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All Blog Articles, Data Models and Free Source Code by Simon Greener, The SpatialDB Advisor is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

A simple spike finder for Spatial/Locator

Tuesday October 11 2011 at 23:35

When investigating geometric or topological errors for linear or polygonal geometry objects, a range of tools are needed. Oracle has basic tools which can be used as the building blocks for additional tests. These are:

One test that is not clearly provided for is the discovery of “spikes” in the linestring defining an outer or inner ring of a polygon (or any linestring).

Dictionary definitions of a spike include:

noun
1. a naillike fastener, 3 to 12 inches (7.6 to 30.5 cm) long and proportionately thicker than a common nail, for fastening together heavy timbers or railroad track.
2. something resembling such a nail; a stiff, sharp-pointed piece or part: to set spikes in the top of a cement wall.
3. a sharp-pointed piece of metal set with the point outward, as on a weapon.
4. an abrupt increase or rise: a chart showing a spike of unusual activity in the stock market; a sudden spike of electrical current.
5. a rectangular or naillike metal projection on the heel and sole of a shoe for improving traction, as of a baseball player or a runner.

9. a pointed portion of a continuous curve or graph, usually rising above the adjacent portion: a spike in the value of the voltage.

Applying these to geographic/geometric linear objects we could say that a “spike” is:

is a sharp, abrupt and narrow deviation from the normal course of a linestring

Graphically:

However, in the context of this article, the spike being discussed is in no way related to Mr Milligan!:

How can we detect spikes?

One simple (the point of this article) method for finding a spike is to see if the gap between the two “vectors” that compose the spike collapse at the specified tolerance of a table’s geometry column (user_sdo_geom_metadata) causing a topological error (often 13349). Here are two examples of spikes.

Spike 1 Spike 2

A simple algorithm for finding spikes is to walk all the adjacent “vectors” of a linestring and send each pair into SDO_GEOM.RELATE with a “determine” mask. If the result is anything other than TOUCH we must have an error.

