Why are my area calculations in Python so small with .area()?

Question

I'm trying to calculate the area of a flood with Python, but the results seem way too small:

iceye_breaks = iceye_breaks.to_crs("+proj=cea +lon_0=0 +x_0=0 +y_0=0 +lat_ts=45 +ellps=WGS84 +datum=WGS84 +units=m +no_defs")
iceye_breaks['area'] = iceye_breaks['geometry'].area
iceye_breaks

These are the results:

The area such be much bigger than that. Could it be that geopandas doesn't calculate MultiPolygon areas correctly? Based on what I've read it should be able to calculate MultiPolygons.

I'd like the result in km2.

I've tried changing the projection and still get the same results. I can't use local epsg's since the same script needs to work worldwide, small errors aren't a worry since it's just supposed to be an estimation.

@BERA They have been projected to Cylindrical equal-area ("proj=cea") — UniStudent, Apr 23 '22 at 15:10
Is your original data in a geographic CRS (ie. lat/lons)? If so instead of converting to a projected CRS you can use methods outlined here https://gis.stackexchange.com/q/413349 — Shawn, Apr 23 '22 at 15:41

BERA · Accepted Answer · 2022-04-25T12:01:19.257

You can create a function to find the utm zone for the centroid of the lat, long point then project to that zone and calculate area. I think it should work worldwide

import geopandas as gpd
import utm #pip install utm
from pyproj import CRS
df = gpd.read_file(r'/home/bera/Desktop/GIStest/4_buildings_4326.shp')
df.geometry.area
#0    1.499222e-06
#1    1.783689e-06
#2    7.173407e-07
#3    1.983493e-06
def findtheutm(aGeometry):
"""A function to find a coordinates UTM zone"""
    x, y, parallell, latband = utm.from_latlon(aGeometry.centroid.y, aGeometry.centroid.x)
    if latband in 'CDEFGHJKLM': #https://www.lantmateriet.se/contentassets/379fe00e09d74fa68550f4154350b047/utm-zoner.gif
        ns = 'S'
    else:
        ns = 'N'
    crs = "+proj=utm +zone={0} +{1}".format(parallell, ns) #https://gis.stackexchange.com/questions/365584/convert-utm-zone-into-epsg-code
    crs = CRS.from_string(crs)
    _, code = crs.to_authority()
    return int(code)
epsg = findtheutm(df.geometry.iloc[0]) #Input the dfs first geometry to the function and get utm epsg code back
df['correctArea'] = df.to_crs(epsg).area # /1e6 for km2
df
id  ...   correctArea
0   585331  ...   9140.157424
1   585332  ...  10874.176357
2   585337  ...   4373.700636
3  2488763  ...  12093.625510

Why are my area calculations in Python so small with .area()?

1 Answers1

df.geometry.area

df

id ... correctArea

0 585331 ... 9140.157424

1 585332 ... 10874.176357

2 585337 ... 4373.700636

3 2488763 ... 12093.625510