Ten Objectives of Model Origin and Setup

Updated 29/10/2016

BIMFix Blog is pleased to announce the release of a "BIMFix Framework for Shared Model Establishment" which can be downloaded for free from here. This framework document shows how the below can be implemented on a project.

Premise:
When it comes to the model origin, coordinates and project setup; Three Dimensional (3D) virtual models (BIModels) have to meet many goals. This includes:
  1.  Limited survey data available at early project design stage
  2. World Geodetic System
  3. Limitations of computer computation algorithms and units
  4. Different project team member needs
  5. Different BIM deliverable needs
  6. Scalable within projects sizes
  7. Building relocation management
  8. Allocation of responsibility and risk mitigation
  9. Allowance for human error, and follow-up resolution
  10. Work within international standards


Do your project model processes and workflows take all these into account? If they don't; you are potentially limiting the use of the model, or adding risk for the party needing to relocate or modify the model to meet the omitted goal?

Over the past two decades, 3D models are commonly circulated between project teams to enhance coordination, communication and meet deliverables. When dealing with 2D (CAD files & Paper), very few drafters were concerned with CAD file origins accommodating other parties workflows. Other parties drawings had the sole purpose of providing backgrounds or visual coordination underlay references.

(Australia is given as an example in this article, however, the same principles are applicable globally)

Today, however; there are many potential uses of models:

  • Components from one discipline can be hosted directly on another disciplines element,
  • Identical components across discipline models may have spatial monitoring engaged,
  • Consultants may be obtaining geodetic coordinates from the Architect’s model,
  • Components may be tagged and annotated across discipline models,
  • Components of the model can be directly used for element set-out (e.g. Trimble robotic total station),
  • Models are becoming the center of team communication, with file formats such as BCF (BIM Collaboration Format). This may need to work between relevant applications involved.
  • There is greater use of models across multiple deliverables and they are expected to seamlessly integrate between disciplines and applications,


An inappropriate origin set-up can have a direct impact on project teams. Let’s elaborate on the above list and understand the challenges and goals:


1. Limited survey data available at early project design stages
It is rare to have a comprehensive site survey at project kick-off, or even concept / sketch design stage. Ideally, a CAD survey is available from the Land Surveyor, about 8 weeks before the end of Sketch Design.

On the first few days of building modeling conception, relative levels and spatial coordinates are not a high priority on most projects. There are, of course, exceptions. Designers need to evolve the design principles with limited site information. Model users may not understand the potential consequences of a bad project set-up, and thus, a simple, user-friendly approach (plan) is likely to succeed.


2. World Geodetic System
Geocentric Datum
Australia’s official geodetic datum is Geocentric Datum of Australia,1994 (GDA94). 
This coordinate system fits into the global coordinate system. It came into use in 1994 and is compatible with the Global Positioning System (GPS). It includes Grid coordinates (Universal Transverse Mercator (UTM), using the GRS80 ellipsoid); called Map Grid of Australia (MGA94). Note: This replaces the former grid system Australian Map Grid (AMG).
UTM was first developed by the United States Army Corps of Engineers.



          Above image: Map Grid of Australia 1994
Each country, and in the US; States, have their own UTM or similar Map Grid, In the UK it's called the Ordnance Survey National Grid, in the US it's the State Plane Coordinate System.

MGA94 fits within the global “Longitude” grid (vertical) and is 6⁰ of separation. It starts at the International Date Line and works anti-clockwise around the earth. Each grid sector is allocated a Zone number. Australia is between Zone 49 (West W.A) and Zone 56 (East N.S.W. / QLD.). It uses the Cartesian coordinate system in a positive X,Y (Easting’s and Northing’s) to locate a point/position. A helpful converter from UTM Coordinates to Geographic (latitude, longitude) coordinate system: AWSM website.

When in the Northern Hemisphere the Northing origin is measured from the Equator line. For the Southern Hemisphere in order to maintain positive values, the origin is measured 10,000KM south (down) of the Equator line (very close to the South-Pole). The measurement takes the North/South curvature of the earth into account. All coordinates are in meters.
Location examples (approximate):

Melbourne    MGA Zone 55    E 320,000 (Easting)      N 5,800,000 (Northing)
Sydney         MGA Zone 56    E 330,000 (Easting)      N 6,250,000 (Northing)


                   Above image: MGA Zone origins - 500,000m West of the Central Meridian.

Universal Transverse Mercator (UTM) 
UTM represents ellipsoidal positions (latitude & longitude) as Grid coordinates (Easting and Northing) on a cylindrical surface, resulting in a number of zones. Uniform scale factor, false origins, and zone size and numbering have been adopted for the Universal Transverse Mercator Projection.


