tf_geo_rasterize_slope

Similar to tf_geo_rasterize, but also computes the slope and aspect per output bin. Aggregates point data into x/y bins of a given size in meters to form a dense spatial grid, computing the specified aggregate (using agg_type) across all points in each bin as the output value for the bin. A Gaussian average is then taken over the neighboring bins, with the number of bins specified by neighborhood_fill_radius, optionally only filling in null-valued bins if fill_only_nulls is set to true. The slope and aspect is then computed for every bin, based on the z values of that bin and its neighboring bins. The slope can be returned in degrees or as a fraction between 0 and 1, depending on the boolean argument to compute_slope_in_degrees.

Note that the bounds of the spatial output grid will be bounded by the x/y range of the input query, and if SQL filters are applied on the output of the tf_geo_rasterize_slope table function, these filters will also constrain the output range.

SELECT * FROM TABLE(
  tf_geo_rasterize_slope(
      raster => CURSOR(
        SELECT 
           x, y, z FROM table
      ),
      agg_type => <'AVG'|'COUNT'|'SUM'|'MIN'|'MAX'>,
      bin_dim_meters => <meters>, 
      geographic_coords => <true/false>, 
      neighborhood_fill_radius => <radius in bins>,
      fill_only_nulls => <true/false>,
      compute_slope_in_degrees => <true/false>
    )
 )

Input Arguments

Parameter
Description
Data Types

x

Input x-coordinate column or expression.

Column<FLOAT | DOUBLE>

y

Input y-coordinate column or expression.

Column<FLOAT | DOUBLE>

z

Input z-coordinate column or expression. The output bin is computed as the maximum z-value for all points falling in each bin.

Column<FLOAT | DOUBLE>

agg_type

The aggregate to be performed to compute the output z-column. Should be one of 'AVG', 'COUNT', 'SUM', 'MIN', or 'MAX'.

TEXT ENCODING NONE

bin_dim_meters

The width and height of each x/y bin in meters. If geographic_coords is not set to true, the input x/y units are already assumed to be in meters.

DOUBLE

geographic_coords

If true, specifies that the input x/y coordinates are in lon/lat degrees. The function will then compute a mapping of degrees to meters based on the center coordinate between x_min/x_max and y_min/y_max.

BOOLEAN

neighborhood_fill_radius

The radius in bins to compute the box blur/filter over, such that each output bin will be the average value of all bins within neighborhood_fill_radius bins.

BIGINT

fill_only_nulls

Specifies that the box blur should only be used to provide output values for null output bins (i.e. bins that contained no data points or had only data points with null Z-values).

BOOLEAN

compute_slope_in_degrees

If true, specifies the slope should be computed in degrees (with 0 degrees perfectly flat and 90 degrees perfectly vertical). If false, specifies the slope should be computed as a fraction from 0 (flat) to 1 (vertical). In a future release, we are planning to move the default output to percentage slope.

BOOLEAN

Output Columns

Name
Description
Data Types

x

The x-coordinates for the centroids of the output spatial bins.

Column<FLOAT | DOUBLE> (same as input x column/expression)

y

The y-coordinates for the centroids of the output spatial bins.

Column<FLOAT | DOUBLE> (same as input y column/expression)

z

The maximum z-coordinate of all input data assigned to a given spatial bin.

Column<FLOAT | DOUBLE> (same as input z column/expression)

slope

The average slope of an output grid cell (in degrees or a fraction between 0 and 1, depending on the argument to compute_slope_in_degrees).

Column<FLOAT | DOUBLE> (same as input z column/expression)

aspect

The direction from 0 to 360 degrees pointing towards the maximum downhill gradient, with 0 degrees being due north and moving clockwise from N (0°) -> NE (45°) -> E (90°) -> SE (135°) -> S (180°) -> SW (225°) -> W (270°) -> NW (315°).

Column<FLOAT | DOUBLE> (same as input z column/expression)

Example

/* Compute the slope and aspect ratio for a 30-meter Copernicus 
Digital Elevation Model (DEM) raster, binned to 90-meters */

select
  *
from
  table(
    tf_geo_rasterize_slope(
      raster => cursor(
        select
          st_x(raster_point),
          st_y(raster_point),
          CAST(z AS float)
        from
          copernicus_30m_mt_everest
      ),
      agg_type => 'AVG',
      bin_dim_meters => 90.0,
      geographic_coords => true,
      neighborhood_fill_radius => 1,
      fill_only_nulls => false,
      compute_slope_in_degrees => true
    )
  )
order by
  slope desc nulls last
limit
  20;
  
x|y|z|slope|aspect
86.96533511629579|27.96534132281817|6212.096|78.37033|18.09232
87.23751907091268|27.78489838800869|3793.584|78.17864|125.03
87.23660262662104|27.78408922686605|3929.989|78.06877|127.629
86.96625156058742|27.96534132281817|6041.277|78.00574|19.00616
87.2356861823294|27.78328006572341|3981.662|77.53327|127.3175
86.96441867200414|27.96615048396082|5869.373|77.3751|20.82031
86.95800356196267|27.96857796738875|6083.791|77.13709|29.89468
86.96350222771251|27.96615048396082|6081.35|77.08266|21.6792
87.23843551520432|27.78570754915134|3630.32|77.04676|125.2154
86.96441867200414|27.96534132281817|6378.94|76.95021|17.77107
87.22468885082972|27.81321902800121|4771.554|76.71017|253.2764
87.2356861823294|27.78247090458076|3520.049|76.63997|113.6511
87.23660262662104|27.78328006572341|3445.282|76.38319|127.2889
86.96716800487906|27.96534132281817|5864.711|76.16835|19.27573
87.23476973803776|27.78166174343812|3945.683|76.13519|102.7789
86.95708711767104|27.96857796738875|6336.072|76.13168|24.90349
87.22468885082972|27.81240986685857|4732.937|76.07494|264.7046
87.23751907091268|27.78408922686605|3367.659|76.0099|126.7463
86.9589200062543|27.9677688062461|6223.083|75.46346|26.85898
87.22377240653809|27.81402818914385|4704.619|75.41299|205.3219
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