Setting Up in the Real World – GeoReferencing Update

This article is an update of the earlier Setting Up in the Real World – GeoReferencing piece with a slightly simplified workflow to create the transformed GCS and using different example data.

Background

The accuracy of 3D modelling operations can be affected by the limitations of floating point arithmetic. This is a fundamental limitation of current technology. When many complex calculations are involved rounding errors can accumulate affecting modelling results. This was previously discussed in Setting Up in the Real World or "Where do we model?".

To minimise these effects 3D modelling engines are generally designed to operate in a limited volume near to the centre of the design space. Some applications limit this space, MicroStation has a very large design space to accommodate the requirements of infrastructure users. 2D and less complex 3D models can use the space available to model in real world locations and orientations as required by civil engineering projects.

However more complex 3D operations are subject to the same floating point accuracy limitations that apply to other 3D applications. To minimise these effects models should be created within the Solids Working Area, near to the Design File Centre (0,0,0).

Building Models and more complex 3D operations also generate more complex geometry and benefit from higher level of accuracy to resolve the detail of smaller scale building components and relationships. AECOsim Building Designer's (ABD) seed files and dataset content are therefore delivered using a higher resolution than those used by some larger scale civil projects.

All DGN files recognise the true scale of their content regardless of resolution, a 1m cube created in an ABD file with a resolution of 1000 UOR/mm will be correctly represented in a DGN file with a different resolution, e.g. the 10 UOR/mm used by some civil engineering  seed files. However we do recommend using the 1000 UOR/mm resolution for all files in ABD project if possible.

Modelling buildings un-rotated also reduces the complexity of calculations. Where buildings have a clear rectilinear form we recommend orienting a model’s primary grid to the application’s x/y axes. Obviously in cases involving complex building forms this may not be relevant, an obvious orientation may not be available.

To relate building models and their civil context we use Geo-Referencing to display:

  1. The context aligned to the building
  2. The building aligned to the context

This is effected by one or more geospatial transformations particular to a building, facility or group of buildings/facilities.

A number of sites can be related to linear infrastructure using this method, each site along the route having its own transform as shown in this simplified example at the end of this piece.

The Local Transform Method

The Local Transform method is a simple way to use Geographic Coordinate Systems to relate building models, which should be modelled within the Solids Working Area and near the Design File Centre, to real world site location data.

To set up a Local Transform you will need to create a site survey plan that references survey or mapping data in its correct location and orientation to the regional GCS. In the UK the mapping data is generally in an OS DXF file. Survey information can obviously vary in format. The best GCS for the UK is:

See Setting Up in the Real World – UK Coordinate Systems for more details on this GCS and how to add it to your GCS library.

The examples below were complied using the legacy BritishNatGrid GCS.

The 0,0,0 point of an OS DXF file is at the National Grid Datum so when referenced an OS DXF file will normally appear in the correct location relative to the National Grid Datum.

The elevation will be zero, OS DXF data is 2D.

The transformation process we are discussing is aimed at AEC designers not civil engineering designers so occurs in the xy plane ignoring the curvature of the earth.

Site Alignment Plan

Create a new 3D model file to contain the project site survey plan: Geo_RailProject-2_Site_GCS.dgn.

Attach the appropriate regional GCS to this file, British National Grid is used in this example (see the help topic "To Select a Geographic Coordinate System from the Library" for guidance on this step):

When GB/UK Dataset seed files are used the working units will be mm. This will trigger the warning below when you attach the GCS. Accept the default, no change, option as the working (graphic) units are correct:

Attach the site location data, this can be OS mapping or site survey that must be in its correct real world location, by reference > Coincident.

Draw a piece of geometry, possibly the actual site boundary or preferably some easily identifiable arbitrary geometry, for instance, a rectangular bounding box for the site with rational dimensions. We call this the Alignment Geometry. In the example below the Alignment Geometry is a red rectangle rotated to match the primary project grid, with a couple of diagonal lines inside the box for verification of orientation later in the process.

