I'm using geopandas, and I'm trying to get the areas of the shapefile I uploaded, but I'm unsure of what the unit of measurement is for the result of the .area attribute. Here is my code:
water = gp.read_file("/Users/user/Downloads/tlgdb_2020_a_us_areawater.gdb")
lakeSup = water.loc[water['FULLNAME'].str.contains('Lk Superior', na=False)]
lakeSup = lakeSup.to_crs(epsg=4269)
lakeSup['area'] = lakeSup.area
print(lakeSup.head(10))
Which outputs
ANSICODE ... area
233938 None ... 0.000821
629973 None ... 0.215539
629974 None ... 0.043184
629975 None ... 0.358674
629976 None ... 0.533665
629977 None ... 0.035854
629978 None ... 0.054233
629979 None ... 0.737469
629980 None ... 0.101494
629981 None ... 0.035499
According to geopandas' docs, the .area attribute "Returns a Series containing the area of each geometry in the GeoSeries expressed in the units of the CRS." I'm very confused as to what this means, and how I can use this information to simply get a defined area for the geometries of my shapefile (e.g. meters^2). Thank you!
The most important part of area computation by geopandas is that you must use the CRS that corresponds to an equal-area projection. In this case, the simple cylindrical equal-area projection (epsg=6933) is good to use.
The data files can be obtained from:- data_source
The relevant code should becomes:
water = gp.read_file("/Users/user/Downloads/tlgdb_2020_a_us_areawater.gdb")
# check the CRS
water.crs
# you should find that it is already `epsg=4269`
# select some rows of data
lakeSup = water.loc[water['FULLNAME'].str.contains('Lk Superior', na=False)]
# lakeSup = lakeSup.to_crs(epsg=4269) # already is
# convert CRS to equal-area projection
# the length unit is now `meter`
eqArea_lakeSup = lakeSup.to_crs(epsg=6933)
# compute areas in sq meters
areas = eqArea_lakeSup.area
# set the area units to sq Km.
# and add it as a new column to geodataframe
eqArea_lakeSup["area_sqKm"] = areas.values/10e6
# print some result
print(eqArea_lakeSup[["FULLNAME","area_sqKm"]].head(10))
Output:
FULLNAME area_sqKm
233938 Lk Superior 0.695790
629973 Lk Superior 181.784389
629974 Lk Superior 36.563379
629975 Lk Superior 303.028792
629976 Lk Superior 446.529784
629977 Lk Superior 30.418959
629978 Lk Superior 45.785750
629979 Lk Superior 621.004141
629980 Lk Superior 86.142490
629981 Lk Superior 30.186412