Questions similar to this appear on the Bentley Discussion Groups. This appeared in the VBA discussion group.

Q You want to be able to load maps in response to a user-supplied datapoint in MicroStation^® GeoGraphics™. You are looking for answers to questions like this …

• How can I get a GeoGraphics map ID from a specified point?

A First, let's be sure we understand how GeoGraphics knows about maps and their extents. When you register a map with GeoGraphics, you define a map extent and associate the extent with the map ID. That's one of the administrative tasks in setting up a GeoGraphics project. The map ID is the MSLINK value in the maps database table.

It is likely that, unless your project is very simple, multiple maps will surround a query point. You need to build a list of map IDs.

A map extent can be defined in two ways …

• a shape element in the project's index file (e.g. index.dgn)
• a coordinate range in the database maps table

Map Index Shape

If a map is irregular in outline, then it makes sense to use a shape in the index file. The index file is a DGN file containing a number of shapes. Each shape circumscribes a map, contained in another DGN file. That is, if you were to attach a map as a reference model to the index file, you would see a shape surrounding all the graphics elements of that reference.

You need the outline shape to register a map. You can either draw the outline yourself, in index.dgn, or let GeoGraphics project setup generate it for you using the map registration wizard.

The shape in the index file has a database linkage. The MSLINK value is the ID of the map. If you want to find the ID of a map having a shape in the index file, then your approach would be …

• Scan the index file for shapes
• For each shape …
  • Test for inclusion of your point
  • Get the MSLink from the shape
  • Add the MSLink to a list of map IDs

Database Range

If a map is regular in shape (i.e. a square or rectangle), then it's preferable to use the database. When storing the map extent in the database, you don't need a shape in the index file. It's better because …

• You're storing less data (a couple of XY coordinates)
• You can search for a map using an SQL query, which is probably faster than scanning the index file

The map extent coordinates are stored in MicroStation units-of-resolution (UORs) in the maps table columns (xlow, ylow) and (xhigh, yhigh). Your approach to obtain a map ID when map extents are stored in the database would be like this …

• Convert the query point to an XY coordinate in UORs
• Compose an SQL statement that searches for maps whose extents enclose your point
• Build a list of map IDs from the recordset resulting from your query