Parsing MSDN Geometry Data Type
I have a database where one field gives spatial coordinates. I have learned the field is a serialised MSDN geometry Data Type (http://msdn.microsoft.com/en-us/library/bb933973.aspx).
I want to access this database from Python and was wandering if anyone knew the format of the Geometry Data Type, or any libraries capable of parsing it out into a set of Geo Coordinates in Python.
The link states that Microsoft used the "Open Geospatial Consortium (OGC) standard" in designing this data type, does this mean the spatial coordinates are defined by this standard?
Does any one else have any experience with this?
Any help would be much appreciated!
Asked by: Adelaide498 | Posted: 06-12-2021
As figured out in the comments below (thanks MarkJ!):
- geometry is a .NET datatype but uses its own custom serialization format; you could select the column whole and then reimplement this by opening Microsoft.SqlServer.Types.dll in Reflector and starting from
- or you can use the SQL server support for the type to read out the properties of the geometry data from the database, e.g.
select geocolumn.STX, geocolumn.STY from myTable;
- or you can export the whole value as GML with e.g.
select geocolumn.AsGml() from myTable;which can be processed by Python geometry libraries such as http://gispython.org/ http://mapnik.org/ http://www.qgis.org/wiki/Python_Bindings
I had originally thought SQL Server stored CLR data types as serialized .NET objects directly in the table but this turned out to be wrong.Answered by: Jared684 | Posted: 07-01-2022
"…if anyone knew the format of the Geometry [data type]…"
The binary serialization format for SQL Server's spatial
GEOGRAPHY types is specified here:
"Specifies the binary format of the GEOGRAPHY, GEOMETRY, HIERARCHYID, and CLR user-defined type (UDT) structures that are managed by SQL Server."
This specification is well written and the binary format is easy to understand, so it shouldn't be much of a problem implementing a basic parser for the binary format yourself.
"…or any libraries capable of parsing it out into a set of [geo coordinates] in Python…"
Microsoft.SqlServer.Types via .NET interop to deserialize these types:
If you don't want to implement your own de-serializer (which should be fairly simple), but you can find a way to interact with a .NET assembly from Python — perhaps via pythonnet? —, then the following hints may be of interest:
The two T-SQL types
GEOGRAPHY are implemented as a combination of a .NET assembly (
Microsoft.SqlServer.Types) that performs the de-/serialization from/to the binary format just mentioned above, and an unmanaged DLL (
SqlServerSpatial….dll) which contains almost everything else (i.e. the routines for spatial operations).
If you're only interested in de-serializing SQL Server spatial data, and you're careful not to invoke any spatial functions on
SqlGeography, then you might be able to use
Microsoft.SqlServer.Types to de-serialize spatial binary data for you, then inspecting it with an implementation of
IGeometrySink110 that you have to provide to e.g. the
SqlServerSpatial….dll are available either as a .NET project-wide NuGet package, or as a system-wide MSI installation package (
SQLSysClrTypes.msi). AFAIK the DLLs are also automatically installed with SQL Server.
Well-Known Text (WKT) and Well-Known Binary (WKB):
One more option would be to let SQL Server translate spatial values to Well-Known Text (WKT) or
Well-Known Binary (WKB) using
SELECT geometryColumn.STAsText() or
SELECT geometryColumn.STAsBinary(), then look for a Python library that can parse these standard interchange formats.
(One word of caution: If you go down that route, just be careful if your data contains circular arcs. There are different versions of the WKT and WKB data format. They were first specified as part of the Simple Features Access specification of the Open Geospatial Consortium; that version doesn't understand about circular arcs. Support for circular curve segments was added in the SQL/MM Part 3: Spatial standard, which SQL Server implements.)Answered by: Marcus472 | Posted: 07-01-2022
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