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Just a short question to lighten up my understanding of some PostGIS-code:

Regarding the creation of user-defined grids I often find the expressions CEILING and FLOOR. Like in this non-representative example:

SELECT floor(st_xmax(the_polygon)/1500) FROM ...

What are they exactly designated for?

Thanks in advance for lightening me up!

knutella

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1 Answer 1

up vote 10 down vote accepted

Floor and ceiling are classical computer and math functions that basically round numbers down or up.

select x, floor(x), ceil(x), trunc(x)
from (select unnest(ARRAY[-4.8, -4.2, 4.2, 4.8]) as x) as f;

  x   | floor | ceil | trunc
------+-------+------+-------
 -4.8 |    -5 |   -4 |    -4
 -4.2 |    -5 |   -4 |    -4
  4.2 |     4 |    5 |     4
  4.8 |     4 |    5 |     4
(4 rows)

In the context of building a grid, it can be used to define the extents of the grid to aesthetically-pleasing numbers. For example, if the x- extents of a layer go between Eastings 2754324.254 and 2770756.610, they can be "rounded" to the bounding 1 km grid with:

select floor(xmin/1000) * 1000 as xmin, ceil(xmax/1000) * 1000 as xmax
from (select 2754324.254 as xmin, 2770756.610 as xmax) as f;

  xmin   |  xmax
---------+---------
 2754000 | 2771000
(1 row)
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Hello Mike, thanks for the extensive answer! It makes sense now in the context, that I’m using it. I guess my main mistake was, to think FLOOR and CEILINING are special PostGIS-functions. That was also the reason, that Google didn't told me smth useful. If I look for it in connection with PostgreSQL the results are 'slightly' different.. %) Nevertheless thanks a lot for explaining it directly to the grid-manner=) –  knutella Jan 21 '13 at 14:53
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