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MeasUDF_module
meas package (libcasa_meas)

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## Topics

| MeasUDF_module_internal_classes
| Internal MeasUDF_module classes and functions.

## Classes

class | casacore::BaseEngine
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class | casacore::DirectionEngine
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class | casacore::DirectionUDF
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class | casacore::DopplerEngine
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class | casacore::DopplerUDF
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class | casacore::EarthMagneticEngine
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class | casacore::EarthMagneticUDF
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class | casacore::EpochEngine
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class | casacore::EpochUDF
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class | casacore::FrequencyEngine
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class | casacore::FrequencyUDF
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class | casacore::MeasEngine< M >
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class | casacore::PositionEngine
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class | casacore::PositionUDF
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class | casacore::RadialVelocityEngine
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class | casacore::RadialVelocityUDF
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## Detailed Description

TaQL user defined functions handling measures

See below for an overview of the classes in this module.

### Prerequisite

- UDFBase
- Measures module

### Synopsis

This module extends TaQL (the Table Query Language) with functions handling measures. Currently it can handle MDirection, MEpoch, MPosition, MFrequency, MRadialVelocity, MDoppler and MEarthMagnetic.

These functions make it possible to convert one or more measures from one reference type and frame to another. For example, to convert a direction from J2000 to apparent one can specify the direction in J2000 as well as a time and position to define the measure frame like:

calc meas.app ([4h23m32.7, 34d11m54.8], "J2000",
datetime(), "UTC", POSITION) from my.ms/ANTENNA
casacore::datetime
TableExprNode datetime(const TableExprNode &node)
Functions for date-values.
Definition ExprNode.h:1501

The above example converts the given J2000 direction to apparent coordinates for the given time (current time is used) and for all positions in the POSITION column in the given ANTENNA table.

As shown in the example an argument of a meas package (libcasa_meas) function can be a constant, a table column in a table. or any expression. If a table column is given, it is recognized if the column has a reference type attached to it (using the TableMeasures). In this example it would be recognized that the positions in the POSITION column are given as, say, WGS84.
For constants the reference type can be given in case it differs from the default type. In the example UTC is specified for the time (was not necessary because it is the default).

Note: The meas library will be loaded dynamically by TaQL when such a function is used; Therefore it is important that the library and the other casacore libraries are built shared;
It is also important that the library can be found in the (DY)LD_LIBRARY_PATH;

### Motivation

It is very handy to be able to convert measures in tools that deal with various table columns (e.g. TaQL, TablePlot, pyrap).