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Overlay rules

Metview’s overlay settings determine which data are combined on the same plot rather than appearing sequentially in the frame list. For example, we can plot with temperature, geopotential and wind overlaid together. Metview has some default rules, which can be overridden using the Overlay Control parameter in the view icons.

To understand what Metview does with data contained in a single data icon, see Rule 1:

titleRule 1

 Multiple data contained in a single icon or file (data unit) are never overlaid

In practice, this means that a GRIB file with 5 fields is visualised as 5 separate plots which you can scroll through.

If you wish to overlay data, then you must provide separate data icons for each ‘layer’. Then we are subject to Rule 2:

titleRule 2

Multiple data provided in several data icons are overlaid according to user or system defined overlay control parameters

Overlay settings

The Geographical View icon contains a parameter Map Overlay Control, which determines the overlay behaviour when plotting multiple data icons together. Its possible values are:

  • Always: fields from different data units are always overlaid; the first fields from each data unit form the first plot frame, the second fields form the second frame and so on
  • Never: fields are never overlaid, but will appear sequentially
  • By Date: fields with the same valid date and time will be overlaid; there will be one plot frame for each distinct date and time across all the data units
  • By Level: fields with the same vertical level will be overlaid; there will be one plot frame for each distinct level across all the data units

The default behaviour is Always. This can be changed by modifying the default Geographical View icon.