David Baker (Geoscience) wrote:
Frank and the List,
I have a geodetic problem I hope GDAL/OGR can solve.
The setup:
I have two radio towers each the same height and at the same elevation
being 10,000 meters apart. I know the latitude and longitude of the
first tower and those coordinates reference WGS84 (though they could be
any geographic CRS, NAD27, NAD83, etc.). I also now the baring to the
second from the first measured from true north at the first tower.
The problem:
Given the above information and using the prebuilt tools, GDAL/OGR
utilities, what workflow would one use to calculate the latitude and
longitude of the second tower?
The hints I have gotten so far say to use a conformal projection (UTM
maybe?) centered at the first point with a scale factor of 1, a local
projection. Calculate the Cartesian offsets from the first point to the
second based on the azimuth of the given baring. Using the baring
calculate the x and y offsets to the second point and use offsets to
find the x and y of the second point in the local coordinate system.
Then, reproject the x and y of the second point to the latitude and
longitude in the CRS of the first points latitude and longitude. If
this is correct, I guess the question is, what would the proj4
parameters be for such a local projections.
David,
The above will give a pretty good approximation, degrading as the
distance increases. However, the proper way to do this is using
great circle operations. The PROJ.4 "geod" program can do this:
http://trac.osgeo.org/proj/wiki/man_geod
There is more information on geodesic operations at:
http://trac.osgeo.org/proj/wiki/GeodesicCalculations
Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, [email protected]
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent
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