How to find the area of polygon in geodjango?

Question

I need to calculate the area of the Building (which the coordinates represents). I used transform() to calculate the area, but the calculated area is not accurate. Please help me calculating the area using the lat/lon coordinates.

          Class Parcel(models.Model):
              geom = models.MultiPolygonField(srid=3492)
              street_nbr = models.CharField(max_length=80)

          Class Primary(models.Model):
              geom  = models.MultiPolygonField(srid=4326)


      >>>parcel = Parcel.objects.filter(street_nbr__contains="2730", street_nam__contains="11TH AVE",city__contains="SACRA").values()
      >>>parcel[0]['geom'].geojson


     >>>build = Primary.objects.filter(goem__contained= parcel[0]['geom'].geojson).values()

     >>>build[0]['geom'].geojson
     '{"coordinates": [[[[-121.476756, 38.542154], [-121.47686, 38.542152], [-121.47686, 38.542136], [-121.476904, 38.542135], [-121.476904, 38.542132], [-121.476962, 38.542131], [-121.476966, 38.542295], [-121.476893, 38.542296], [-121.476892, 38.542278], [-121.476845, 38.542279], [-121.476845, 38.542281], [-121.476759, 38.542282], [-121.476756, 38.542154]]]], "type": "MultiPolygon"}'


      >>>x = build[0]['geom']
      >>>y = x.transform(3492,Clone=True)
      >>>y.area
      1636.8046 

Here i'm getting the area as 1636.8046, I was supposed to get 2,327 sqft . I'm not able to find what I am doing wrong.

Answer 1

If your coordinates are in lat/long, then the SRID should be 4326. So just change the transformation parameter in x.transform.

Try changing y = x.transform(3492,Clone=True) to y = x.transform(4326,Clone=True)