Mapping Borehole observations using a true 1-D CRS

By Florence Tan with Ben Caradoc-Davies and Simon Cox

For a long time, the use of spatial representations other than a conventional 2D cartographic view has been a rather dark corner of the GIS world. However, some recently adopted OGC standards promise to allow communities that need these, such as facilities and engineering, urban planning and design, hydro-geology, and the solid-earth geosciences, to integrate their world with the view normally presented in the geospatial community. These include

• CityGML 2.0 [1] – a GML 3.2-compliant upgrade to OGC’s first community application model for representing urban objects and landscapes in 3D
• GML 3.3 [2], which includes an XML representation of (spatial) Linear Referencing, as defined in OGC Abstract Specification Topic 19 [3] 

Boreholes are features appearing in several environmental science disciplines, particularly to support observations and monitoring of subsurface phenomena. Observations are made in boreholes using down-hole logging instruments, with data indexed by distance along the hole [8]. Thus a 1D spatial reference system is the natural frame for borehole observations, allowing them to be correlated and easily visualized together for analysis [4].

These developments have been used in a “Secret Friday-morning Project” in Commonwealth Scientific and Industrial Research Organisation (CSIRO). A small question from one of the developers to one of the scientists – “How to manage position in boreholes?” – has lead to the use of a Linear Coordinate Reference System for indexing borehole data in production systems, using OGC standards.

In practice the use of a 1D spatial representation may require some workarounds. In particular, as popular spatially enabled databases such as PostGIS only support 2D coordinates, positions along the borehole must be represented as 2D coordinates in a ‘Lateral Offset Linear Coordinate Reference System’ [5], with the second coordinate set to 0.0 so that the position coincides with the ‘linear’ element. 

The theory was tested using the OpenGeo GeoServer software [6].  The test turned out to be almost trivial to execute, thanks to recent upgrades to GeoServer that added support for (a) GML 3.2 (b) complex ‘application schemas’ based on GML 3.2. Since the new schema elements in GML 3.3, such as gmllro:LateralOffsetLinearSRS, are formalized as ‘applications’ of GML 3.2, GeoServer can be configured for GML 3.3 support merely by pointing it at the published schema [7].  

The experiment vindicated a significant investment made by CSIRO into the OpenGeo GeoServer software, and also validated the OGC GML 3.3 standardized implementation of linear referencing.  A challenge is now to the developers of spatially-enabled datastores to generalize their spatial support to topological dimensions other than 2D.

[1] http://www.opengeospatial.org/standards/citygml

[2] http://www.opengeospatial.org/standards/gml

[3] http://www.opengeospatial.org/standards/as

[4] https://www.seegrid.csiro.au/wiki/CGIModel/BoreHolesAndObservation#Linear_Referencing_40Borehole_41

[5] http://www.opengis.net/doc/gml/3.3#clause-9.5

[6] http://geoserver.org/display/GEOS/What+is+Geoserver
The Geoserver version used for testing was GeoServer Trunk Rev16340 http://files.ivec.org/geoserver/geoserver-trunk/2011-09-25/

[7] http://schemas.opengis.net/gml/3.3/linearRefOffset.xsd