[Proj] A rHealPix bug or I am misunderstanding something

[Rubén Béjar]

Hi people,

I am having some problems with the rHealPix projection and maybe you can 
help me.

In (0,0)-rHealPix (i.e. north_square = south_square = 0), I understand 
that the top left corner is (-PI, 3*PI/4).

However trying to project that point to lon-lat WGS84 fails.

I am running proj 5.2.0 (September 15th, 2018) in a terminal in Ubuntu 
18.04 (I have compiled it from sources, I am not using the older version 
from the repositories).

I assumed that it could be due to some tolerances/epsilon issues. 
However it does not seem to be so. I have tried with this command:

proj +proj=rhealpix -f '%.4f' -I +lon_0=0 +a=1 +ellps=WGS84 
+north_square=0 +south_square=0 -E

and some of the results I get are these:

-3.14 2.35
-3.14 2.35      *       *
-3.13 2.34
-3.13 2.34      90.3176 42.4654
-3.138 2.348
-3.138 2.348    90.3144 41.9431
-3.139 2.349
-3.139 2.349    *       *


So even after "pushing" the top left corner to the right (from -PI to 
-3.14, big difference) and down (from 3*PI/4 to 2.35, again big 
difference) I still don't get a conversion. The closest to the "real" 
point that seems to work is around (-3.138, 2.349) which is far from 
that corner (much further than a tolerance).

This corner is a simple, easily reproducible case. I am having similar 
problems close to the edges of the N square (and possibly close to some 
of the edges of other squares too).

Am I doing something wrong, or is this a problem with rHeallPix in Proj.4?

Thanks in advance!

   Rubén


-- 

Dr. Rubén Béjar
*Universidad* Zaragoza <https://www.unizar.es>
Advanced Information Systems Laboratory <https://www.iaaa.es/>
Aragon Institute of Engineering Research <http://i3a.unizar.es/>

rbejar at unizar.es <mailto:rbejar at unizar.es> (@rubejar in Twitter) 
<https://twitter.com/rubejar>
*rubenbejar*.com <https://www.rubenbejar.com>
----

Departamento de Informática e Ingeniería de Sistemas

María de Luna 1 / 50018 / Zaragoza (Spain)
Tlf: +34 876 555534 / Fax: +34 976 761914

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