When MicroStation is aware of the geographic location of the data in a model, it can very conveniently reference data that is also geographically aware. Because MicroStation knows the location and orientation of both models, it can calculate the coordinates of any point in the reference model in the master model (as long as the geographic area of the reference model is within the useful range of the Geographic Coordinate System of the master model). MicroStation provides two different ways of making use of geolocation. The first step is to select a Reference for attachment, using the Attach Reference tool. In the file selection dialog, make sure that "Interactive" Attachment Method is chosen:
If both your active model and the chosen reference have a Geographic Coordinate System, MicroStation adds two new attachment choices to the "Orientation" list box in the Reference Attachment Settings dialog:
The more rigorously correct georeferencing method is "Geographic - Reprojected". When this is chosen, all of the data in the reference model is reprojected from the reference model's Geographic Coordinate System to the active model's Geographic Coordinate System as described in section 4.0, Reprojection Settings, above. The reprojected data is stored only in memory (since the reference is not changed), so the reprojection calculations happen each time the reference is loaded, which increases the time required to open the active model.
The second georeferencing method is "Geographic - AEC Transform". When this is chosen, rather than reproject all data in the reference, MicroStation calculates the linear transform that gives the best approximation to the results that you would get by performing the full reprojection algorithm. The approximation is acceptable for smaller scale data, such as most man-made structures that occupy less than a square kilometer. For geographic data, the results are unlikely to be acceptable. To help evaluate whether the "Geographic - AEC Transform" method is an acceptable approximation, the maximum error is displayed in the "Description" field of the list box. It is calculated by applying the exact calculation to each corner of the reference range and comparing that position to the position calculated from the transform. For references that cover geographically small areas, the errors are typically small fractions of a meter. The primary advantage of using the Geographic - AEC Transform georeferencing method is that it gives the same performance as any other reference attachment, since reprojection is not necessary.
The reference dialog has several new features that identify georeferenced attachments and help manage them. There is now a column in the list box labeled "GeoCS" that shows the identifier for the Geographic Coordinate System if the reference has one. The contents of the field are red if there's a GCS but the reference is attached without one of the georeferencing attachment modes. The information panel below the list contains a "Georeferenced" combo box that allows changes to the Georeference mode:
The Settings pulldown menu contains a new entry that allows changes to the Reprojection settings for the selected reference(s).
The entry is enabled only if the Georeference mode is "Reprojected". When a reference is attached with Geographic Reprojected mode, the current Reference Reprojection Settings (see Section 4.0) are copied into the reference attachment and stored there so that every user who opens the master file is using a consistent set of Reprojection Settings and thus gets the same result.
Note: References attached with earlier versions of MicroStation do not have a value for the georeferenced mode. For compatibility with earlier versions of the MicroStation Geospatial Extension, such references are treated as Georeferenced in Geographic - Reprojected mode if both the attachment and the master model contain a Geographic Coordinate System.