# Using JTS’s Comparison Functions – HausdorffSimilarityMeasure & AreaSimilarityMeasure with SDO_GEOMETRY

There is often a need to compare SDO_GEOMETRYs to see if a geometry has changed or not. The SDO_GEOM.RELATE function is useful in this regards where two geometries are EQUAL but it is not fine-grained enough to help understand how different two things are. The Java Topology Suite (JTS) has two very useful match algorithms in its com.vividsolutions.jts.algorithm.match package.

I have extended my Spatial Companion For Oracle (SC4O) package and installer to expose the two methods JTS makes available:

• HausdorffSimilarityMeasure (Supports lines and polygons)
• AreaSimilarityMeasure (Supports only areas)

Martin Davis has a good article on the HausdorffSimilarityMeasure on his website.

Here are some examples.

1. Call to both functions using two identical area sdo_geometry

``` WITH myGeoms AS (
SELECT SDO_GEOMETRY(2003, NULL, NULL, SDO_ELEM_INFO_ARRAY(1,1003,3), SDO_ORDINATE_ARRAY(100.0, 100.0, 500.0, 500.0)) AS area1,
geom.rectangle2polygon(SDO_GEOMETRY(2003, NULL, NULL, SDO_ELEM_INFO_ARRAY(1,1003,3), SDO_ORDINATE_ARRAY(100.0, 100.0, 500.0, 500.0))) AS area2
FROM dual
)
SELECT SC4O.ST_HausdorffSimilarityMeasure(area1,area2,3) AS HSM,
SC4O.ST_AreaSimilarityMeasure(area1,area2,3)      AS ASM
FROM myGeoms;
.
HSM ASM
--- ---
1   1
```

2. Call to both functions using two slightly different area sdo_geometrys

``` WITH myGeoms AS (
SELECT SDO_GEOMETRY(2003, NULL, NULL, SDO_ELEM_INFO_ARRAY(1,1003,3), SDO_ORDINATE_ARRAY(100.0, 100.0, 500.0,  500.0)) AS area1,
geom.rectangle2polygon(SDO_GEOMETRY(2003, NULL, NULL, SDO_ELEM_INFO_ARRAY(1,1003,3), SDO_ORDINATE_ARRAY(100.0, 100.0, 500.01, 500.01))) AS area2
FROM dual
)
SELECT SC4O.ST_HausdorffSimilarityMeasure(area1,area2,3) AS HSM,
SC4O.ST_AreaSimilarityMeasure(area1,area2,3)      AS ASM
FROM myGeoms;
.
HSM               ASM
----------------- -----------------
0.999975000624984 0.999950001874937
```

3. Call to HausdorffSimilarityMeasure function only using two slightly different line sdo_geometrys

``` WITH myGeoms AS (
SELECT SDO_GEOMETRY(2002, NULL, NULL, SDO_ELEM_INFO_ARRAY(1,2,1), SDO_ORDINATE_ARRAY(0,0, 10,10, 20,0, 30,30)) AS line1,
SDO_GEOMETRY(2002, NULL, NULL, SDO_ELEM_INFO_ARRAY(1,2,1), SDO_ORDINATE_ARRAY(0,0, 10.01,10, 20,0.1, 30,30.07)) AS line2
FROM dual
)
SELECT SC4O.ST_HausdorffSimilarityMeasure(line1,line2,3) AS HSM
FROM myGeoms;
.
HSM
---------------
0.9976457256498
```

See my Spatial Companion For Oracle (SC4O) package header description for the call signatures for these functions.