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Creating a Geographic Coordinate System in BIM Projects
This Wiki article describes a best practice workflow for incorporating a BIM Model with other geographically located information. It assumes site or survey data is available and can be related to a Geographic Coordinate System (GCS). In many cases files used for geospatial purposes are drawn in Cartesian coordinate systems that ties directly to a known geographic projection system. In these cases, a coordinate system for the supplied library can simply be applied to the file. MicroStation’s georeferencing functionality can then allow files with varying projection systems to be attached and re-projected on-the-fly relative to the master file’s active coordinate system.
When BIM projects are located in a design file to optimize the design process (near the 0,0 point of the file), a custom coordinate system may need to be created within the file. This can be easily accomplished by creating a series of Placemark Monument points that containing Latitude and Longitude point coordinates, and subsequently creating an Azimuthal Equal Area Projection coordinate system based on optimization of these points. This coordinate system can then be used to align other geospatially enabled data.
Placemark Monument points can created several different ways: Manually entering Latitude/Longitude values, importing a KMZ file from Google Earth, or by selecting a point in a reference file that contains geospatially enabled data. The process described below will be based on the later method and will use a system of two files: a Master GCS Alignment file and a Project GCS file.
Definition of Terms:
Alignment Geometry: Geometry in a geospatially located DGN file that represent design centerlines, structural edges, or offsets that can be used for alignment of the project to non-coincident files. In short, it is geometry that is needed to specifically locate a project on a site.
Master GCS File: A file that contains alignment geometry. This file may reference other geospatially enabled files (examples: regional boundary files, public rail transportation alignment files, individual survey files projected to a coordinate system) in order to appropriately create the alignment geometry. This file may also contain alignment geometry for multiple projects. Making this a separate file allows geometry to be created without compromising original survey data.
Project GCS file: This file is used to create and store the Projects GCS for distribution to other files in a specific project as needed. This file will normally have two file attached: the Master GCS file well as the project column grid file.
1) Identify the building location by alignment geometry in a file with the Master GCS (Geographic Coordinate System) file.
a. Begin with file that has been created relative to a known geographic coordinate system (Master GCS). This could be file that is drawn in Cartesian coordinate system that aligns with a defined GCS. (i.e. MTM System, State Plane Coordinate System, etc.). If the GCS has not already been applied, this can be done so simply by selecting the appropriate GCS "from Library" though the Select Geographic Coordinate System>From Library tool. Choose File>Save Settings (or ctrl-s) to save this change. This file should have the same working units as the GCS being used.
b. Reference attach (using the coincident method) the file which contains project survey data or other landmark data being used to locate the project (this data should be drawn in its file, relative to the GCS coordinates)
c. Relative to the project survey reference file, add simple line work to the Master GCS to represent the location of the project. This should be created in such manner so that the primary and secondary monument points can be identified by snapping. It is recommended to use monument points that directly represent the building geometry; (i.e. column lines, defined edge point of structure, center line of building, etc.)
2) Create the Project GCS file (or model) for storing the specific project’s Geographic Coordinate System.
a. Create a new DGN file. The working units of this file should match the working units of the BIM models
3) Attach both the "Master GCS" and a "BIM file" containing respective alignment geometry to the Project GCS file, that contains respective snappable geometry (example: Column center lines).
a. Assuming the primary and secondary monument points relate to the structural grid, attach both the structural grid file as coincident reference.
b. Reference attach the Master GCS file. This file may be optionally moved and rotated to align with the project geometry, but this is not required. Simply moving this file closer to the BIM file geometry may aid in the following steps.
4) Use the Geo-Coordination tools to select points in each of the files to create Placemark Monument points. Note: this step assumes the Master GCS reference has not been moved or rotated
a. Fit structural grid information in View 1 (oriented as "top view")
b. Fit Master GCS information for this project in View 2 (oriented as "top view")
c. Select Define Placemark Monument tool with the following settings:
Source: "Reference with Geographic Coordinate"
Reference: select the Master GCS reference.
Note: this list will only display references that contain a valid GCS.
Placemark Name: Using default is acceptable, this will increment on placement
d. Snap to and accept the Primary Monument point location in Master GCS (View 2)
e. A Placemark Monument icon will appear on your cursor. Move the cursor to the relatively same location of the Structural grid, and then establish the Placemark Monument point by snapping and accepting.
f. Follow the same procedure with the Secondary Monument point.
5) Create a resulting Azimuthal Equal Area GCS in the "Project GCS" file, based on the Placemark Monument points..
a. Using Select Geographic Coordinate System> From Placemarks, create GCS from the two monument points just created.
b. Review the dialog box for acceptable tolerances of projection.
c. Save Settings in the file. (or ctrl-f )
6) Change the reference orientation setting and attach survey data
a. Change the reference orientation to be Geographic-Reprojected on the Master GCS file. The file will then rotate to align with the building geometry.
b. Attach the Site data to the file with Geographic-Reprojected orientation also. This step will orient the site data relative to the active file as well.
The Project GCS file can now be used to push the created GCS to any file that will need to reference. It can also be referenced to a file that will use Geo-referencing for reference alignment. Not all of your files will need to contain the Project GCS definition.
To reference a file with projected data, simply select the Orientation to be either "Geographically Re-projected" or "AEC Transform".
Important Notes about Geographic projection and referencing:
1) Measured distances in projected files are to be considered "approximations" due to the distortion encountered by projecting a curved surface into a Cartesian coordinate system.
2) As a GCS is an approximation/mathematical projection, it is suggested that the primary monument point be relatively close to the center of the project design plane. The further the geometry is away from the primary monument point, the greater the distortion there will be when using geo-referencing methods. The secondary monument point will be used to orient the rotation of the project. It is also recommended that the useable area of an Azimuthal Equal Area GSC be limited to about 2 kilometers.
3) Geo-referenced files will load slower than non-projected files due the recalculation of points to align the master file's projection system.