Concave Hull Geometries in Oracle 11gR2

I have always had an interest in the possibilities that concave (not convex) hull geometries provide for geoprocessing (not just in databases).

There is work going on by Regina Obe to create a concave hull function for PostGIS. In addition, there has been some great work by Glenn Hudson and Matt Duckham in creating an open source Java version. And, of course, you could also use the commercial version. And, finally, while not yet in JTS, there is an underlying Alpha Shape implementation that would form the basis of any implementation.

But what about Oracle?

Well, at 11gR2, there are two new functions in the SDO_GEOM package:

• SDO_GEOM.SDO_CONCAVEHULL
• SDO_GEOM.SDO_CONCAVEHULL_BOUNDARY

(The is a third, underlying function, SDO_GEOM.SDO_ALPHA_SHAPE, but I do not cover its use in this article.)

But how do I use them?

I grabbed the sample points for the letter S from the source code distributed by Hudson and Duckham and converted them to an Oracle SDO_GEOMETRY object. In the following example, I convert them via WKT as that provides a more accessible method for understanding what is going on than creating the SDO_GEOMETRY elements directly. This SDO_GEOMETRY object is then visualised using GeoRaptor for SQL Developer.

I then computed the CONCAVEHULL and CONCAVEHULL_BOUNDARY shapes and visualised them. Here is the complete code:

 VARIABLE geom varchar2(2000);
 begin
   :geom := 'MULTIPOINT(
 (120 -224), (185 -219), (190 -234), (200 -246), (212 -256), (227 -261), (242 -264), (257 -265), (272 -264), (287 -263),
 (302 -258), (315 -250), (323 -237), (321 -222), (308 -213), (293 -208), (278 -205), (263 -202), (248 -199), (233 -196), 
 (218 -193), (203 -190), (188 -185), (173 -180), (160 -172), (148 -162), (138 -150), (133 -135), (132 -120), (136 -105), 
 (146  -92), (158  -82), (171  -74), (186  -69), (201  -65), (216  -62), (231  -60), (246  -60), (261  -60), (276  -60), 
 (291  -61), (306  -64), (321  -67), (336  -72), (349  -80), (362  -89), (372 -101), (379 -115), (382 -130), (314 -134), 
 (309 -119), (298 -108), (284 -102), (269 -100), (254  -99), (239 -100), (224 -102), (209 -107), (197 -117), (200 -132), 
 (213 -140), (228 -145), (243 -148), (258 -151), (273 -154), (288 -156), (303 -159), (318 -163), (333 -167), (347 -173), 
 (361 -179), (373 -189), (383 -201), (389 -215), (391 -230), (390 -245), (384 -259), (374 -271), (363 -282), (349 -289), 
 (335 -295), (320 -299), (305 -302), (290 -304), (275 -305), (259 -305), (243 -305), (228 -304), (213 -302), (198 -300), 
 (183 -295), (169 -289), (155 -282), (143 -272), (133 -260), (126 -246), (136 -223), (152 -222), (168 -221), (365 -131), 
 (348 -132), (331 -133), (251 -177), (183 -157), (342  -98), (247  -75), (274 -174), (360 -223), (192  -85), (330 -273), 
 (210 -283), (326  -97), (177 -103), (315 -188), (175 -139), (366 -250), (321 -204), (344 -232), (331 -113), (162 -129), 
 (272  -77), (292 -192), (144 -244), (196 -272), (212  -89), (166 -236), (238 -167), (289 -282), (333 -187), (341 -249), 
 (164 -113), (238 -283), (344 -265), (176 -248), (312 -273), (299  -85), (154 -261), (265 -287), (359 -111), (160 -150), 
 (212 -169), (351 -199), (160  -98), (228  -77), (376 -224), (148 -120), (174 -272), (194 -100), (292 -173), (341 -212), 
 (369 -209), (189 -258), (198 -159), (275 -190), (322  -82))';
 end;
 /
 select sdo_geometry(:geom,NULL) as geom from DUAL
 union all
 select sdo_geom.sdo_concavehull(sdo_geometry(:geom,NULL),0.01) as cgeom from dual
 union all
 select sdo_geom.sdo_concavehull_boundary(sdo_geometry(:geom,NULL),0.01) as cgeom from DUAL;

The images that display the results are.

The S SDO_GEOMETRY Shape

The SDO_CONCAVEHULL

The SDO_CONCAVEHULL_BOUNDARY

I hope this is of use to someone.