Image above – concept of Universal Mercator and Universal Transverse Mercator system – I.e. the attempt to flatten an Oblate Spheroid (the Earth) onto a flat surface. 


Typically, World Maps use the Universal Mercator, however, MGA maps use the Transverse Mercator system enabling less distortion in a North / South direction. There is an East / West distortion and it can be between a Scale Factor of 0.9996 (Point A) and 
1.00071 (Point C) as shown in the below sectional diagram.
Above diagram: Extract from The Map Grid of Australia 1994A Simplified Computational Manual,  showing the Scale Factor will vary with the distance of the survey from the Central Meridian allowing for the East-West curvature of the Earth. 

The Australian Government provides a free to use Calculator, which computes the scale factor (Point Scale) East-West deviation. Thus an MGA survey with an Eastings of 510,200 (Close to the Central Meridian), as per the above calculator, will have a Point Scale of 0.99960128. Thus 100 meters measured East-West in the CAD drawing will be in fact 100/0.99960128 = 100.040 meters on site. About +40mm out.

Some local areas are a sub-section of MGA. Perth projects often work to Perth Coastal Grid (PCG). These smaller local grids allow for greater accuracy as they reduce some of the unfolding deviations which occur within the Universal Transverse Mercator (UTM).
The bottom paragraph of this link provides guidance when working on large linear projects to minimize distortion.


Elevation (Altitude)
The “Z” coordinates (Elevation), Australian Height Datum (AHD) is used. This is the mean sea level (Geoid) for 1966-1968 and was assigned a value of 0.000m with the surveying of 30 stations around Australia. All surveys refer to AHD, and again the units are in meters.
Please also note the height above mean sea level will also affect the output scale factor.



Above diagram: Shows, as the Elevation increases the Ground Distance increases vs. the Grid Distance. 

Building Set-out (Layout)
When annotating the set-out of a standalone commercial building, it is common to give Easting and Northing for the primary building set-out Grid (See Figure A.1 image below). and an AHD relative level for each floor level.

Civil Engineers, Land Surveyors, Transport Engineers all work to this official coordinate system, and it is an imperative any building information exchange integrate with them.


3. Limitations of computer computation algorithms and units
The above mentioned Geodetic coordinates can become very large (6,000KM = 6,000,000,000mm). Most software applications (including CAD applications and MS Excel) have limitations and these include the number of calculated figures they can effectively work with, due to Floating Point precision engaged via the IEEE 754 Standard. To achieve the required level of accuracy,  software usually like 8 decimal places before and after the decimal point (i.e. a total of 15 to 16 figures) in relation to the desired output units. When figures go above the limitations, rounding errors occur.  And thus if the geometry is modeled outside the software coordinate limitations, it starts to produce numerical, such as radial arcs and tangential lines won't trim correctly, and graphical or rendering errors; such as lines may appear to jump on selection, hatches won't display/print correctly or renders are missing lights or objects.

Also, consider some US CAD Building applications (e.g. Autodesk Revit) are hard-coded in decimal feet, and changing the units does not change how the coordinate units are stored. 

With all of the above, this is one reason land surveyors work in meters. The software is too unpredictable if they worked in millimeters. 
Thus, Building Design and Construction teams who want to work in millimeters must use a known (Easting and Northing identified) "Local" datum point.

References to the Above problem:

Autodesk AutoCAD:
Autodesk Revit:
Bentley:
Graphisoft ArchiCAD:
Microsoft Excel:
Vectorworks:


Trimble Sketchup:


4. Different project team member needs
Most building projects can be split up into two camps; those dealing with the site or civil, and those dealing with the building itself. 

All site related geodetic references (MGA & AHD) are in meters.  As these disciplines are engaging large dimensional figures, it makes a lot of sense to work in meters. However, within the Building and Fabrication disciplines, component set-outs and measurements are in millimeters.

Site Disciplines: 
(units meters)
Building Disciplines: 
(units millimeters)
Land Surveyors
Architects and Interior Designers
Civil Engineers
Structural Engineers
Transport Engineers
Building Services Engineers
Landscape Architects
Building Surveyor / BCA Consultants

Façade Engineers

Acoustics Engineers

Other building specialist consultants

Building Component fabrication modelers
NOTE: Landscape Architects may prefer to work in millimeters and thus must work off the local site origin.

As disciplines use different units (meters / millimeters) and thus different origin points, an integrated workflow allowing for a coordinate data exchange needs to be established between all parties.


