Fixed a few issues in previous commit.

class CensusZipcode(NamedModel):

    "Model for a few South Texas ZIP codes (in original Census NAD83)."

    name = models.CharField(max_length=5)

    poly = models.PolygonField(srid=4269)

class SouthTexasZipcode(NamedModel):

    "Model for a few South Texas ZIP codes."

    name = models.CharField(max_length=5)

    poly = models.PolygonField(srid=32140, null=True)

class Interstate(NamedModel):

    "Geodetic model for U.S. Interstates."

    name = models.CharField(max_length=10)

    path = models.LineStringField()

class SouthTexasInterstate(NamedModel):

    "Projected model for South Texas Interstates."

    name = models.CharField(max_length=10)

    path = models.LineStringField(srid=32140)