Hi Ben,
I don't think PROJ API is the best fit for DGGS implementations.
Probably https://github.com/ecere/dggal would be a better host (I
haven't used it myself)
And AFAIK, WKT / ISO-19111 hasn't been designed with DGGS use cases in
mind either.
Even
Le 22/03/2026 à 15:32, Ben Griffin via PROJ a écrit :
I am working on a CRS of the work I’ve been doing involved a series of novel
projections. (The current python module is hhg9),
My current PROJ (C++) implementation shows an RTE (round-trip error) of about
~15nm - and has been extensively tested.
The natural encoding of HEX9 a co-ordinate is hybrid, and involves a separation of the WGS84 ellipsoid into octants (carved at 0,90, etc).
This lends itself to a 3-tuple x,y, octant_id. Therefore I am currently using
PROJCRS["HEX9 Octahedral Barycentric",
BASEGEOGCRS["WGS 84”, …. ],
CONVERSION["H9 Octahedral Barycentric",
METHOD["PROJ h9_boct",
ID["HEX9","h9_boct"]]],
CS[Cartesian,3],
AXIS["hex9 x (X)",east,
ORDER[1],
LENGTHUNIT["metre",1]],
AXIS["hex9 y (Y)",north,
ORDER[2],
LENGTHUNIT["metre",1]],
AXIS["hex9 octant (O)",unspecified,
ORDER[3],
SCALEUNIT["unity",1]],
ID["HEX9","Oct”]]
(This is the registration of the underlying continuous projection — the octant
component plays the same role as, for example, a UTM zone number.
The discrete hex-cell addressing scheme of the supported DGG built on top of
this CRS is not what's being registered here).
The octant component is ordinal/nominal, not a third spatial axis — there's no meaningful
interpolation between octant 3 and octant 4. I'm encoding it as a 3rd Cartesian axis with
orientation=unspecified and SCALEUNIT["unity",1] because OrdinalCRS isn't
composable with ProjectedCRS in current PROJ. Is there a better-supported pattern for
this?
Is there any precedent for face-indexed projections (eg, HEALPix has the same
issue — 12 faces, each with a local Cartesian frame)? I considered using
EngineeringCRS but that seems to be a mistake - it is strongly tied to WGS84,
and lends itself better to DerivedGeographicCRS.
So, the last axis is not strictly Z.
The X, Y axes are also octant-specific and while X is ‘east’, and Y is on the
north/south axis, the direction of north is inverted on some octants.
I will be adding ProjectedCRS to the above base - but I thought it wise to post
a line to the list at this point.
Thoughts/Suggestions welcome!
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