GeoDjango: Calculate and save Polygon area in units upon object creation - area

Question

I am struggling to save the area of a polygon to the database. I would like to be able to calculate the area, convert it to the right units and finally save it in the table, but until now I have not found the correct way to do it.
So far I have the following model, where I calculate and save the area in square degrees.
class Parcel(models.Model):
srid = settings.SRID
geometry = models.GeometryField(srid=srid, geography=True)
area = models.FloatField(blank=False, null=True)
def save(self, *args, **kwargs):
self.area = self.geometry.area
super(Parcel, self).save()
In order to use the area in acres I do elsewhere:
p = Parcel.objects.annotate(area_=Area('geometry')).get(id=parcel_id)
parcel_area = p.area_.standard/1000
But this operation is a bit heavy since it calculates the area for all parcels and after that is gets the desired parcel and also it does not use or save the area in acres in the database.
I have seen that some people transform to an srid that has the right units but that would not work for me because my polygons are from all around the earth.
Thanks!

Answer 1

I think the best way to do it is to calculate the area in m² based on a metric coordinate system like utm.
Here is a function to calculate it:
def get_sqm_by_wgs84_polygon(geom: GEOSGeometry) -> float:
def get_utm_by_wgs_84(cent_lon, cent_lat):
utm_zone_num = int(math.floor((cent_lon + 180) / 6) + 1)
utm_zone_hemi = 6 if cent_lat >= 0 else 7
utm_epsg = 32000 + utm_zone_hemi * 100 + utm_zone_num
return utm_epsg
lon = geom.centroid.x
lat = geom.centroid.y
epsg_code = get_utm_by_wgs_84(lon, lat)
transformed_geom = geom.transform(epsg_code, clone=True)
area = transformed_geom.area
return area
In your Model you can save it on creation:
class Parcel(models.Model):
srid = settings.SRID
geometry = models.GeometryField(srid=srid, geography=True)
area = models.FloatField(blank=False, null=True)
def save(self, *args, **kwargs):
self.area = get_sqm_by_wgs84_polygon(self.geometry)
super(Parcel, self).save()
This method only works for WGS84 (EPSG:4326) as input. From degrees to meters.