Comma Seprated Character Field to store Geographic Coordinates

from django.db import models
from math import acos, cos, sin

class GeoCoordinate(object):
    def __init__(self,latitude=0.00,longitude=0.00):
        self.latitude = latitude
        self.longitude = longitude
                
    def within_radius(self,r,geocoordn_obj):
        print geocoordn_obj.__dict__
        if acos(sin(geocoordn_obj.latitude)*sin(self.latitude) + \
            cos(geocoordn_obj.latitude)*cos(self.latitude)*cos(self.longitude -(geocoordn_obj.longitude)))*6371 <=r:
            return True
        else:
            return False
        
        
    def __repr__(self):
        return str(self.latitude) + ',' + str(self.longitude)
    
    
class GeoCoordinateField(models.Field):
    description = "A latitude,longitude pair"
    __metaclass__ = models.SubfieldBase
    
    def __init__(self,help_text=("A comma-separated latitude longitude pair"),verbose_name='geocordfield', *args,**kwargs):
        self.name="GeoCoordinateField",
        self.through = None
        self.help_text = help_text
        self.blank = True
        self.editable = True
        self.creates_table = False
        self.db_column = None
        self.serialize = False
        self.null = True
        self.creation_counter = models.Field.creation_counter
        models.Field.creation_counter += 1
        super(GeoCoordinateField, self).__init__(*args, **kwargs)
        
        
    def db_type(self, connection):
        return 'varchar(100)'
    
    def to_python(self,value):
        if value in ( None,''):
            return GeoCoordinate()
        else:
            if isinstance(value, GeoCoordinate):
                return value
            else:
                args = [float(value.split(',')[0]),float(value.split(',')[1])]
                if len(args) != 2 and value is not None:
                    raise ValidationError("Invalid input for a GeoCoordinate instance")
                return GeoCoordinate(*args)
        
    
    def get_internal_type(self):
        return 'CharField'
    
    def value_to_string(self, obj):
        value = self._get_val_from_obj(obj)
        return self.get_prep_value(value)