[GRASS-dev] Adding hexagonal rasters to GRASS

[Vaclav Petras]

Dear Luís,

On Sun, Jul 23, 2017 at 11:50 AM, Luí­s Moreira de Sousa <
luis.de.sousa at protonmail.ch> wrote:

>
> I presented hex-utils at the GISTAM conference in April, here is the video:
> https://www.youtube.com/watch?v=uLO4HDCVBp0
>
>
thank you, this is very informative and exiting!


>
> But as Luca said during the Q/A, it is probably better to start outright
> within GRASS. This will require the abstraction of the raster concept,
> together with operations like neighbourhood, number of neighbours, distance
> to neighbours, etc. With that done most of the raster modules would
> function the same way for squared and hexagonal rasters. Still, this would
> require a complete revision of all the raster modules.
>

Sounds good. I think there is a way in GRASS GIS to make it revolutionary
and at the same time keeping everything in place. See below.


>
> For a programmer, the main difference between hexagonal and squared
> rasters is the cell addressing system: with hexagons the axis are not
> orthogonal.
>

If there would be a way, like the abstraction you mentioned, which would,
e.g. make the current rectangle-only code work with hexagonal data (e.g. by
presenting converting hexagons as rectangles on the fly), that would be
helpful, but on the other side, I don't think it is necessarily the first
step if there is an other way how to connect with rectangular rasters (even
if it involves converting back and forth or less precision).


> However, it is possible to store an hexagonal raster in a bi-dimensional
> array like you would store a squared raster.
>

That would help the implementation and hopefully also the integration and
adoption, again see below.


> I wrote an article last year with full details on the HexASCII file format
> and how to deal with hexagonal cell adressing, but it is still under
> review. I can not send it to the list, please send me a personal message if
> you wish to read it.
>

If you can share it, I would like to see it.


>
> It would be great if we can take this forward. I imagine it is a load of
> work, but it will make GRASS an even more awesome system ;)
>
>
Load of work and resulting code to maintain should be part of the
consideration. Using existing functionality (algorithms, formats) and
integrating with it will be crucial for both users and developers alike.

Can you implement it this as a layer on top of existing functionality in
GRASS GIS? There is several examples and concepts in GRASS GIS of this. For
example, the temporal (t.*) modules work with series of rasters and vectors
registered as spatio-temporal datasets and the image processing (i.*)
modules work with series of rasters registered as imagery groups and
subgroups ("multiband image"). In both cases standard storage of rasters
(or vectors) is used, so there is no new backend format in GRASS database
except for the additional data (metadata) needed for these tools. What is
important is that you can work with the existing tools on the data handled
by temporal or imagery modules, i.e. you can use the existing tools to
implement the new tools (e.g. average of spatio-temporal raster dataset can
be implemented through a tool for average of multiple rasters) and if the
imagery or temporal modules miss specific functionality you need, you can
fallback to the standard modules (e.g. loop over members of imagery group).

I was thinking that the hexagonal rasters can be implemented in GRASS GIS
using two rasters which would be shifted against each other, but aligned
with the hexagonal cells, i.e. two rectangular rasters making up one
hexagonal raster/grid (but I didn't test that or examined that more). After
seeing your video with slightly rotated hexagonal grid, I'm thinking if it
is possible to fit only one raster but with different resolution to the
hexagons. The idea is that 1) you don't need another storage format and 2)
you can use the raw data without the hexagon-aware algorithms. Of course we
can settle just with re-using the storage format (1).

Another approach is to threat hexagonal rasters/grids as another basic data
type next to raster maps, vector maps, and 3D raster maps (in GRASS GIS
there are also the aforementioned spatio-temporal datasets and imagery
groups). They would be connected to the rest where it makes sense. For
example, (2D) rasters and 3D rasters are independent, but there is also
several connections: there are modules for conversion in between the two
types, a lot of code for handling colors is the same, and computational
region applies to both and is handled from the same module.

If I may ask, what would you need to create a prototype and what do you
envision as a final goal?

Best,
Vaclav
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