Apparent geoToH3 or H3ToGeo bug on master

[travisbrady]

I used geoToH3 to encode the coordinates for NYC and then ran the H3 hex string through H3toGeo and got a confusing result.
This is with a fresh clone of master built per readme instructions and running on a Mac.

$ ./bin/geoToH3 12
40.7128 -74.0060
8c2a107289061ff

Then

$ ./bin/h3ToGeo
8c2a107289061ff
40.7127470097 285.9940373051

Unless I misunderstand the purpose of these tools, 285.9940373051 is obviously an illegal value and far from the expected -74.0060.

$ uname -a
Darwin HA002727 16.7.0 Darwin Kernel Version 16.7.0: Thu Jun 15 17:36:27 PDT 2017; root:xnu-3789.70.16~2/RELEASE_X86_64 x86_64

[sahrk]

285.9940373051 is the same longitude as -74.0060. h3ToGeo gives longitude values in the range 0-360 degrees.

[travisbrady]

Ahh, I see. Still somewhat confusing to me as I'd typically expect ThingToInt(IntToThing(blah)) == blah.

Does H3 provide any docs or functions for these sorts of conversions? I think many people often use latitudes in the range -180 to 180, so this kind of thing would tend (I'd think) to confuse other users too. But I'll close for now.

[dfellis]

I will note that the bindings (that haven't been released, yet, hi again @isaacbrodsky :) ) do convert back to -180 -> 180 (and -90 -> 90 for latitude) because that's what people tend to expect. I don't recall why we haven't pushed this down to the C code, but I do know it will cause some churn with the test suite.

[travisbrady]

@dfellis good to know as I'm currently writing some bindings of my own (in OCaml). I expect to have them on github in the next week or so.

[isaacbrodsky]

I'm OK with changing the C library/applications/examples to return negative angles, since that's what the bindings do anyways. (hi @dfellis :) ) Or, we should add this to the bindings documentation.

@travisbrady Please feel free to open a PR to add your bindings to the bindings list when you feel they're ready.

[nrabinowitz]

I'd support updating the C lib here. I think it subverts user expectations, and agree with @travisbrady's point that it breaks assumptions about round-trip stability.

[sahrk]

I should point-out that, unless you limit geoToH3 to only accept longitudes in the range +/-180 (which I don't recommend), then geoToH3/h3ToGeo can't be completely round-trip stable, in the sense that the user will necessarily get back their input longitude value in the original input range.

[nrabinowitz]

True, but we could guarantee stability for a defined valid input range, and I think it would be closer to user expectations to define that range as -180 - 180.

…

On Thu, Mar 29, 2018 at 9:52 AM sahrk ***@***.***> wrote: I should point-out that, unless you limit geoToH3 to only accept longitudes in the range +/-180 (which I don't recommend), then geoToH3/ h3ToGeo can't be completely round-trip stable, in the sense that the user will necessarily get back their input longitude value in the original input range. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#38 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAKAZNJCePNvlFXnoLXBQHSHGOpSLYyLks5tjRFUgaJpZM4S70pQ> .

[gacafe]

Resolved this generally... the input was incorrect (130 is not a lat), but bit concerning that the two libraries give different results for the same input.
There is a problem still.
Using the example, I expect h3_to_geo(geo_to_h3(lat,lon), resolution) to return the original lat,lon input, or something quite nearby... If I use the example, this is true, but if I do it in Australia, its not true -- the [-180,180] bounds are respected but the answer is shifted by 180:

11:37 AM gacafe ∈ h3 (master) ./bin/geoToH3 -r 15
130.90938479800002 -12.365644588
8f16c6d624a25a8
11:38 AM gacafe ∈ h3 (master) ./bin/h3ToGeo      
8f16c6d624a25a8
49.0906178199 167.6343623402

Using the Python library --

>>> h3_address = h3.geo_to_h3(37.3615593, -122.0553238, 15)
>>> hex_center_coordinates = h3.h3_to_geo(h3_address)
>>> hex_center_coordinates
[37.36156313952111, -122.05532505846305]
>>> h3_address = h3.geo_to_h3(130.90938479800002, -12.365644588,15)
>>> hex_center_coordinates = h3.h3_to_geo(h3_address)
>>> hex_center_coordinates
[-49.09061440581596, -12.36565153299568]