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This icon interpolates irregularly spaced point data (in Geopoints format) into a GRIB field, which can then be plotted, saved or combined with other GRIB data. The method used to compute the values of the grid points from the input data depends on the Interpolation Method parameter. The resulting GRIB field can be a regular lat/lon grid of defined size, or else based on a template GRIB file supplied by the user. Note that only the first parameter of a double-valued geopoints vector will be used in the calculations.

The macro language equivalent is geo_to_grib()

The Geopoints To GRIB Editor

Geopoints

Specifies the input data, which must be in Geopoints format (e.g. a geopoints data icon or an Observation Filter icon returning geopoints).

The Bufr Picker Editor

Data

Drop any icon containing or returning BUFR data. This may be, for example, a MARS Retrieval (of observations) icon, a BUFR file icon or an Observation Filter icon (provided it outputs BUFR, not geopoints).

Grib Definition Mode

If set to User, the output grid will be a regular lat/lon matrix defined by the parameters Area and Grid; if set to Grib, an example GRIB file should be specified by the parameter Template Grib.

Area

Specifies a geographical area over which to carry out the interpolation, the default being for the whole globe. Enter coordinates (lat/lon) of an area separated by a "/" (top left lat and lon, bottom right lat and lon); alternatively, use the assist button to define the area graphically. Only available if Grib Definition Mode is User.

Grid

Specifies a resolution in degrees, thus together with Area, determining the limits and density of the regular grid for interpolation of the point data values. Enter the longitude and latitude resolution as numbers separated by a "/". Only available if Grib Definition Mode is User.

Template Grib Definition

Specifies a GRIB file on which to base the output GRIB. The resulting GRIB field will be a copy of the template, with the values and parameter identifier modified.

Tolerance

Specifies a neighbourhood in degrees around each grid point. All geopoints data within this neighbourhood are used to interpolate the value at the central grid point. E.g. if Tolerance is 2 then all geopoints within a +/-2 degrees square around the grid point are used.

If your geopoints data has high spatial density then you can afford to specify a short neighbourhood, if the density is sparse you should use a wide neighbourhood. Remember that the wider the neighbourhood the smoother the resulting interpolated field (and the slower the computation).

Interpolation Method

Specifies how the values of the geopoints with in the window around a resulting grid point will be combined to produce the resulting value. The available algorithms are:

  • Reciprocal: the mean of the values, weighted by the inverse of their distance from the target point. If one of the geopoints lies exactly on the target point then its value is used.

  • Exponential Mean: computes the mean of the following:

    • if Threshold is not zero: \( e^{-distance/tolerance^2} \)

    • if Threshold is zero: 1 if the point is on the target point, 0 otherwise
    • note that this method, combined with setting Threshold to zero computes the proportion of points which lie exactly on the target point
  • Exponential Sum: performs the same computation as Exponential Mean, but does not finally divide by the total weight. With a Threshold of zero, this method will compute the number of input points that lie exactly on each target point.

 

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