ARTICLES

Reverse Geocoding with MapPoint 2002

Reverse Geocoding is finding out information about a given coordinate. This article shows how to determine the country, state, city, nearby places and maybe even street address for a location known by its lat/lon.

Where in the world is N47.4560212902604 W122.262495768687 ?

Interactively, it's easy to find out where a spot given by its lat/lon coordinates is located - just launch MapPoint, press Ctrl-F, switch to the "Lat/Lon" tab of the Find dialog (enter S and W coordinates as negative values) and press Enter.

MapPoint will take you to the corresponding spot, and you'll be able to see at a glance where it is (e.g. in the Location and Scale toolbar) and what is around it.

But what if you need this sort of information programmatically, e.g. to add additional info to a list of GPS coordinates captured using a GPS receiver ?

Finding "points of interest" around your given point is relatively easy - the "FindNearby" method of the location object can be used.

Here's an example:

   ' Attach to running instance of MapPoint 2002
   Set oMpApp = GetObject(, "MapPoint.Application")
   
   ' Retrieve the active map
   Dim oMap As MapPoint.Map
   Set oMap = oMpApp.ActiveMap

   ' Create location object   
   Dim oLoc As MapPoint.Location
   set oLoc = oMap.GetLocation(47.4560212902604, _
                             -122.262495768687)
                           
   ' Find nearby places 
   Dim oResults as MapPoint.FindResults
   Set oResults = oLoc.FindNearby(0.1)

FindNearby does this by returning a "FindResults" collection that contains both Location as well as Pushpin objects. All that remains to be done is to iterate over the results:

   Dim oObj As Object
   
   ' Loop over collection
   For Each oObj In oResults
      MsgBox oObj.Name
   Next oObj

• Displaying just the name of a match is not enough, other data like the match type or the note attached to a pushpin may be interesting. As it turns out, the real problem with displaying this sort of information is that the objects in the "FindResults" collection are not of the same type - they are a mix of Locations and Pushpins that support different methods and properties. They happen to share the "Name" property so the above loop works, but that's about all interesting properties these two objects share.

• Sometimes more precise information about the location in question is required than what is just close to it - ideally a street address of the location itself.

   If Typeof Object Is Type Then

The second problem is a little bit more difficult to solve. MapPoint does not provide a direct way to convert from a coordinate to an address. However, the "ObjectsFromPoint" method of the map object is the programmatic equivalent to the user (double-)clicking on the map - it will yield a list of potentially interesting objects sorted by increasing size (or if you prefer, decreasing specificity).

ObjectsFromPoint needs a screen coordinate on input, but that can be obtained using the "LocationToX" and "LocationToY" methods. Since screen coordinates are used here, it is important to ensure the map is zoomed to the location in question - therefore, a height of 1 is used in the GetLocation method, and the map is centered on the location using its "GoTo" method. This is one way to code this (initial code same as above):

   set oLoc = oMap.GetLocation(47.4560212902604, _
                             -122.262495768687, 1)
   oLoc.GoTo
                                
   ' Retrieve ObjectsFromPoint
   Dim oResults as MapPoint.FindResults
   Set oResults = oMap.ObjectsFromPoint( _
     oMap.LocationToX(oLoc), _
     oMap.LocationToY(oLoc))

As you can see, ObjectsFromPoint also returns a FindResults collection, just like FindNearby (and a couple other methods, actually).

The complete sample below illustrates both techniques.

To try it, create a VB project and do the following:

• set a reference to your version of MapPoint 2002
• add the following elements to the Form:
  • a "Command1" CommandButton
  • a "Text1" TextBox
  • a large "List1" Listbox
• paste the snippet below into the code for the form
• add the DmsVal routine (posted in an earlier article, left out here for brevity).

  DmsVal is used to support user-friendlier coordinate input formats, if you do not need this, simply replace it with the built-in Val() function.

The "working horse" that does this is the ShowResults subroutine - it accepts a title (for the listbox), a FindResults collection, and the original spot as a location object. The latter is used to compute the distance from the original spot to the locations MapPoint found - the larger the distance, the less "specific" the location found probably is.

After adding a title to the list (so that the user can tell apart the "FindNearby" and the "ObjectsFromPoint" results, ShowResults displays the ResultsQuality property of the collection. It can have the following values:

Constant name	Value	Description
geoAllResultsValid	0	Used only for results obtained from the FindNearby and ObjectsFromPoint methods, where all returned results match the method criteria
geoAmbiguousResults	2	At least the first two results are good matches, but it is not clear which one was intended
geoFirstResultGood	1	The first result is a good match to the method criteria
geoNoGoodResult	3	None of the results is a good match to the method criteria, but the results are the best available
geoNoResults	4	No results

With FindNearby and ObjectsFromPoint, I have always only seen quality codes of either 0 (all valid) or 4 (nothing found) though.

ShowResults then iterates over the objects in the FindResults collection. Depending on the object type, the following information is added to the list:

• Location object:
  • Location name
  • Place category (if available)
  • Location type code
  • Street address (if available)
  • Distance to original location
• Pushpin object:
  • Pushpin name
  • Pushpin note

Place category and location type are of particular interest. The former provides (textual) information about "nearby places" locations, e.g. "Hotels and Motels", "Restaurants - Italian". The latter is numerically coded and describes the type of object you found - for example, 19 means "country", 18 means state (or similar subdivision, e.g. région in France or Bundesland in Germany), 17 means county (or département in France, Landkreis in Germany etc.)

Check out the GeoShowDataBy values in MapPoint help for a detailed description of these codes. Please find the complete sample code below.

Mapping and especially GPS-related topics are a hobby - Gilles enjoys developing solutions for Microsoft MapPoint and his favorite outdoor occupation is confluence hunting.

As always, please direct questions to the newsgroup.

	MP2Kmag Internet