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#### 8.3.31 `skycorr`

Plots an error ellipse (or rectangle or other similar figure) on the sky defined by errors in the longitude and latitude directions, and a correlation between the two errors.

The error in longitude is considered to be premultiplied by the cosine of the latitude, i.e. both errors correspond to angular distances along a great circle.

The supplied correlation is a dimensionless value in the range -1..+1 and is equal to the covariance divided by the product of the lon and lat errors. The covariance matrix is thus:

        [  lonerr*lonerr       lonerr*laterr*corr  ]
        [  lonerr*laterr*corr  laterr*laterr       ]

The dimensions of the plotted ellipses are given by the `lonerr` and `laterr` coordinates. The units of these values are specified using the `unit` option. If only the relative rather than the absolute sizes are required on the plot, or if the units are not known, the special value `unit=scaled` may be used; this applies a non-physical scaling factor to make the ellipses appear at some reasonable size in the plot. When `unit=scaled` ellipses will keep approximately the same screen size during zoom operations; when one of the angular units is chosen, they will keep the same size in data coordinates.

Additionally, the `scale` option may be used to scale all the plotted ellipses by a given factor to make them all larger or smaller.

This plot type is suitable for use with the `ra_error`, `dec_error` and `ra_dec_corr` columns in the Gaia source catalogue. Note that Gaia positional errors are generally quoted in milli-arcseconds, so you should set `unit=mas`. Note also that in most plots Gaia positional errors are much too small to see!

Usage Overview:

       layerN=skycorr ellipseN=ellipse|crosshair_ellipse|...
                      unitN=scaled|radian|degree|minute|arcsec|mas|uas
                      scaleN=<number>
                      shadingN=auto|flat|translucent|transparent|density|aux|weighted <shade-paramsN>
                      lonN=<deg-expr> latN=<deg-expr> lonerrN=<num-expr>
                      laterrN=<num-expr> corrN=<num-expr> inN=<table>
                      ifmtN=<in-format> istreamN=true|false icmdN=<cmds>

All the parameters listed here affect only the relevant layer, identified by the suffix `N`.

Example:

       stilts plot2sky in=tgas_source.fits
                       lon=ra lat=dec
                       icmd='select ra>245.1&&ra<245.9&&dec>-17.8&&dec<-17.2'
                       color=blue
                       layer1=mark
                       unit=mas scale=2e5
                       ra2=ra_error rb2=dec_error posang2=90
                       color2=orange shading2=transparent
                       layer2a=skyellipse ellipse2a=filled_rectangle opaque2a=6
                       layer2b=skyellipse ellipse2b=crosshair_rectangle opaque2b=2
                       layer3=skycorr
                       lonerr3=ra_error laterr3=dec_error corr3=ra_dec_corr
                       ellipse3=crosshair_ellipse

`corrN = <num-expr>` (String)
  Correlation between the errors in longitude and latitude. This is a dimensionless quantity in the range -1..+1, and is equivalent to the covariance divided by the product of the Longitude and Latitude error values themselves. It corresponds to the `ra_dec_corr` value supplied in the Gaia source catalogue.

  The value is a numeric algebraic expression based on column names as described in Section 10.

`ellipseN = ellipse|crosshair_ellipse|...` (ErrorRenderer)
  How ellipses are represented.

  The available options are:

  - `ellipse`
  - `crosshair_ellipse`
  - `filled_ellipse`
  - `rectangle`
  - `crosshair_rectangle`
  - `filled_rectangle`
  - `open_triangle`
  - `filled_triangle`
  - `lines`
  - `capped_lines`
  - `arrows`

  [Default: `ellipse`]

`icmdN = <cmds>` (ProcessingStep[])
  Specifies processing to be performed on the layer N input table as specified by parameter `inN`. The value of this parameter is one or more of the filter commands described in Section 6.1. If more than one is given, they must be separated by semicolon characters (";"). This parameter can be repeated multiple times on the same command line to build up a list of processing steps. The sequence of commands given in this way defines the processing pipeline which is performed on the table.

  Commands may alteratively be supplied in an external file, by using the indirection character '@'. Thus a value of "`@filename`" causes the file `filename` to be read for a list of filter commands to execute. The commands in the file may be separated by newline characters and/or semicolons, and lines which are blank or which start with a '`#`' character are ignored.

