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#### A.4.4.6 Area Layer Control

The Area layer control () allows you to plot a two-dimensional shape from each row of a table. This might typically be something like an instrument coverage footprint corresponding to an observation. The shape may be specified in various different ways: as an STC-S region specification string (as found for instance in the `s_region` column of ObsCore or EPN-TAP query results), as an array specification of polygons or circles, or as an ASCII-encoded MOC. You can add one of these controls to the stack by using the Add Area Control () button in the control panel toolbar or the corresponding item in the Layers menu for suitable plot types (Plane, Sky or Sphere).

Areas specified in this way are generally intended for displaying relatively small shapes such as instrument footprints. Larger areas may also be specified, but there may be issues with use, for instance auto-determination of the initial plot region may not work so well, and the rendering of shapes that are large relative to the sky may be inaccurate. These issues may be addressed in future releases.

Sky Plot Window with an Area control

This control is a Table Data control as described in Appendix A.4.2.2. That section explains the Subsets tab; the Position and Form tabs are described in more detail below.

Position tab of Area layer control, for Sky Plot

In the Position tab, you just need to select the input table using the Table selector, and then supply the expression for the region specification in the Area selector. This area value will typically be a table column containing the relevant information, for instance the s_region column in tables resulting from an ObsCore or EPN-TAP query. However, it's also possible to enter a suitable expression, for instance "`array(RA,DEC,RADIUS)`" if you want to specify a CIRCLE-format shape given position and radius columns in the table.

The area may be specified in various formats, currently:

STC-S
  Region description using STC-S syntax; see TAP 1.0, section 6. Note there are currently some restrictions: `<frame>`, `<refpos>` and `<flavor>` metadata are ignored, polygon winding direction is ignored (small polygons are assumed) and the `INTERSECTION` and `NOT` constructions are not supported. The non-standard `MOC` construction is supported.
POLYGON
  2N-element array (x1,y1, x2,y2, ..., xN,yN); a NaN,NaN pair can be used to delimit distinct polygons.
CIRCLE
  3-element array (x,y,r).
POINT
  2-element array (x,y)
MOC-ASCII
  Region description using ASCII MOC syntax; see MOC 1.1 section 2.3.2. Note there are currently a few issues with MOC plotting, especially for large tiles.
UNIQ
  Region description representing a single HEALPix cell as defined by a UNIQ value; see MOC 1.1 sec 2.3.1.

The selector to the right of the Area selector defines which of these formats the selected value or expression will be understood as. TOPCAT tries to guess the right value when the Area selector is filled in; if it guesses wrong, you should select the correct option manually. If the Area value is changed, it will update the guess accordingly; if you don't want it to do that, use the Lock () button to stop it happening.

Form tab of Area layer control

The Form tab lets you define how the specified region data will be plotted. The list on the left gives a list of forms currently being plotted, and the panel on the right shows the detailed configuration for the currently selected form.

The following forms are currently available for the Area plot:

- Area: draws an outlined or filled area
- Central: plots the central point of an area
- AreaLabel: adds a text label near the center of an area

It can be useful to have multiple instances of the Area form, for instance to paint the outline and fill the interior separately. The Central and AreaLabel forms just plot points/text in the same way as Mark/Label, but they work with an Area coordinate rather than normal positional ones.

You can add a new form using the Forms button. Each such form can be configured separately using a panel divided into two parts:

Shading
  The shading mode controls how points are shaded based on their chosen colour. The various options are described in Appendix A.4.6. Depending on the mode there may be more settings to fill in here.
Global/Subset Styles
  Controls the style details for the chosen form; see Appendix A.4.2.2.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk