-x, src.shape[0])
r2 = src.read(1, window=w2)
w2_transform = src.window_transform(w2)
rasterio.plot.show(r2, transform=w2_transform);

Raster to points

Question

How can we efficiently create a point layer from a raster?

Sample data

import rasterio
import rasterio.plot
src = rasterio.open('output/carmel2.tif')
rasterio.plot.show(src);

Example

import numpy as np
import pandas as pd
import geopandas as gpd
import rasterio
import rasterio.plot

src = rasterio.open('output/carmel2.tif')
height = src.shape[0]
width = src.shape[1]
cols, rows = np.meshgrid(np.arange(width), np.arange(height))
x, y = rasterio.transform.xy(src.transform, rows, cols)
x = np.array(x).flatten()
y = np.array(y).flatten()
z = src.read(1).flatten()
geom = gpd.points_from_xy(x, y, crs=src.crs)
dat = gpd.GeoDataFrame(data={'value':z}, geometry=geom)
dat

	value	geometry
0	NaN	POINT (662362 3658367)
1	NaN	POINT (662452 3658367)
2	NaN	POINT (662542 3658367)
3	NaN	POINT (662632 3658367)
4	NaN	POINT (662722 3658367)
...	...	...
332587	NaN	POINT (709882 3602297)
332588	NaN	POINT (709972 3602297)
332589	NaN	POINT (710062 3602297)
332590	NaN	POINT (710152 3602297)
332591	NaN	POINT (710242 3602297)

332592 rows × 2 columns

dat.plot(column='value');

arcpy

Listing all layers on current map

import arcpy
aprx = arcpy.mp.ArcGISProject("CURRENT")
map = aprx.listMaps()[0]
layers = map.listLayers()
print([i.dataSource for i in layers])

Fig. 82 Listing layers on current map with arcpy

Transforming ecw to tif

arcpy.env.workspace = r"\\VBOXSVR\Downloads\ortho_2015"
files = arcpy.ListFiles("*.ecw")
for i in files:
    arcpy.management.CopyRaster(i, i.replace(".ecw", ".tif"), '', None, "256", "NONE", "NONE", '', "NONE", "NONE", "TIFF", "NONE", "CURRENT_SLICE", "NO_TRANSPOSE")