MicroStation reprojects design data from one Geographic Coordinate System to another in two cases:
- When a new Geographic Coordinate System is selected, and the Reproject option is chosen in the confirmation dialog. In that case the data within the model to which you are assigning the GCS is permanently reprojected.
- When a reference attached with the "Geographic Reprojection" option is loaded. In that case the data within the referenced model is reproject ed temporarily, and not written back to the file.
Geographic Reprojection involves these steps for each point that must be reprojected from the source GCS to the destination GCS:
- If necessary, the model coordinates are converted from the model's linear units to Cartesian coordinates in linear units of the source GCS.
- The Cartesian coordinates are converted to longitude, latitude, and elevation in the source GCS datum.
- If the datum of the source GCS is different from the datum of the destination GCS, the datum shift calculation is preformed to give the longitude, latitude, and elevation in the datum of the destination GCS.
- The longitude, latitude, and elevation are converted to the Cartesian coordinates in the linear units of the destination GCS.
- If necessary, the Cartesian coordinates in the units of the destination GCS are converted to the model's linear units.
Some of the steps above are not necessary if the source or destination GCS is nonprojected rather than projected, since the "Cartesian" coordinates in those situations are actually longitude and latitude rather than actual linear values.
This reprojection algorithm is performed for each point of elements that contain discrete points such as line strings, lines, Bsplines, and point strings. For other element types, the reprojection technique varies and can be influenced by the reprojection settings that are in effect.
The modal Reprojection Settings dialog is opened when you select the Reprojection Settings tool from the Geographic Coordinate System dialog.
There are separate reprojection settings for reference reprojection and active model reprojection. Since reference reprojection happens every time a reference is loaded, it makes sense to select faster, more approximate settings than those that are used when the active model is reprojected when you select a different GCS.
The meaning of the settings is as follow
Stroke Tolerance
The stroke tolerance specifies the maximum allowable separation between the chord that represents a line string approximation of a curved element and the exact curve. The units are master units of the active model. The smaller the number, the finer the approximation and the slower the algorithm. The Stroke tolerance is used for all of the Stroke options.
Reproject Cell components individually
When a MicroStation cell is encountered, there are two alternatives for reprojecting it:
- Each component element can be individually reprojected
- The best linear transform at the cell origin can be calculated, and that transform applied to all components, essentially treating the cell as a single entity.
The first option is best suited for cells that represent a man made unit, such as a transformer box or a parking lot light. The second option is best suited for geographic features that have been grouped as a cell for convenience. There are three options that can be chosen. Always reprojects every element in every cell. Never treats all elements as single entities. The default setting is If Spatially Large, which reprojects every component of cells that occupies more than an area of a .2 square kilometers in the X-Y plane, and treats smaller cells as single entities.
Reproject Multiline Text components individually
When Multiline text (text node) is encountered, there are two possible reprojection methods. In the first, the multiline text is considered to be a single element for reprojection purposes, and in the second, each individual text element is separately reprojected. As for Cells, the three options are to Always or Never consider the components separately, or to do that only when the Multiline text covers a large area. The default is If Spatially Large.
Rotate Cells
You can elect to rotate cells by the Geographic Coordinate System convergence angle calculated at the cell origin, or to leave the rotation as is. The default is to rotate.
Scale Cells
You can elect to scale cells by the Geographic Coordinate System scale factor calculated at the cell origin, or to leave the scale as is. The default is to scale.
Rotate Text Elements
As with Cells, Text elements can be rotate or not as desired. The default setting is to apply rotation.
Scale Text Elements
As with Cells, Text elements can be scaled or not as desired. The default setting is to apply the scale.
Stroke Arcs to Line Strings
In many reprojections, the path of an arc does not necessarily remain an arc. You can choose to stroke the arc to a line string approximation (with the accuracy of the approximation depending on the Stroke Tolerance you set) and then reproject points in that line string. In that case, the arc loses its identity as an arc. If the arc isn't stroked, the "best fit" reprojected arc is calculated. The options for when to use the stroking algorithm are Always, Never, and If Spatially Large. The default is "If Spatially Large".
Stroke Ellipses to Line Strings
This is analogous to the Stroke Arcs setting, except that it pertains to Ellipses.
Stroke Curves to Line Strings
This is analogous to the Stroke Arcs setting, except that it pertains to Akima curve elements (element type 11).
Add Points If Needed
In some cases, a line segment that appears straight in one projection appears curved in another. For example, lines of constant latitude in an Equidistant Cylindrical projection are straight horizontal lines, but in a Lambert Conformal Conic projection they are arcs of varying radius. Thus if a particular boundary that lies exactly along a parallel is represented by a long line in a design using an Equidistance Cylindrical projection, the end points would get reprojected but it would still appear as a straight line when reprojected to Lambert Conformal Conic projection. The Add Points if Needed option tells MicroStation to insert points along such a line to get it to stay along the correct path within the Stroke Tolerance setting. This improves quality at the expense of processor time required. The options are Yes or No.
Display Errors to Message Center
You can optionally turn on error logging to see any errors that occur during the reprojection process. The most common error is applying a projection to longitude/latitude data that is outside of the optimal range of the Geographic Coordinate System.