Cartesian Maps and Design Files

Maps are projected, meaning that x and y coordinates have been determined for every point throughout the map. Further, x and y coordinates are Cartesian, meaning that x is perpendicular to y and each unit has the same length. All design files are Cartesian.

Latitude and longitude coordinates are not Cartesian--they are angular units of unequal value, since all longitudes converge to a single point at the north and south poles. Despite this, it is possible to save latitude/longitude coordinates in a design file. When you do, the characteristics of convergence and equal length are lost and the the result looks similar to cylindrical projections.

General Notes:

• MicroStation GeoCoordinator includes the seed file \geocoord\seed\s_latlng.dgn as an example
• Most maps will use "Geographic Latitude Longitude" as a reference coordinate system

 

Design File Setup for Latitude Longitude

This example will demonstrate the use of "Geographic Latitude Longitude" as a master coordinate system. The first step is to set up the resolution of an empty design file (Settings > Design File > Working Units). Generally, the master design file unit corresponds to the Master Coordinate System unit, unless paper scale is being used (which is not typical). A master unit of "dg" simply assigns a two letter abbreviation to design file units. Since all transformations will be done in master units (double precision floating point), the breakdown of sub-units and positional units is not important, as long as su*pu provides enough resolution. For example,

  mu: dg  su:    su/mu 3600000  mu:    pu/su 1

is equivalent to

  mu: dg  su: se su/mu 3600  mu:    pu/su 1000

Both cases give 1/3600000 dg resolution.

Always perform File > Save Settings (or key in filedesign) after changing working units.

Next, save the latitude/longitude MicroStation GeoCoordinator element with the file (in MicroStation GeoCoordinator, choose Master > Edit). Toggle the Target Coordinate Systems to "Standard" and "Geographic (Latitude/Longitude)". Choose Master > Save.

For other examples of setting up design file resolution, please refer to MicroStation GeoCoordinator Help, "Design File Setup and Resolution". You can also refer to the MicroStation GeoCoordinator Help, "Getting Started". Once the file is set up for latitude/longitude, operations are similar to all other transform operations.

 

Common Errors

•  A design file is used which was set up for linear units (meter, foot, etc), but is used for degrees latitude/longitude by simply changing the unit name abbreviation. This leads to jagged lines result after transform, due to inadequate resolution. Remember, 0.1 meter resolution is not comparable to 0.1 degree, since one degree can be as large as 100 km.
• If you save a coordinate system to the design file, then subsequently change resolution, you will be warned that the design file setup has changed since the coordinate system was established. If this occurs, put the master coordinate system into edit mode and simply re-save it.