[Proj] Finally: geodesic-1.0 is available

Gerald I. Evenden geraldi.evenden at gmail.com
Thu Jan 8 18:11:02 EST 2009

On Thursday 08 January 2009 5:10:07 pm Charles Karney wrote:
> >> I find the mini command language within the program (as opposed to
> >> a complex set of commandline options) an odd approach.
> >
> > I do not find it too odd.  For example, I use gp to compute the
> > constants and what all and it is often used in the same manner.  Even
> > maxima.  The difference is that I do not evaluate math or complex
> > expressions.  ...
> I note that the following "works":
>  echo "earth ellps=WGS84; p1 10 20; p2 30 40" | ./geodesic -n
> so that geodesic can be used as part of a pipe.  A slight complication
> is that the output comes in multi-line form which will typically need to
> be decoded.  In this way tabular data can be fed into the tool and
> tabular results can be generated.

Yes indeed, that is a quick way to use geodesic.  Admittedly, that mode of 
usage was not what I had in mind.

My first problem is that unlike projections where I can imagine someone 
processing untold millions of points in a data-in->data-out, I could not 
image that usage in the case of geodesic information other than the two 
interpolation operations I included.  Secondly, the format of result of 
massive processing is difficult for me to predict.

My only intention was to make an easily understood routine where one or two 
operations were expected.  Your example was imaginative but it rather falls 
back into the area that many users feel frustrated by: the "jargon" of Unix 
commands and methodology.

> If you did want to facilitate this mode of usage, then I would suggest
> something like
> (1) ellipse can be specified on command line
> (2) flag -f to specify forward calc
>     input line = lon1 lat1 dist azi1
>     output line = lon2 lat2 azi2
> (3) flag -r to specify reverse calc
>     input line = lon1 lat1 lon2 lat2
>     output line = dist azi1 azi2

Somewhat like the old geod program.  But again, I wanted something a little 
more for casual usage.  For example, I personally do not do geodesic 
calculations very often so that if I was faced with the above usage I'd have 
to dig out the manual or ponder a *.1 file and fuss around getting the 
operation going.  In geodesic's case I feel I could come back to the program 
months from now and only have to remember to key in "geodesic" and the rest 
of the operation is a piece of cake.

> A few other comments:
> (1) Decimal degrees are nice (especially in a pipe).  How do I get them?
> (2) Prec changes the precision of lat/lon angles but not azimuths.

I know.  One of those things to do.  Some experimentation is required to 
figure out how azimuth precision is related to distance precision.  It is not 
the easy problem of the geographic coordinates.

> (3) How is the "height" used?  It doesn't seem to affect the results.

It has no effect with this method.  I was thinking of including NGS's 3D 
method but their application was only for one ellipse and not very general.  
I was getting into an area of expertise I was uncomfortable with.

I left h in as I wanted the point structure to be general enough to be usable 
in other application where perhaps XYZ information was either input or 

> (4) No word on the accuracy?  For Vincenty, I've read 1 mm except for
> near antipodal in which case don't believe the results.  Does this apply
> here?

I have compare a few points earlier but deferred this until later.  From other 
communications I feel that Vincenty is accurate enough for most practical 

> On the last point:
> Does anyone know of a large set of test data?  The papers on the
> geodesic paper typically present a handful of cases.  Instead I'm
> thinking of a large test set mostly with randomly chosen points but with
> some significant fraction of "hard" examples (near antipodal, points
> near the poles, etc.).  I'm thinking that these could be produced by
> doing the forward calculation to high precision (using as many terms in
> the expansion of the integral to get to 1 nm precision or better on the
> WGS84 ellipsoid).  The result data could be used to test both forward
> and reverse geodesic calculations.

I know that the antipod is a problem and that is what seems to attract the 
interest, but, for the life of me, I cannot see what the fuss is about.  
Sorry, that's just my attitude about it.

Thank-you very much for the evaluation and comments.

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist

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