`ifmtN = <in-format>` (String)
  Specifies the format of the input table as specified by parameter `inN`. The known formats are listed in Section 5.1.1. This flag can be used if you know what format your table is in. If it has the special value `(auto)` (the default), then an attempt will be made to detect the format of the table automatically. This cannot always be done correctly however, in which case the program will exit with an error explaining which formats were attempted. This parameter is ignored for scheme-specified tables.

  [Default: `(auto)`]

`inN = <table>` (StarTable)
  The location of the input table. This may take one of the following forms:
  - A filename.
  - A URL.
  - The special value "`-`", meaning standard input. In this case the input format must be given explicitly using the `ifmtN` parameter. Note that not all formats can be streamed in this way.
  - A scheme specification of the form `:<scheme-name>:<scheme-args>`.
  - A system command line with either a "`<`" character at the start, or a "`|`" character at the end ("`<syscmd`" or "`syscmd|`"). This executes the given pipeline and reads from its standard output. This will probably only work on unix-like systems.
  In any case, compressed data in one of the supported compression formats (gzip, Unix compress or bzip2) will be decompressed transparently.
`istreamN = true|false` (Boolean)
  If set true, the input table specified by the `inN` parameter will be read as a stream. It is necessary to give the `ifmtN` parameter in this case. Depending on the required operations and processing mode, this may cause the read to fail (sometimes it is necessary to read the table more than once). It is not normally necessary to set this flag; in most cases the data will be streamed automatically if that is the best thing to do. However it can sometimes result in less resource usage when processing large files in certain formats (such as VOTable). This parameter is ignored for scheme-specified tables.

  [Default: `false`]

`latN = <deg-expr>` (String)
  Latitude in decimal degrees.

  The value is a numeric algebraic expression based on column names as described in Section 10.

`laterrN = <num-expr>` (String)
  Error in the latitude coordinate. The units of this angular extent are determined by the `unit` option.

  The value is a numeric algebraic expression based on column names as described in Section 10.

`lonN = <deg-expr>` (String)
  Longitude in decimal degrees.

  The value is a numeric algebraic expression based on column names as described in Section 10.

`lonerrN = <num-expr>` (String)
  Error in the longitude coordinate. The supplied value is considered to be premultiplied by cos(Latitude). The units of this angular extent are determined by the `unit` option.

  The value is a numeric algebraic expression based on column names as described in Section 10.

`scaleN = <number>` (Double)
  Scales the size of variable-sized markers like vectors and ellipses. The default value is 1, smaller or larger values multiply the visible sizes accordingly.

  The main purpose of this option is to tweak the visible sizes of the plotted markers for better visibility. The `unit` option is more convenient to account for the units in which the angular extent coordinates are supplied. If the markers are supposed to be plotted with their absolute angular extents visible, this option should be set to its default value of 1.

  [Default: `1`]

`shadingN = auto|flat|translucent|transparent|density|aux|weighted <shade-paramsN>` (ShapeMode)
  Determines how plotted objects in layer N are coloured. This may be influenced by how many objects are plotted over each other as well as the values of other parameters. Available options (Section 8.4) are:
  - `auto`
  - `flat`
  - `translucent`
  - `transparent`
  - `density`
  - `aux`
  - `weighted`
  Each of these options comes with its own set of parameters to specify the details of how colouring is done.

  [Default: `auto`]

`unitN = scaled|radian|degree|minute|arcsec|mas|uas` (AngleUnit)
  Defines the units in which the angular extents are specified. Options are degrees, arcseconds etc. If the special value `scaled` is given then a non-physical scaling is applied to the input values to make the the largest markers appear at a reasonable size (a few tens of pixels) in the plot.

  Note that the actual plotted size of the markers can also be scaled using the `scale` option; these two work together to determine the actual plotted sizes.

  The available options are:

  - `scaled`: a non-physical scaling is applied based on the size of values present
  - `radian`: radians
  - `degree`: degrees
  - `minute`: arcminutes
  - `arcsec`: arcseconds
  - `mas`: milli-arcseconds
  - `uas`: micro-arcseconds

  [Default: `degree`]

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Next: skydensity
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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk