Assigning A Geographic Coordinate System


Structure-scale Data

The first case, often used when designing buildings, stadiums, industrial plants, and other structures, is to use a coordinate system that is most convenient for the structure. In that case, the design team might elect to place the origin at the front left corner of the building at ground level, with the positive X axis aligned with the front of the building, positive Y aligned with the side of the building and Z representing elevation above ground level. All data entry and readout is relative to that coordinate system. The geographic location and orientation of the structure is not considered in selecting the coordinate system. The advantage of using such a coordinate system is that it the most convenient for the project design team. However, if there are multiple structures, and each is drawn in such a coordinate system, without Geo-Coordination it is  inconvenient to reference those designs to each other and to a site or campus map. Geo-Coordination greatly simplifies that process. For data relative to such a coordinate system, there will not be a predefined Geographic Coordinate System (such a State Plane) that matches the Cartesian coordinate system used, but MicroStation provides a method for creating a GCS that matches the existing coordinate system, as described in Section 3.2, "Creating a Geographic Coordinate System from Placemarks".

Earth-scale Data

 

The second case, most often used when designing larger scale infrastructure, mapping, and surveying is to select an appropriate projected Geographic Coordinate System that results in a Cartesian coordinate system that is appropriate for the location of the design. Generally, the GCS's used are defined by governing bodies. It is not necessary for project participants to be constantly aware of the details of the GCS; they are generally accustomed to working in such a coordinate system. When the same GCS is used for multiple design files, they can be easily referenced to each other, but referencing data that uses a different GCS requires special calculations.

 

In previous versions of MicroStation, even when a design was executed in a particular GCS, there was no way of specifying the GCS that was in use, nor was there any functionality included that took advantage of the fact that the data conformed to that GCS - that functionality required the MicroStation Geospatial Extension. Section 3.1 describes MicroStation Athens' mechanism for selecting a GCS from a library of predefined, standard Geographic Coordinate Systems. Once that association is made, all of the abovementioned benefits of Geo-Coordination are realized.

Selecting A GeoCoordinate System from the Standard Library

There are three situations when selecting a Geographic Coordinate System from MicroStation's extensive library of predefined Geographic Coordinate Systems is appropriate:

1. When you have existing data that was drawn in a Geographic Coordinate System (for example a state plane or country grid coordinate system) and you want to make MicroStation aware of that GCS.

2. When you have data that is correctly drawn in one Geographic Coordinate System, and that GCS has been already been specified, but you want to reproject that data to a different Geographic Coordinate System.

3. When you create a new design file and you want to designate the Geographic Coordinate System that you intend to use.


Selecting a Geographic Coordinate System from the Library:

Click  the "Select Geographic Coordinate System" tool on the Geographic Toolbox (the left-most tool).

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That opens the nonmodal Geographic Coordinate System Dialog:

If the active model already has a GCS assigned, a few of its attributes are displayed. To Select a GCS from the library, use the "Select Geographic Coordinate System From Library" tool (second from left). That opens the modal "Select Geographic Coordinate System" dialog.

There are two tabs, "Library" and "Search". Under the Library tab, you will see a tree showing groups and GCS's. Each group can contain subgroups and/or Geographic Coordinate Systems, distinguished by icon. The topmost group is always "Favorites". These are the GCS's that you, or perhaps an administrator at your site, have opted to set aside for quick access. To add a GCS to Favorites, you can locate it in the tree and drag it to the Favorites folder, or use the right-click menu from the selected GCS to add it to Favorites. You can use the right-click menu to create subgroups (to any level of nesting desired) of Favorites. Favorite GCS's can be dragged between Favorites groups.

The Library Group is organized into Subgroups by category Geographic GCS's, (which use Latitude and Longitude rather than Cartesian coordinates) and Projected GCS's, and then further grouped by geography.

As a GCS is selected in the tree, the properties of the GCS are displayed, read-only, in the right pane.

To select a GCS, hit the OK button, or double click on the GCS.

The Search tab allows you to quickly locate a GCS in the library by entering a string or strings that you expect to be in its name, description, source, datum name, or ellipsoid name. The first such search takes a few seconds while data is read from the library file, but subsequent searches are quite fast. You can specify whether the GCS data must include all words that you specify, or any word. If you specify more than one word, the results are ranked according to number of words that match. The search strings are not case sensitive, but matches that contain the exact case entered are ranked higher.

To select a GCS, double click on its name, or click on it and click on Ok. You can also add it to your Favorites or show Properties for it.

Linear units (e.g., meters) are part of the definition of a projected Geographic Coordinate System for two reasons. First, some of the Geographic Coordinate System parameters are linear measures or Cartesian coordinates, such as the False Easting and False Northing, and second, the projection mathematics need to know the units of Cartesian coordinates that serve as inputs to or outputs from the calculations. However, because MicroStation's unit system is capable of converting the linear units of your model to those of any projected Geographic Coordinate System, the units of the GCS do not have to match the units of your model. When there are two Geographic Coordinate Systems that differ only in the units (for example, two Pennsylvania State Plane North entries, one of which has units of Meters, and the other which has units of Survey Feet), either can be chosen to give the same results. However, if you are planning to use the file with the MicroStation Geospatial Extension or Bentley Map version 8.9.xx or earlier, you should select the GCS that has linear units that match those of the model.

When you select a GCS, MicroStation checks to see whether the active model already has a GCS specified. If it does, then you are presented with a dialog that asks whether you are correcting the GCS, or want to reproject the data in the model from the existing GCS to the newly selected GCS:

The second option changes the data in the design file, so proceed with caution. If you select Cancel, the existing GCS is retained and no changes are made.

Creating a Geographic Coordinate Systems From Placemarks

For users that use structure-centric coordinate systems, selecting a standard GCS from the Library is not possible. Instead, given some information about geographic positioning of your model, MicroStation can calculate an Azimuthal Equal Area GCS that will allow you to realize all the benefits of Geo-Coordination.

You tell MicroStation about the geographic positioning of your design using Geographic Placemarks. A Placemark is a cell that contains text fields labeled Name:, Longitude:, Latitude:, and Altitude:. The longitude, latitude, and altitude fields specify the geographic position relative to the WGS 84 datum, which is the datum reported by GPS devices and also used by Google Earth. The corresponding position in the design file is specified by placement point of the cell. The scale and rotation of the cell does not affect its meaning as a Geographic Placemark.
 

Placemarks are created using the Define Placemark Monument tool (fifth from the left) in the Geographic toolbox:

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At least two Placemarks are needed to define a GCS, and more are preferred. It is best to have placemarks that span the entire range of interest (for example, at all four corners of a building footprint).

After defining Placemarks, you open the Geographic Coordinate System dialog and select the Define Geographic Coordinate System From Placemarks Tool (third from the left). That tool is disabled if there are no Placemark Monuments in the active model.


 

That opens the Geographic Coordinate System From Placemarks dialog:

This modal dialog lists all of the Placemarks in your model and shows the geographic and Cartesian coordinates for each one. The Placemark for which there is a dot in the second column is used for center of the Azimuthal Equal Area GCS that is calculated from the placemarks, which you can verify by looking at the values for Longitude, Latitude, False Easting and False Northing in the calculated parameters at the bottom of the dialog. Use the most important Placemark for the origin. The Distance and Azimuth error columns give you a good idea as to whether the set of Placemarks is internally consistent. Ideally, all of the error values are small. The Maximum Angle Distortion and Maximum Scale Distortion give you the maximum distortion values that occur considering the entire range of all of the elements in your model. If the Maximum scale distortion is larger than .01%, or the Maximum Angular Distortion is greater than about 2', then the area covered by the model is too large for an Azimuthal coordinate system to work properly.

To select the calculated GCS, hit OK. The confirmation dialog box discussed in Section 3.1 will appear if there is an existing GCS.

Note that a Geographic Coordinate System calculated from Placemarks is only as accurate as the placemark data used to calculate it. If you use such a GCS to reference other geolocated designs, errors could be cumulative. You must treat measurements between features in such referenced designs as approximate only.

Selecting A Geographic Coordinate System from an Attached Reference

There are a few situations where you can take advantage of one of your correctly oriented references to quickly select a GCS for your active model:

To use the attached reference's geographic coordinate system, open the Geographic Coordinate System dialog and click the Select Geographic Coordinate System "From Reference"  tool (fourth from the left). That tool is disabled if there are no reference attachments from which a GCS can be extracted or calculated.

 

That opens the modal Geographic Coordinate System From Reference dialog:

The dialog shows all directly attached references for which display is turned on. The Comment column gives more information, including the reason that a particular attachment cannot be used as a GCS source and thus is disabled. Properties of the GCS that would result from accepting the currently selected reference are displayed in the bottom part of the dialog. Note that you if you currently have a Geographic Coordinate System, and a reference is attached in one of the Geographic Referencing modes discussed in Section X.xx below, that reference cannot be used as a source for the GCS because its position is calculated from its GCS and the active model's current GCS, so selecting it as a source could not change the current GCS.

Setting the GeoCoordinate System in Attached References

When you have a GCS assigned to the current model, it is sometimes possible to set the GCS of directly attached references based on that GCS:

To set the GCS in attached references, open the  Geographic Coordinate System dialog and click the Set Geographic Coordinate System "To Reference" tool (fifth from the left). That tool is disabled if there are no reference attachments that are eligible for the operation.

That opens the modal Set Geographic Coordinate System In References dialog:

Multiple references can be selected for the operation. The "Comment" column provides a reason if the operation is not applicable to a particular reference. For example, if the current model and reference both have a GCS, and the reference is attached using one of the Geo Reference attachment modes discussed in Section X.xx below, its position is calculated from the its GCS and that of the active model, so selecting it as a destination would not change its GCS.

The Apply button attempts to write the GCS to the selected references. If the referenced model already has a GCS, the dialog box described in Section 3.1 is displayed to allow you to specify whether the new GCS is a correction or the data should be reprojected. This operation writes to the reference file(s) using MicroStation's reference activation feature, and has all of the requirements and characteristics of reference activation. Changes are undoable until a writable reference is released.

When you are done, exit the dialog by clicking the Done button. The Done button does cause any change to the selected references.

Selection a Geographic Coordinate Systems From an Unrelated Design

Sometimes you may want to use the same GCS as is used in a model of another design file, even if you don't have it attached as a reference. In that case, open the Geographic Coordinate System dialog and click the "Select Geographic Coordinate System From File" Tool.

That opens a modal dialog that allows you to select a MicroStation design file:

If there are multiple models that contain a Geographic Coordinate System, a second modal dialog allows you to select the model from which to source the Geographic Coordinate System:

If there are no models that contain a Geographic Coordinate System, MicroStation informs you:

And then reopens the file selection dialog.

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