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Geopoints To Grib

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.

For the reverse computation, see Grib To Geopoints.

From Macro/Python this icon can be called as 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)

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 within 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 directly and the rest of the values discarded.
• Exponential Mean: computes the mean of the values weighted (multiplied) by the following:

• if Tolerance is not zero: $$e^{-distance/tolerance^2}$$

• if Tolerance is zero: 1 if the point is on the target point, 0 otherwise
• note that this method, combined with setting Tolerance 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 Tolerance of zero, this method will compute the number of input points that lie exactly on each target point.
• Nearest Gridpoint Mean: for each target grid point, computes the unweighted mean value of the geopoints for whom this is the closest grid point; any grid point which is not the closest to any geopoints will be given a missing value
• Nearest Gridpoint Sum: for each target grid point, computes the unweighted sum of the values of the geopoints for whom this is the closest grid point; any grid point which is not the closest to any geopoints will be given a missing value
• Nearest Gridpoint Count:  for each target grid point, computes the number of geopoints for whom this is the closest grid point. Note that for a regular target grid, this essentially produces a 'heat map', where the value of a grid point will be the number of geopoints within its grid box. This is not necessarily true for quasi-regular grids, e.g. reduced Gaussian, reduced lat/lon or octahedral (which is just a specific type of reduced Gaussian).

Parameter

If not set to 255, then the paramId GRIB_API key on the output field will be set to this value.

Grib Table2 Version

If Parameter is not set to 255, then the table2Version GRIB_API key on the output field will be set to this value.

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