Composing raster tiles?

[Thiemo Kellner]

Thanks for your nudge

07.11.2024 23:00:04 David Haynes <haynesd2 at gmail.com>:

> 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[http://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[http://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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