  1.    /** ----------------------------------------------------------------------------------------
  2.     * @function   : FindSpikes
  3.     * @precis     : Function that implements a simple "spike" finder.
  4.     * @version    : 1.0
  5.     * @usage      : SELECT b.* FROM test a, TABLE(FindSpikes(a.geom,0.005) b;
  6.     * @param      : p_geometry  : MDSYS.SDO_GEOMETRY : An sdo_geometry line or polygon object.
  7.     * @param      : p_tolerance : NUMBER             : Oracle sdo_tolerance value eg 0.005 meters for geodetic.
  8.     * @history    : Simon Greener - October 2011 - Original coding.
  9.     * @copyright  : Licensed under a Creative Commons Attribution-Share Alike 2.5 Australia License.
  10.     *               http://creativecommons.org/licenses/by-sa/2.5/au/
  11.    **/
  12.    CREATE OR REPLACE
  13.    FUNCTION FindSpikes(p_geometry    IN mdsys.sdo_geometry,
  14.                        p_tolerance   IN NUMBER DEFAULT 0.005)
  15.     RETURN CODESYS.SDO_ERROR.T_VectorSet pipelined
  16.   AS
  17.     v_vector       CODESYS.t_vector;
  18.     v_vectors      CODESYS.SDO_ERROR.T_VectorSet;
  19.     v_num_elements pls_integer;
  20.     v_element_no   pls_integer;
  21.     v_element      mdsys.sdo_geometry;
  22.     v_num_rings    pls_integer;
  23.     v_ring_no      pls_integer;
  24.     v_ring         mdsys.sdo_geometry;
  25.     v_relate       varchar2(100);
  26.     NULL_GEOMETRY  EXCEPTION;
  27.     NOT_LINE_POLY  EXCEPTION;
  28.   BEGIN
  29.      IF ( p_geometry IS NULL ) THEN
  30.        Raise NULL_GEOMETRY;
  31.      ElsIf ( p_geometry.get_gtype() NOT IN (2,5,3,7) ) THEN
  32.         RAISE NOT_LINE_POLY;
  33.      END IF;
  34.      -- Process Line/Multiline and Polygon/Multipolygon
  35.      --
  36.      v_num_elements := MDSYS.SDO_UTIL.GETNUMELEM(p_geometry);
  37.      <<Process_All_Elements>>
  38.      FOR v_element_no IN 1..v_num_elements LOOP
  39.          v_element := MDSYS.SDO_UTIL.EXTRACT(p_geometry,v_element_no,0);
  40.          IF ( v_element IS NOT NULL ) THEN
  41.              -- Polygons
  42.              -- Need to check for inner rings
  43.              --
  44.              IF ( v_element.get_gtype() = 3) THEN
  45.                 -- Process all rings in this single polygon have?
  46.                 v_num_rings := GetNumRings(v_element);
  47.                 <<All_Rings>>
  48.                 FOR v_ring_no IN 1..v_num_rings LOOP
  49.                     v_ring            := MDSYS.SDO_UTIL.EXTRACT(p_geometry,v_element_no,v_ring_no);                    
  50.                     -- Now walk ring's vectors
  51.                     --
  52.                     IF ( v_ring IS NOT NULL ) THEN
  53.                        -- Process 1st/2rd vectors looking for spike
  54.                       SELECT CODESYS.T_Vector(v.id,V.Startcoord,V.Endcoord) AS vector
  55.                         BULK COLLECT INTO V_Vectors
  56.                         FROM TABLE(CODESYS.Sdo_Error.GetVector(v_ring)) V
  57.                        ORDER BY v.id;
  58.                       IF NOT ( V_Vectors IS NULL OR V_Vectors.COUNT = 0 ) THEN
  59.                          FOR I IN V_Vectors.FIRST..V_Vectors.LAST Loop
  60.                              IF ( i = V_Vectors.LAST ) THEN
  61.                                  v_vector := v_vectors(1);
  62.                              ELSE
  63.                                  v_vector := v_vectors(i+1);
  64.                              END IF;
  65.                              -- Check if the interact in a way other than TOUCH
  66.                              --
  67.                              v_relate := mdsys.sdo_geom.relate(Vector2Geom(v_vectors(i),p_geometry.sdo_srid),'DETERMINE',Vector2Geom(v_vector,p_geometry.sdo_srid),p_tolerance);
  68.                              IF ( v_relate <> 'TOUCH' ) THEN
  69.                                 PIPE ROW ( CODESYS.T_Vector(v_vectors(i).id,v_vectors(i).startCoord,v_vectors(i).endCoord)  );
  70.                                 PIPE ROW ( CODESYS.T_Vector(v_vector.id,v_vector.startCoord,v_vector.endCoord) );
  71.                              END IF;
  72.                          END Loop;
  73.                       END IF;
  74.                     END IF;
  75.                 END LOOP All_Rings;
  76.              -- Linestrings
  77.              --
  78.              ElsIf ( v_element.get_gtype() = 2) THEN
  79.                    -- Process 1st/2rd vectors looking for spike
  80.                   SELECT CODESYS.T_Vector(v.id,V.Startcoord,V.Endcoord) AS vector
  81.                     BULK COLLECT INTO V_Vectors
  82.                     FROM TABLE(CODESYS.Sdo_Error.GetVector(v_element)) V
  83.                    ORDER BY v.id;
  84.                   IF NOT ( V_Vectors IS NULL OR V_Vectors.COUNT = 0 ) THEN
  85.                      FOR I IN V_Vectors.FIRST..(V_Vectors.Last-1) Loop
  86.                          v_vector := v_vectors(i+1);
  87.                          -- Check if the interact in a way other than TOUCH
  88.                          --
  89.                          v_relate := mdsys.sdo_geom.relate(Vector2Geom(v_vectors(i),p_geometry.sdo_srid),'DETERMINE',Vector2Geom(v_vector,p_geometry.sdo_srid),p_tolerance);
  90.                          IF ( v_relate <> 'TOUCH' ) THEN
  91.                              PIPE ROW ( CODESYS.T_Vector(v_vectors(i).id,v_vectors(i).startCoord,v_vectors(i).endCoord)  );
  92.                              PIPE ROW ( CODESYS.T_Vector(v_vector.id,v_vector.startCoord,v_vector.endCoord) );
  93.                          END IF;
  94.                      END Loop;
  95.                   END IF;            
  96.              END IF;
  97.          END IF;
  98.      END LOOP;
  99.     RETURN ;
  100.     EXCEPTION
  101.       WHEN NULL_GEOMETRY THEN
  102.         raise_application_error(c_i_null_geometry, c_s_null_geometry,TRUE);
  103.         RETURN;
  104.       WHEN NOT_LINE_POLY THEN
  105.         raise_application_error(c_i_not_line_poly,   c_s_not_line_poly,TRUE);
  106.        RETURN ;
  107.   END FindSpikes;

Applying this function to the two spikes above we get.

Linestring with Spike 1 Linestring with Spike 2

I hope this is of use to someone.

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Comment [1]

Hi Simon;

I try to compile your ‘FindSpikes’, but get the following errors:

[Error] PLS-00201 (14: 12): PLS-00201: ‘GREG2.SDO_ERROR’ must be declared (this one actually complained about ‘greg2.SDO_ERROR.T_VectorSet’. I don’t have CODESYS in my 11.2 db, but I do have the ‘GEOM’ package, so I changed ‘GREG2.SDO_ERROR’ to ‘GREG2.GEOM’ and this one error was gone, allowing the rest to appear:)
[Error] PLS-00201 (66: 64): PLS-00201: ‘VECTOR2GEOM’ must be declared
[Error] PLS-00201 (66: 64): PLS-00201: ‘VECTOR2GEOM’ must be declared
[Error] PLS-00201 (101: 33): PLS-00201: ‘C_I_NULL_GEOMETRY’ must be declared
[Error] PLS-00201 (104: 33): PLS-00201: ‘C_I_NOT_LINE_POLY’ must be declared

Your guidance would be much appreciated…

PS1: I had quite a few difficulties in installing your packages (had to eventually bypass the install.bat and do it manually), so I’ve undertaken the task to attempt to correct it and send it to you – fingers crossed.

PS2: I’m also a GeoRaptor addict. I hope, and wish, that the project will not go down the drain!

— Gregory Stefanakos · 7 February 2012, 16:17 · #