gml:BSpline 

A B-Spline is a piecewise parametric polynomial or rational curve described in terms of control points and basis functions as specified in ISO 19107:2003, 6.4.30. Therefore, interpolation may be either "polynomialSpline" or "rationalSpline" depending on the interpolation type; default is "polynomialSpline".
degree shall be the degree of the polynomial used for interpolation in this spline.
knot shall be the sequence of distinct knots used to define the spline basis functions (see ISO 19107:2003, 6.4.26.2).
The attribute isPolynomial shall be set to "true" if this is a polynomial spline (see ISO 19107:2003, 6.4.30.5).
The attribute knotType shall provide the type of knot distribution used in defining this spline (see ISO 19107:2003, 6.4.30.4).
The content model follows the general pattern for the encoding of curve segments.

Element Information

Model

(gml:pos | gml:pointProperty | gml:pointRep | gml:posList | gml:coordinates) , gml:degree , gml:knot{2,unbounded}
Children: gml:coordinates, gml:degree, gml:knot, gml:pointProperty, gml:pointRep, gml:pos, gml:posList

Attributes

QName Type Fixed Default Use Inheritable Annotation
interpolation gml:CurveInterpolationType polynomialSpline optional
isPolynomial boolean optional
knotType gml:KnotTypesType optional
numDerivativeInterior integer 0 optional
numDerivativesAtEnd integer 0 optional
numDerivativesAtStart integer 0 optional

Source 

<element name="BSpline" type="gml:BSplineType" substitutionGroup="gml:AbstractCurveSegment">
  <annotation>
    <documentation>A B-Spline is a piecewise parametric polynomial or rational curve described in terms of control points and basis functions as specified in ISO 19107:2003, 6.4.30. Therefore, interpolation may be either "polynomialSpline" or "rationalSpline" depending on the interpolation type; default is "polynomialSpline". degree shall be the degree of the polynomial used for interpolation in this spline. knot shall be the sequence of distinct knots used to define the spline basis functions (see ISO 19107:2003, 6.4.26.2). The attribute isPolynomial shall be set to "true" if this is a polynomial spline (see ISO 19107:2003, 6.4.30.5). The attribute knotType shall provide the type of knot distribution used in defining this spline (see ISO 19107:2003, 6.4.30.4). The content model follows the general pattern for the encoding of curve segments.</documentation>
  </annotation>
</element>

Sample