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Dion Moult
96 lines
5.2 KiB
ReStructuredText
96 lines
5.2 KiB
ReStructuredText
Georeferencing
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==============
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In the AEC industry, works in the built environment are split between vertical
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construction (such as buildings and sites), and horizontal construction (such as
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transit, transmission, and subterranean networks). Blender and the BlenderBIM
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Add-on is only suitable for vertical construction.
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IFC4 onwards supports georeferencing. The BlenderBIM Add-on has full support for
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IFC georeferencing. Here are the potential scenarios you will encounter in the
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wild for vertical construction.
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1. Correctly georeferenced IFC4 with a map conversion transformation
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2. Correctly georeferenced IFC4 with a map conversion without a transformation
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3. Non-georeferenced IFC4
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4. Non-georeferenced IFC2X3
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The first scenario is desired for most disciplines, such as architects and all
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engineers (except for civil). Typically, this results in small local engineering
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coordinates, which work well in Blender.
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Correct georeferencing requires a projected CRS and a map conversion.
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Unfortunately, many users may be under the impression that their file is
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correctly georeferenced, but this is rarely the case. You can check whether your
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file is correctly georeferenced in the ``IFC Georeferencing`` scene panel. If
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you see "Not Georeferenced", your file is not correctly georeferenced. If your
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file is georeferenced, it is still no guarantee that the georeferencing data is
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actually correct, but how to determine this is out of scope of this article.
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For scenarios 2, 3, and 4, coordinates may be quite large. To present this data
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in Blender, the BlenderBIM Add-on will automatically attempt to create a false
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origin to preserve the precision of the model. The first coordinate greater than
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1km will be detected and used as an offset coordinate. You can see the false
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origin, if any, in the ``Blender Offset`` section of the ``IFC Georeferencing``
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scene panel.
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Instead of relying on an automatic false origin, you can also specify a custom
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origin coordinate. To do this, choose ``Enable Advanced Mode`` when loading an
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IFC project. Then enable the ``Import and Offset Model`` option and specify a
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new coordinate to replace the origin's default of 0,0,0 in the ``Model Offset
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Coordinates`` option.
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Dealing with large coordinates
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------------------------------
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In scenarios 2, 3, and 4, a BIM vendor will typically choose from two possible
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methods to offset their coordinates into large map coordinates.
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The first method is to shift the origin point of objects in the model relative
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to the global coordinate system. We call this the ``OBJECT_PLACEMENT`` method.
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The second method is to shift the coordinates of geometry within the objects
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themselves relative to the object placement. We call this the
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``CARTESIAN_POINT`` method. Sometimes, BIM applications combine both of these
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methods in a single IFC project. To see which workaround was used on an object,
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check the "Blender Offset" property in the ``Transform`` object panel.
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Sometimes, a model shifts their coordinates for some objects, but not all. For
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example, the walls in a model may have their object placement or cartesian
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points shifted to map coordinates, however, the object placement of the site is
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still at 0, 0, 0. Since these coordinates are so far apart, this creates a
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problem, because Blender needs to choose between displaying the walls accurately
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and sacrificing precision at the site placement, or vice versa, but it is
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impossible to satisfy both simultaneously in the same Blender session.
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Many IFC viewers only show geometry, and don't show object placements. This may
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give users the false impression that their coordinates in their IFC project do
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not have such a large range. However, because the BlenderBIM Add-on is a full
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authoring platform, we do need to show these placements and thus it is the users
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responsibility to reconcile this inconsistency in their coordinates. Either the
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user needs to fix their file to consistently offset all coordinates, or the user
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needs to manually tell the BlenderBIM Add-on the coordinates of the desired
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false origin. In the absence of manual intervention, the BlenderBIM Add-on will
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make an intelligent guess, but it may be wrong.
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Coordinates and precision limits
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--------------------------------
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Blender, and subsequently the BlenderBIM Add-on, is not designed for map
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coordinate systems. Blender internally uses single precision floating point
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calculations. A full description of the precision implications are described in
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the `Blender working limits documentation
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<https://docs.blender.org/manual/en/latest/advanced/limits.html>`__.
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From a software perspective, lengths greater than 5,000 meters start to
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accumulate precision errors that affect the nearest millimeter. Therefore, from
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a software perspective, it is unwise to embark on a project with coordinates
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ranging greater than +/- 5km.
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However, if working in local engineering coordinates, a single transformation is
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required to convert from local engineering coordinates to map coordinates. This
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transformation includes a scale factor. The scale factor is only assumed to be
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constant for small sites (defined approximately as less than 1km square). This
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practical limit of georeferenced vertical construction is smaller than the
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software limit, so this surveying convention is the actual limiting factor, not
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the software.
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