ST_Aspect
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Nome

ST_Aspect — Retorna o aspecto (em graus) de uma banda raster de elevação. Útil para analisar terrenos.

Sinopse

raster ST_Aspect(raster rast, integer band=1, text pixeltype=32BF, text units=DEGREES, boolean interpolate_nodata=FALSE);

raster ST_Aspect(raster rast, integer band, raster customextent, text pixeltype=32BF, text units=DEGREES, boolean interpolate_nodata=FALSE);

Description

Retorna o aspecto (em graus) de uma banda raster de elevação. Utiliza mapa algébrico e aplica o aspecto de equação para pixeis vizinhos.

units indica as unidade do aspecto. Possíveis valores são: RADIANOS, GRAUS (padrão).

Quando units = RADIANOS, valoers são entre 0 e 2 * pi radianos medidos sentido horário a partir do Norte.

Quando units = GRAUS, valores são entre 0 e 360 graus medidos a partir do Norte.

Se o declive de pixel for zero, o aspecto do pixel é -1.

[Nota]

Para maiores informações sobre declive, aspecto e sombreado, por favor recorra a ESRI - How hillshade works e ERDAS Field Guide - Aspect Images.

Disponibilidade: 2.0.0

melhorias: 2.1.0 Usa ST_MapAlgebra() e foi adicionado uma função parâmetro opcional interpolate_nodata

Alterações: 2.1.0 Nas versões anteriores, os valores retornados eram em radianos. Agora, eles retornam em graus

Exemplos

Variant 1.

This symmetric peak highlights the expected compass directions at the corners and the -1 aspect at the summit. The generated PNG shows the full aspect surface without relying on a manual illustration. The generated source raster is large enough for the PNG to show the directional field around the summit.

Code
WITH pixels AS (
    SELECT
        x,
        y,
        (20 - greatest(abs(x - 9), abs(y - 9)))::double precision AS value
    FROM generate_series(1, 17) AS y
    CROSS JOIN generate_series(1, 17) AS x
), rows AS (
    SELECT
        y,
        array_agg(value ORDER BY x) AS row_values
    FROM pixels
    GROUP BY y
), source AS (
    SELECT ST_SetValues(
        ST_AddBand(
            ST_MakeEmptyRaster(17, 17, 0, 0, 1, -1, 0, 0, 0),
            1, '32BF', 0, -9999
        ),
        1, 1, 1,
        array_agg(row_values ORDER BY y)::double precision[][]
    ) AS rast
    FROM rows
), aspect AS (
    SELECT ST_Aspect(rast, 1, '32BF') AS rast
    FROM source
)
SELECT
    round(ST_Value(rast, 1, 1, 1)::numeric, 3) AS northwest_degrees,
    round(ST_Value(rast, 1, 9, 9)::numeric, 3) AS summit_degrees,
    round(ST_Value(rast, 1, 17, 17)::numeric, 3) AS southeast_degrees,
    ST_AsPNG(ST_ColorMap(rast, 1, 'bluered')) AS image
FROM aspect;
Raster Outputs
northwest_degrees | summit_degrees | southeast_degrees | image
-------------------+----------------+-------------------+------------------------
           315.000 |         -1.000 |           135.000 | PNG image, 17 x 17 pixels
Figure
Geometry figure for visual-rt-st-aspect-01

Variant 2.

Complete example of tiles of a coverage.

Code
WITH foo AS (
    SELECT ST_Tile(ST_SetValues(ST_AddBand(ST_MakeEmptyRaster(6, 6, 0, 0, 1, -1, 0, 0, 0),
                1, '32BF', 0, -9999
            ),
            1, 1, 1, ARRAY[
                [1, 1, 1, 1, 1, 1],
                [1, 1, 1, 1, 2, 1],
                [1, 2, 2, 3, 3, 1],
                [1, 1, 3, 2, 1, 1],
                [1, 2, 2, 1, 2, 1],
                [1, 1, 1, 1, 1, 1]
            ]::double precision[]
        ),
        2, 2
    ) AS rast
)
SELECT
    t1.rast,
    ST_Aspect(ST_Union(t2.rast), 1, t1.rast)
FROM foo t1
CROSS JOIN foo t2
WHERE ST_Intersects(t1.rast, t2.rast)
GROUP BY t1.rast;