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WERA HF Radar WGS84 Coordinates
As of December 2007, the WERA processing software takes the locations of the sites, angle and range, as well as the grid points
defined by Make_Grid to be given in WGS84 coordinates. This is the coordinate reference system
used by the GPS (Global Positioning System) in most cases.
The Fortran code used for the WGS84 calculations has been derived from source coded hosted by the
National Geodetic Survey
[look for 'INVERSE/FORWARD3D (Version 2.0)']. The code is based on a paper by Vincenty [1975].
Advantages of WGS84 Use
There are several advantages if the results of WERA current maps, ocean wave spectra, and wind direction is directly
given in WGS84 coordinates:
- There is no deviation between the location given by WERA and the location of ships or moored instruments, which usually
have been located using GPS.
- The electromagnetic wave is travelling along great circles on the earth's surface and not following straight lines in
Mercator-like maps. The propagation physics of the EM waves is now taken into account in a more accurate way.
- And finally, the data supplied to the US coastal radar network are required to be based on WGS84 coordinates.
Compatibility with pre-WGS84 Results
The coordinates of the WERA sites and the measurement grid points are now interpreted to be in the WGS84 reference system.
With increasing range, the deviation between WERA's old reference system and WGS84 also increases. For distances of ~200 km
a deviation in the order of a few kilometres can occur.
It is impossible to convert an "old" measurement grid to WGS84 coordinates in a general way. It can only be done using one
WERA location as a reference point. The transformation would be from the old carthesian measurement grid to a radial grid
originated at the WERA site using the old Mercator-like transformation. From this radial grid, the new WGS84-based carthesian
grid can be calculated using the WGS84 transformation. Obviously, the resulting WGS84-based grid coordinates depend on the
WERA site location and can not be calculated in a general way. If significant deviations are observed when processing pre-WGS84 and
WGS84 data sets together, the best way is to reprocess the old .SORT data as required.
Modified Processing Programs
The following programs have been modified to use the WGS84 reference system:
Please check the programs for the modifications.
Comments to:
gurgel@ifm.uni-hamburg.de
(Klaus-Werner Gurgel)