Using ConstPointer here returns an compiler error:

ITKv4/Modules/Filtering/DisplacementField/include/itkTimeVaryingVelocityFieldTransform.h:100:12:
"return this->GetVelocityField();"

Error is:
no viable conversion from 'const VelocityFieldType *' (aka 'const Image<OutputVectorType, VelocityFieldDimension> *') to 'typename VelocityFieldType::Pointer' (aka 'SmartPointer<Self>')