[GIS] the difference between ST_GEOMETRY and WKT

postgispostgresqlst-geometrywell-known-text

What is the difference between ST_GEOMETRY and WKT? Is ST_GEOMETRY just a database type and WKT (well-known text) is a textual representation of the data?

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WKT is a text interchange format (Wikipedia) (PostGIS), and if you use PostGIS, you might use it like I do occasionally in SQL queries, to get a quick non-visual check on the geometry - see the ST_AsEWKT function.

ESRI seems to use 'ST_Geometry' to be the whole class of 'Spatial SQL' concepts and functions laid out in the OGC/ISO standard, AND use it to be a geometry storage type, e.g. in contrast to their older closed SDEBINARY format. PostGIS seems to use it as a storage type, as you use functions like ST_GeomFromText to populate spatial columns from interchange formats. Though I can't say I've got a clue what the actual storage format in PostGIS is.

Related Solutions

ArcObjects – Converting Between Esri Geometry and WKT

Using the IWkb interface does a nice job at converting between an IGeometry and WKB. From a WKB you can use the Microsoft.SqlServer.Types library to convert a WKB to SqlGeometry then back to WKT.

IWkb wkb = geometry as (IWkb); //(Where geometry is an instance of IGeometry)
byte[] wkb_bytes = new byte[wkb.WkbSize];
int byte_count = wkb.WkbSize;
wkb.ExportToWkb(ref byte_count, out wkb_bytes[0]);

At this point you have the WKB stored in wkb_bytes. If you want to go the next step to SqlGeometry then to WKT:

SqlGeometry sqlGeom = SqlGeometry.STGeomFromWKB(new SqlBytes(wkb_bytes), srid);
string wkt = sqlGeom.ToString();

WKT Standard – Where to Find Well-Known Text References and Specifications

Well known text for geometry is defined in OGC 01-103r4 (amongst other places). Section 7 of that document provides the syntax. There is also a list of SQL functions relating to those in Part 2 of the Simple Features spec (also an OGC product).

The ISO documents are just a more expensive way to get much the same thing.

There are useful extensions such as the Extended WKT (EWKT) that is defined by PostGIS and supported by various other tools such as SpatiaLite, and provides the SRS, which you probably want.

Update: As pointed out in a comment, there is also WKT representation for Coordinate Reference Systems: OGC 18-010r7. However these two WKT forms are using similar syntax to describe different things - one is for a geometry (e.g. a linestring), and the other for a CRS (e.g. for a geometric or temporal datum).

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Related Solutions

ArcObjects – Converting Between Esri Geometry and WKT

WKT Standard – Where to Find Well-Known Text References and Specifications

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