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 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 > >
