Thanks for your nudge 07.11.2024 23:00:04 David Haynes <hayne...@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 <thi...@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 >>
