Class Acts::Measurement

template<typename indices_t, size_t kSize>
class Measurement

A measurement of a fixed-size subspace of the full parameters.

The measurement intentionally does not store a pointer/reference to the reference object in the geometry hierarchy, i.e. the surface or volume. The reference object can already be identified via the geometry identifier provided by the source link. Since a measurement must be anchored within the geometry hierarchy, all measurement surfaces and volumes must provide valid geometry identifiers. In all use-cases, e.g. Kalman filtering, a pointer/reference to the reference object is available before the measurement is accessed; e.g. the propagator provides the surface pointer during navigation, which is then used to lookup possible measurements.

The pointed-to geometry object would differ depending on the parameter type. This means either, that there needs to be an additional variable type or that a pointer to a base object is stored (requiring a dynamic_cast later on). Both variants add additional complications. Since the geometry object is not required anyways (as discussed above), not storing it removes all these complications altogether.

Template Parameters
  • indices_t – Parameter index type, determines the full parameter space

  • kSize – Size of the parameter subset.

Public Types

using CovarianceMatrix = ActsSymMatrix<kSize>

Matrix type for the measurement covariance.

using ExpansionMatrix = ActsMatrix<kFullSize, kSize>
using FullParametersVector = ActsVector<kFullSize>

Vector type containing all parameters in the same space.

using ParametersVector = ActsVector<kSize>

Vector type containing for measured parameter values.

using ProjectionMatrix = ActsMatrix<kSize, kFullSize>
using Scalar = ActsScalar

Public Functions

template<typename parameters_t, typename covariance_t>
inline Measurement(SourceLink source, const std::array<indices_t, kSize> &indices, const Eigen::MatrixBase<parameters_t> &params, const Eigen::MatrixBase<covariance_t> &cov)

Construct from source link, subset indices, and measured data.


The indices must be ordered and must describe/match the content of parameters and covariance.

Template Parameters
  • parameters_t – Input parameters vector type

  • covariance_t – Input covariance matrix type

  • source – The link that connects to the underlying detector readout

  • indices – Which parameters are measured

  • params – Measured parameters values

  • cov – Measured parameters covariance

Measurement() = delete

A measurement can only be constructed with valid parameters.

Measurement(const Measurement&) = default
Measurement(Measurement&&) = default
~Measurement() = default
inline bool contains(indices_t i) const

Check if a specific parameter is part of this measurement.

inline const CovarianceMatrix &covariance() const

Measured parameters covariance.

inline ExpansionMatrix expander() const

Expansion matrix from the measured subspace into the full space.

This is equivalent to the tranpose of the projection matrix only in the case of a trivial projection matrix. While this is the case here, it is still recommended to use the expansion matrix directly in cases where it is explicitely used.

inline constexpr std::array<indices_t, kSize> indices() const

The measurement indices.

inline std::ostream &operator<<(std::ostream &os) const
Measurement &operator=(const Measurement&) = default
Measurement &operator=(Measurement&&) = default
inline const ParametersVector &parameters() const

Measured parameters values.

inline ProjectionMatrix projector() const

Projection matrix from the full space into the measured subspace.

inline ParametersVector residuals(const FullParametersVector &reference) const

Compute residuals in the measured subspace.

This computes the difference measured - reference taking into account the allowed parameter ranges. Only the reference values in the measured subspace are used for the computation.


reference – Reference parameters in the full space.

inline const SourceLink &sourceLink() const

Source link that connects to the underlying detector readout.

Public Static Functions

static inline constexpr size_t size()

Number of measured parameters.