This Alignment Geometry must have an established relationship with a point on the site, in this example I have shown a notional site datum point below left of the Alignment Geometry, its real world coordinates are shown.

This example shows a 2D process that ignores z-height. If your project uses 3D terrain data the Alignment Geometry will need to be positioned with a z-height as well.

Facility Alignment Plan

Create a new file Geo_RailProject-2_Facility_GCS.dgn. This will store the facility GCS that will shortly be created.

In this draw an Alignment Geometry, this time in blue, a shape of the same size as (or copied from) that in Geo_RailProject-2_Site_GCS.dgn with the same relationship to the Design File Centre (0,0,0) and unrotated:

Create Local Transform

In this file attach Reference > Coincident Geo_RailProject-2_Site_GCS.dgn

As mentioned above, this example shows a 2D process that ignores z-height. If you are working with 3D site data enter a z offset for the reference attachment matching the z height of the Site Alignment Geometry.

The Site Alignment Geometry will appear at the real world location, perhaps hundreds of km away from the Design File Centre:

Using the reference manipulation tools, reposition Geo_RailProject-2_Site_GCS.dgn so that both red and blue alignment geometries are coincident:

Please note that the alignment between the files is based on the coincident shapes, not on the coincident 'origin' markers, these are included in this example only as additional illustrations.

Once they are aligned use the Select Geographic Coordinate System tool to create a locally transformed GCS in Geo_RailProject-2_Facility_GCS.dgn from the GCS attached to Geo_RailProject-2_Site_GCS.dgn:

Click the From Reference button:

and in the resulting dialog select Geo_Project-1_GCS.dgn, click OK:

This warning will appear to alert you that this method is only supported in applications based on MicroStation 08.11.07.xx or later, click OK:

Accept the no change option as the design files are in mm (or m if those are the units you are working with) and click OK:

The active GCS is then shown in the Geographic Coordinate System selection dialog as:

Note the addition of [Helmert Transformation] to the GCS name.

Click the properties icon to see how the source GCS has been modified:

Note that the offsets match those of the Site datum.

Referencing the site to building models

Having created the transformed GCS it can then be used to locate the site context as referenced underlay to the facility or building design model.

Create a new file Geo_RailProject-2_Facility_Model_01.dgn.

In the same process as above, use the GCS From File tool to attach the transformed GCS to the new file.

Then attach the Site plan, Geo_RailProject-2_Site_GCS.dgn, using the 'Geographic - AEC Transform' method, turning on live nesting, level 1 to include the OS map

Modelling can then take place in orthogonal x/y orientation and well within the Solids Working Area with all real world contextual information being displayed.

Alternatively seed files can be created for each facility transform and used when creating the facility/building model files.

Complex modelling operations and Dynamic View creation should always be carried out in these design models within the Solids Working Area for best (and fastest) results.

Referencing the facility/building models to the site

Once the GCS transform has been applied to a building/facility model simply attach it to any site models that have the source GCS attached, in this case 'British National Grid', using the 'Geographic - AEC Transform' method.

Coordinates can then be labelled in real-world:

Multiple facilities on one site

Repeat the previously described process to Create Local Transforms for each additional facility, then when each one reference is referenced into the site it will use it's own transform to locate the facility correctly on the site.

See Setting Up in the Real World – GeoReferencing : Large Sites for a discussion on how to use GCS's on large sites.

Notes:

Once a transform has been created for a project, first of all ensure that it is documented so that the originating design team have a clear record of what was done. This will reduce the risk of error later in the life of the project. Create a document suitable for sharing with other project participants that clearly communicates how the GCS was established.

Optionally, create a project seed file with this Transformed GCS attached from which container files can be created for referencing to the site.

Alternatively just use the ‘To Reference’ method to attach the Transformed GCS to model files when required.

There is no need to attach the project GCS to all project files, only to those container or model files that actually need to be related to the site plan.