Composing raster tiles?

[Vera Green]

We use command line GDAL for all our Easter processes, if your data is
large I recommend you look into that option.

On Fri, Nov 8, 2024, 6:12 AM David Haynes <haynesd2 at gmail.com> wrote:

> I'll try and help you with A & B
>
> a) Is it more efficient to convert the raster to vector data and calculate
> on the those than to calculate directly on the raster?
> >> I don't think it would be faster to convert to vector because that
> would be dumping the raster into polygons. I think for
> specifically calculating slope, you are better off staying in raster.
>
> b) To my understanding, if I calculate the slope on a raster tile,
> the slope,… of the borders will have accuracy problems. My I idea, was
> to "stitch" a tile with its direct neighbours, calculate on the composed
> tile, and either save a cropped calculated composed tile to its  original
> dimension or save the calculated composed tile as is,  probably the latter.
>
> >> Overall correct, some small adjustments. For any raster operation, the
> tiles are operated on independently, so you need to "stitch" them together.
> I've provided a couple of ways in pseduo code
>
> Way 1
> 1) Use ST_Union and make a big tile,
> 2) Do the spatial operation,
> 3) Break it up using ST_Tile()
> The downside is you might run out of memory doing this. Also consider
> ST_MemUnion()
>
> SELECT ST_Tile(ST_Slope(ST_Union(r1.ras)), 350,350)
> FROM raster_table
>
> Way 2)
> A second option is to basically create an aggregate, which is likely
> faster.
> 1) Union the tiles based on ST_Touch  - make mega_tiles
> 2) Do the spatial operation on the mega_tiles
> 3) Clip the mega_tiles by the old tiles bounding box
>
> WITH rtest as
> (
> SELECT r1.ras, ST_Union (r1.ras) as megatile
> FROM raster_table r1
> LEFT JOIN raster_table r2
> ON ST_Touches (r1.geom, r2.geom)
> GROUP BY r1.ras
> )
> SELECT ST_CLIP(ST_SLOPE(megatile), ST_Envelop(r1.rast) ) as ras
> FROM rtest
>
> Maybe someone wants to make an aggregate for the raster functions?
>
> On Thu, Nov 7, 2024 at 3:31 PM <thiemo at gelassene-pferde.biz> wrote:
>
>> Hi
>>
>> In my project
>>
>> https://sourceforge.net/p/treintaytres/code/HEAD/tree/trunk/code_files/data_storage/
>> I have the
>> table
>>
>> TABLE_SCHEMA    TABLE_NAME      DATA_TYPE       TYPE_NAME
>>  COLUMN_NAME
>> treintaytres    topo_files␟t    1111            uuid            id
>> treintaytres    topo_files␟t    93              timestamptz     entry_pit
>> treintaytres    topo_files␟t    1111            uuid            source_id
>> treintaytres    topo_files␟t    12              text            file_name
>> treintaytres    topo_files␟t    1111            raster          tile
>> treintaytres    topo_files␟t    93              timestamptz
>>  file_creation_pit
>> treintaytres    topo_files␟t    12              text            file_hash
>>
>> TILE contains topographical height raster data from OpenTopography.
>> They are from different regions, let's say, some tiles cover
>> Switzerland, some cover New Zealand. I want to create slope and other
>> data from the height data and I have some questions I hope you can
>> answer or point me to answers.
>>
>> a) Is it more efficient to convert the raster to vector data and
>> calculate on the those than to calculate directly on the raster?
>>
>> b) To my understanding, if I calculate the slope on a raster tile, the
>> slope,… of the borders will have accuracy problems. My I idea, was to
>> "stitch" a tile with its direct neighbours, calculate on the composed
>> tile, and either save a cropped calculated composed tile to its
>> original dimension or save the calculated composed tile as is,
>> probably the latter.
>> Can I compose as follows?
>> with RASTER_NEIGHBORS as (         select R1.TILE   as CURRENT_TILE
>>                                           ,R2.TILE   as NEIGHBOR_TILE
>>                                           ,R1.ID     as CURRENT_ID
>>                                       from TOPO_FILES␟T R1
>>                            left outer join TOPO_FILES␟T R2
>>                                         on ST_Touches(R1.TILE
>>                                                      ,R2.TILE)
>>                                         or ST_Intersects(R1.TILE
>>                                                         ,R2.TILE)
>>                                      where TRUE
>>                                        --and R1.ID =
>> '6b8ca53a-bb5f-4c2b-a9c9-94b6a706e9b0'
>>                                        and TRUE)
>>     ,NION as (select CURRENT_TILE as TILE
>>                     ,CURRENT_ID
>>                 from RASTER_NEIGHBORS
>>               union
>>               select NEIGHBOR_TILE as TILE
>>                     ,CURRENT_ID
>>                 from RASTER_NEIGHBORS)
>>    select ST_Union(TILE) as COMPOSED_TILE
>>          ,CURRENT_ID
>>      from NION
>> group by CURRENT_ID;
>>
>> c) Finally, I want to select all the areas where slope, TRI,… conform
>> certain criteria and have a minium surface size. Do I do it this
>> better on vector data and do I need to do this on data composed of all
>> the contiguous areas?
>>
>> I would be grateful for any nudge into the right direction. Maybe URLs
>> with samples.
>>
>> Kind regards
>>
>> Thiemo
>>
>>
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