Describe the New Feature

This is a request from the UK MetOffice made during the METplus MetOffice telecon on 5/5/2022. @robdarvell ran gen_vx_mask on a rotated lat/lon grid and was surprised by the result. Using a polyline consisting of 4 points, he found that the edges of the resulting mask were curved rather than straight lines.

The existing -type poly option is performed in lat/lon space. The user provides a polyline consisting of lat/lon points. gen_vx_mask loops through all of the grid points, converts each from (x, y) to (lat, lon) space, and determines if that (lat, lon) point falls inside the lat/lon polyline.

The task here is to add a new option for -type poly_xy. While the user still provides a polyline consisting of lat/lon points, those points are converted into grid (x, y) space. gen_vx_mask loops through all of the grid points and determines if each (x, y) grid point falls inside the x/y polyline.

The former produces straight lines on the earth while the latter produces straight lines on the grid. Both are reasonable approaches.

Also note that the MetOffice would like to add a coordinates variable attribute to gridded NetCDF output written by MET. That change would apply to masks written by Gen-Vx-Mask as well as the many other MET tools that write gridded NetCDF output.

Acceptance Testing

• Add new -type poly_xy option and add a unit test to demonstrate it.
• Confirm that the gridded NetCDF output files written by MET include the coordinates = "lat lon"; variable attribute.

Time Estimate

4 hours.

Sub-Issues

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None needed.

Relevant Deadlines

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Funding Source

MetOffice support key (2799991)

Define the Metadata

Assignee

• Select engineer(s) or no engineer required: @JohnHalleyGotway
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Define Related Issue(s)

Consider the impact to the other METplus components.

• METplus, MET, METdatadb, METviewer, METexpress, METcalcpy, METplotpy
• Coordinate with @georgemccabe to determine if an update to the Gen-Vx-Mask wrapper is needed.

New Feature Checklist

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