ST_Densify#

Table of Contents#

  1. What is ST_Densify

  2. ST_Densify Input Data

  3. Using ST_Densify

[1]:

import bdt
bdt.auth("bdt.lic")
from bdt.functions import *
from pyspark.sql.functions import *
from bdt.processors import *

BDT has been successfully authorized!

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BDT python version: v3.5.0-v3.4.0-develop-7-gb089d527
BDT jar version: v3.5.0-v3.4.0-develop-7-gb089d527

Part 1: What is ST_Densify#

  • ST_Densify is a function to add additional vertices along a line or polygon geometry. Vertices are added at an interval that does not exceed a specified distance.

  • If the specified distance is larger than the total length of the geometry, then the distance will default to half the total length of the geometry.

  • See more details here

ST Densify Overview

Part 2: ST_Densify Input Data#

  • Use Spark to create a sample dataframe containing a polyline

  • Below is a visualization of the vertices and polyline

[2]:

line_df = spark.createDataFrame([("LINESTRING (0 0, 8 8)",)], ["WKT"])
line_df = line_df.select(st_fromText(col("WKT")).alias("SHAPE"))
line_df.show()



+--------------------+
|               SHAPE|
+--------------------+
|{[01 05 00 00 00 ...|
+--------------------+

ST Densify Example 1

  • Use Spark to create a sample dataframe containing a polygon

  • Below is a visualization of the vertices and polygon

[3]:

poly_df = spark.createDataFrame([("MULTIPOLYGON(((0 0, 0 2, 2 2, 2 0, 0 0)), ((3 3, 3 5, 5 5, 5 3, 3 3)))",)], ["WKT"])
poly_df = poly_df.select(st_fromText(col("WKT")).alias("SHAPE"))
poly_df.show()

+--------------------+
|               SHAPE|
+--------------------+
|{[01 06 00 00 00 ...|
+--------------------+

ST Densify Example 2

Part 3: Using ST_Densify#

  • The ST_Densify function requires the following arguments:

  • struct – The SHAPE struct with wkb, xmin, ymin, xmax, ymax

  • distance – The maximum distance between vertices to densify by. Can also be a percentage. The distance will be in the units of the input shape struct spatial reference.

  • isPercent – Boolean flag to set if using a percentage for distance param.

First use the densify function with a small distance of 5.0

[4]:

line_out_df = line_df.select(st_asText(st_densify("SHAPE", 5.0, False)).alias("WKT"))
line_out_df.show(truncate=False)

+--------------------------------------------------------------------------------------------------------+
|WKT                                                                                                     |
+--------------------------------------------------------------------------------------------------------+
|MULTILINESTRING ((0 0, 2.6666666666666665 2.6666666666666665, 5.333333333333333 5.333333333333333, 8 8))|
+--------------------------------------------------------------------------------------------------------+

ST Densify Example 3

  • Use the densify function with a larger distance of 75.0

  • In this example with the points (0 0, 8 8), using the distance forumla the total length of the geometry should be ~11.313708498985

  • Since the 75.0 > 11.3, the distance between vertices will default to half the total length of the geometry (11.313708498985 * 0.5)

[5]:

line_out_df = line_df.select(st_asText(st_densify("SHAPE", 75.0, False)).alias("WKT"))
line_out_df.show(truncate=False)

+---------------------------------+
|WKT                              |
+---------------------------------+
|MULTILINESTRING ((0 0, 4 4, 8 8))|
+---------------------------------+

ST Densify Example 4

  • Use the densify function with the polygon df and define a percentage of the total geometry distance

[6]:

poly_out_df = poly_df.select(st_asText(st_densify("SHAPE", 10.0, True)).alias("WKT"))
poly_out_df.show(truncate=False)

+---------------------------------------------------------------------------------------------------------------+
|WKT                                                                                                            |
+---------------------------------------------------------------------------------------------------------------+
|MULTIPOLYGON (((0 0, 1 0, 2 0, 2 1, 2 2, 1 2, 0 2, 0 1, 0 0)), ((3 3, 4 3, 5 3, 5 4, 5 5, 4 5, 3 5, 3 4, 3 3)))|
+---------------------------------------------------------------------------------------------------------------+

ST Densify Example 5