| 53 | | * It would be interesting to have an option to create a second band with the length of the line or the area of polygon intersecting each pixel. This band could then be used in a ST_Union(rast, 'MAX_LENGTH') or a ST_Union(rast, 'MAX_LENGTH') function burning the value of the line having the longest intersection with the pixel or the value of the polygon having the biggest intersecting area. For this to be useful ST_Union should work with a ST_MapAlgebra(rast1, rast2) able to refer to pixel value in any band (e.g. rast1.2 referring to the pixel value in the second band). The ST_Union(rast, 'MAX_LENGTH') aggregate could then be implemented with the following state expression: 'CASE WHEN rast1.2 > rast2.2 THEN rast1.1 ELSE rast2.1 ENDIF'. Otherwise we would have to imbricate four Mapalgebra functions. ST_Union(rast, 'MAX_COMBINED_LENGTH') could use a ST_MapAlgebra(rast1, rast2) able to take a custom user function. The state function would accumulate, in a temporary table, the total intersecting length or area for a same value over every overlapping pixels and the final function would determine which of the values from the temporary table would be the right one to burn in the final raster pixel. |
| | 53 | * It would be interesting to have a 'ADD_PONDERATION_INFO' option to create a second band with the length of the line or the area of polygon (or the distance to the center of points) intersecting each pixel. This band could then be used in a ST_Union(rast, 'MAX_LENGTH') or a ST_Union(rast, 'MAX_LENGTH') function burning the value of the line having the longest intersection with the pixel or the value of the polygon having the biggest intersecting area. For this to be useful ST_Union should work with a ST_MapAlgebra(rast1, rast2) able to refer to pixel value in any band (e.g. rast1.2 referring to the pixel value in the second band). The ST_Union(rast, 'MAX_LENGTH') aggregate could then be implemented with the following state expression: 'CASE WHEN rast1.2 > rast2.2 THEN rast1.1 ELSE rast2.1 ENDIF'. Otherwise we would have to imbricate four Mapalgebra functions. ST_Union(rast, 'MAX_COMBINED_LENGTH') could use a ST_MapAlgebra(rast1, rast2) able to take a custom user function. The state function would accumulate, in a temporary table, the total intersecting length or area for a same value over every overlapping pixels and the final function would determine which of the values from the temporary table would be the right one to burn in the final raster pixel. Variations of ST_Union which could then be easily implemented could be: ST_Union(rast, 'PONDERATE') to ponderate the value with the length, the area or the count, ST_Union(rast, 'MOST_FREQUENT') to burn the value for the most frequent points values. |