5. Different BIM deliverable needs
When looking at the available BIM goals, many project team members will require a different approach to model coordinates and origins. Some examples;
  • Master Planning: When master planning a campus it can include GIS data, multiple buildings, staging strategies and integrate Geodetic Coordinate systems.
  • Clash Detection: 98% of Clash detection on building projects takes place within the building itself (this will be different on infrastructure projects). On a commercial Building construction project, the site clash detection is often minimal. Building relocations during design stages should not temporally hold up the Clash Detection process. Clash Detection may also include fabrication model authoring applications which may not support Geodetic Coordinate systems.
    Communication of Clash detection via (BCF) may require two-way communication. BCF files generally require identical model (hard coded) origins between the applications in use.
  • Sustainability Evaluation: When defining the rating scheme used to check the model, (e.g. Green Star or LEED), the building orientation to true north, longitude, and latitude position, are transferred into the analysis application often via the model.
  • Positive and Small Coordinates: Some analysis applications require the geometry to be close to the origin and for all geometry to be positive figures. This is to minimize and simplify the computations. It will result in the Model origin being in the bottom right of the model. 
Thus, it is necessary for the approach to work with multiple units and coordinate systems.



 6. Scalable within projects sizes
Whatever approach and process are used, it needs to be scalable. That is, to work with:
  • a single green/brown field site building,
  • a multi-building campus environment, all buildings unique
  • a multi-building campus environment, multiple instances of building units
  • a multiple discipline sub-model environment, forming a single federated building model
  • a project containing existing conditions, staging, and/or building extensions


7. Building relocation management
From building design kick off, up until foundation concrete pouring, a building design relocation is likely to occur. This relocation can occur due to new data the design has to integrate. Building movements can occur in any of the X,Y,Z and rotation coordinates. So even if the building takes up 100% of the site, it may still move in the “Z” direction.

If not planned for and managed appropriately; building relocations can have substantial adverse effects on project teams. Potentially adding weeks of documentation re-work for parties affected.

During my time as a BIM consultant, it is rare to come across a design practice who has thoroughly investigated and documented processes on how building relocations are logged and executed. Failure to address this may open up liability to parties involved.

8. Allocation of responsibility and risk mitigation
When it comes to “most” building projects (Infrastructure projects are an exception), the Architect is responsible for documenting where the building is located and set-out (layout) on the site. The Architect will determine this off a site survey provided during the early design stage by the Land Surveyor.

As the design evolves, and the design location of the building updates, it is up to the Architect to take responsibility for relocation change and adequately communicate it to all relevant parties. All necessary processes are to be established to eliminate other design consultants or contractors, being requested to manually move their models or enter coordinates figures to align with an Architectural model. This is an unnecessary risk. 
If no processes and checking systems are in place, and parties get it wrong and costs are incurred, is the Architect going to reimburse them?


9. Allowance for human error, and follow-up resolution
No matter how clever a process is, human error is often the weakest link. Thus, the overall workflow should take on board the following:
  • A protection system in place, to avoid users accidently moving linked models
  • An audit at each deliverable stage to pick up coordinate/origin errors
  • A backup coordinates reference system, to help in resolving any errors
  • If all fails a reset option, which will have minimum effect on other parties


10. Work within international standards
There are not many specific standards in regard to site set-up, so when there is one, it’s worth adhering to it where possible. BS 1192:2007+A2:2016 - Collaborative production of architectural, engineering and construction information – Code of practice has a specific section on site coordinate establishment. Its relative section is Annex A (normative); Project space statement. This section identifies the establishment of a site map grid (engaging Easting’s and Northing’s map grids). It suggests a location for a project site local origin (site grid origin), and the identification of at least two building grid intersection coordinates and rotation. 
Above:  Figure A.1 Geodetic referencing from BS 1192:2007+A2:2016.

This approach is very robust, and it also can be used to facilitate item “9” above. BS 1192:2007 is a requirement of UK BIM Level 1 & 2.




Close
I can confirm, it is possible to establish a site set-up process to include all the above goals. Any Architectural firm sharing BIModels, need to take the initiative to develop and document (flowchart) the final process. 

This workflow is then circulated to all relevant parties.
If you are a design consultant, insist you are provided with such a document by the Architect. Ensure you thoroughly test it and check it does not adversely affect your workflows. The consultants are the party must likely to be directly impacted in the event a building relocation is inadequately planned.
There is no one item mentioned here which is difficult. An all-encompassing plan does take a while to develop and test. Yet it is one sure workflow all lead consultants need to have.

Call for Review
For any readers, reviewing the above and taking issue with any of the premises, I request you please provide Brian Renehan (the author) with the relevant constructive feedback (via below comment or email - see contact info). Identify the background and reason for your counter argument. The above premises “should” be universal for all building construction projects. If it led to universal software specific workflow outcomes, it would greatly benefit the industry.






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