code: func (f *Feature) PropertyFloat64(key string) (float64, error) {
  	if i, ok := (f.Properties[key]).(float64); ok {
  		return i, nil
  	}
  	return 0, fmt.Errorf("type assertion of `%s` to float64 failed", key)
  }
docs: // PropertyFloat64 type asserts a property to `float64`.