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I want to calculate the difference between a raster and vector layer,

I used this querie but it's so slow and I don't know if it's right or not.

I'm not using st_difference because it only support vector layers

SELECT ST_Clip(a.raster,b.geom) FROM my_raster a, my_shape b WHERE not ST_Intersects(a.raster, b.geom)

I want to erase raster region wish is intersected with the vector layer

EDIT 1

Queries listed bellow work but they take a lot of time

To rasterise vector layer

create table rasterise_vector
select ST_AsRaster(st_transform(geom,4326), (select st_union(raster) from my_raster) , 2BUI') as raster
from shp

To calculate difference between rasters

create table diff_rasters as
SELECT ST_MapAlgebra(st_union(a.raster), st_union(b.raster), 'rast1 - rast2', '32BF', 'FIRST', NULL, 'rast1', NULL)
from my_raster a, rasterise_vector b

Table size :

Raster size Rows (estimation) = 1046 tiles

Vector size Rows (estimation) = 9854 items

enter image description here

could you please help me solve this problem?

3
  • Hard to know without any metrics on table sizes, polygon complexity, etc. How big, how long. What does explain show? Jul 3, 2015 at 9:45
  • 1
    How did you import your raster data into your database (100x100 tiles)? From th experiences I've made I think the union function could slow down the query noticable. Can you avoid it? What about tiling the new vector_raster?
    – Stefan
    Jul 4, 2015 at 9:03
  • exactly, I imported my ratser data with 100x100 tiles. I will test it by making tiles and give you my results
    – elmo
    Jul 5, 2015 at 0:28

1 Answer 1

2

Here is my idea:

  1. Use ST_AsRaster to rasterize your vector layer (think about the resolution of the new raster layer)
  2. Then use the ST_MapAlgebra function to create a new raster (new_raster values have raster values where vector_raster values are NULL or something like this.)
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  • 1
    This is a very good idea, I tested it but the difference of rasters takes a lot of time, I added queries to my question
    – elmo
    Jul 3, 2015 at 11:59
  • your answer is right, I made union out of queries and it's done
    – elmo
    Jul 13, 2015 at 13:35

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