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All of this can also be put into a macro, where the resulting netcdf variable can be further manipulated before being written to file (or visualised).
Geopoints
Format overview
Geopoints is the ASCII format used by Metview to handle spatially irregular data (e.g. observations). There are a number of variations on the format, but the default one is a 6-column layout. The columns do not have to be aligned, but there must be at least one whitespace character between each entry.
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Variants of the format allow 2-dimensional variables to be stored (e.g. U/V or speed/direction wind components), and another variant stores only lat, lon and value for a more compact file.
Examining geopoints
Examine the supplied geopoints.gpt icon to confirm the contents of the file. The columns are sortable. You may wish to open the file in an external text editor to see exactly what it looks like.
Visualising geopoints
Visualise the icon. The visdef used for geopoints is Symbol Plotting, and its default behaviour is to plot the actual numbers on the map. This can become cluttered, and text rendering can be slow. Drop the supplied coloured_symbols icon into the Display Window to get a better view of the data.
Manipulating geopoints in Macro
First, we will print some information about our geopoints data. Create a new Macro icon, type this code and run it:
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gp = read('geopoints.gpt')
print('Num points: ', count(gp))
print('Min value: ', minvalue(gp))
print('Max value: ', maxvalue(gp)) |
Perform a simple data manipulation and return the result to Metview's user interface:
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return gp*100 |
Save the macro and see its result by right-clicking on its icon and choosing examine or visualise. We could also have put a write() command into the macro to write the result to a geopoints file.
As a more complex example, we will combine two functions in order to find the locations of the points within a certain distance of a given location.
The distance() function returns a new geopoints variable based on its input geopoint, where each point's value has been replaced by the distance of that point from the given location. The description of this function follows:
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Returns geopoints with the value of each point being the distance in meters from the given geographical location. The location may be specified by supplying either two numbers (latitude and longitude respectively) or a 2-element list containing latitude and longitude in that order. The location should be specified in degrees. |
Choose a location and use this function to compute the distances of the points from it. Assign the result to a variable called distances and return it to the user interface to examine the numbers.
Now we will see a boolean operator in action. The expression distances < 10000 will return a new geopoints variable where, for each point, if the input value was less than 10000, the resulting value will be 1; otherwise the resulting value will be zero. So the resulting geopoints will have a collection of ones and zeros. Confirm that this is the case.
The filter() function, from the documentation:
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A filter function to extract a subset of its geopoints input using a second geopoints as criteria. The two input geopoints must have the same number of values. The resulting output geopoints contains the values of the first geopoints where the value of the second geopoints is non-zero. It is usefully employed in conjunction with the comparison operators :
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Use this in combination with what you have already done to produce a geopoints variable consisting only of the points within 10km of your chosen location.
Converting between geopoints and GRIB
? Needed? Already covered in the Processing Data part! XXXXXX
ASCII Table Data