Database – How to Find Closest Point on a Line from a Point in Oracle Spatial

databasenearest neighbororacle-dbmsoracle-spatial

I want to find the nearest point on a line from a point in Oracle Spatial. I know how to do in SQL Server and PostgreSQL but could not find anything about Oracle.
I have used MDSYS.sdo_nn to find the closest line from a point, but I need the closest point on line as well to calculate distance from the start of that line.

Best Answer

What you are asking for is linear referencing. Check out the SDO_LRS package for that.

First the SDO_NN operator will give you the nearest line to a given point, then SDO_LRS.PROJECT_PT() against that line will give you a point on the line that contains the measure of that point, i.e. the distance from the start of the line.

Here is a complete example. First consider the following two tables

SQL> desc us_interstates
 Name                                     Null?    Type
 ---------------------------------------- -------- ----------------------------
 ID                                       NOT NULL NUMBER
 INTERSTATE                                        VARCHAR2(35 CHAR)
 GEOM                                              MDSYS.SDO_GEOMETRY

SQL> desc us_cities
 Name                                     Null?    Type
 ---------------------------------------- -------- ----------------------------
 ID                                       NOT NULL NUMBER
 CITY                                              VARCHAR2(42 CHAR)
 STATE_ABRV                                        VARCHAR2(2 CHAR)
 POP90                                             NUMBER
 RANK90                                            NUMBER
 LOCATION                                          MDSYS.SDO_GEOMETRY

This gets the closest interstate to the Indianapolis:

select 
  interstate,
  sdo_nn_distance(1) meters
from us_interstates i, us_cities c
where sdo_nn(i.geom, c.location, 'sdo_num_res=1',1) = 'TRUE'
and c.city = 'Indianapolis';

INTERSTATE     METERS
---------- ----------
I65/I70    367.061345

1 row selected.

Let's get the shape of that interstate and turn it into an LRS geometry:

select 
  interstate, 
  sdo_nn_distance(1) meters ,
  sdo_lrs.convert_to_lrs_geom(i.geom) lrs_line
from us_interstates i, us_cities c
where sdo_nn(i.geom, c.location, 'sdo_num_res=1',1) = 'TRUE'
and c.city = 'Indianapolis';

INTERSTATE     METERS LRS_LINE
---------- ---------- ------------------------------------------------
I65/I70    367.061345 SDO_GEOMETRY(3302, 4326, NULL, SDO_ELEM_INFO_ARRAY(1, 2, 1), SDO_ORDINATE_ARRAY(-86.145485, 39.753242, 0, -86.144585, 39.754372, 147.275807, -86.144035, 39.755329, 263.516454, -86.143509, 39.75893, 665.868292, -86.14325, 39.760296, 819.150604, -86.143044, 39.766884, 1550.82981, -86.142845, 39.771091, 2018.24493, -86.14267, 39.77475, 2424.78127, -86.141968, 39.776432, 2620.97959, -86.141388, 39.778408, 2845.93198, -86.140984, 39.779915, 3016.79675, -86.140984, 39.781193, 3158.69363, -86.141083, 39.78175, 3221.11637, -86.141243, 39.781971, 3249.22263))

1 row selected.

Each point in that geometry carries a measure. By default, measures start at 0 for the first point of the geometry and increase with the length of the line. The measure of the last point is the same as the measured length of the line (in meters).

Now let's project the point onto that line. This returns an LRS point on the line, which contains the measure of that point, i.e. the distance from the start of the line.

select 
  interstate, 
  sdo_nn_distance(1) meters ,
  sdo_lrs.project_pt(
    sdo_lrs.convert_to_lrs_geom(i.geom),
    c.location,
    0.05
  ) point_on_line
from us_interstates i, us_cities c
where sdo_nn(i.geom, c.location, 'sdo_num_res=1',1) = 'TRUE'
and c.city = 'Indianapolis';

INTERSTATE     METERS POINT_ON_LINE
---------- ---------- -------------------------------------------------------------------------------
I65/I70    367.061345 SDO_GEOMETRY(3301, 4326, NULL, SDO_ELEM_INFO_ARRAY(1, 1, 1), SDO_ORDINATE_ARRAY(-86.141809, 39.7769737, 2682.64485))

1 row selected.

And finally, extract that measure:

select 
  interstate, 
  sdo_nn_distance(1) meters,
  sdo_lrs.geom_segment_start_measure (
    sdo_lrs.project_pt(
      sdo_lrs.convert_to_lrs_geom(i.geom),
      c.location,
      0.05
    )
  ) measure
from us_interstates i, us_cities c
where sdo_nn(i.geom, c.location, 'sdo_num_res=1',1) = 'TRUE'
and c.city = 'Indianapolis';

INTERSTATE     METERS    MEASURE
---------- ---------- ----------
I65/I70    367.061345 2682.64485

1 row selected.
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