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A More Compact Method for Obtaining Lat/Long

An alternate method for obtaining lat/lon was recently posted to the MapPoint newsgroup by Gilles Kohl that works with pushpins and also works in the European version.

In a previous article Chris Gebhardt provided a workaround allowing developers to determine the lat/long in MapPoint. The details are here. An alternate method for obtaining lat/lon was recently posted to the MapPoint newsgroup by Gilles Kohl that works with pushpins and also works in the European version.

The following code will return the lat/lon of a pushpin (actually, its location object) passed by reference. CalcPos is the actual working horse, Arccos is just a trigonometric helper function. Note that this approach does not require iterations nor does it require a specific units setting (miles vs. kilometers) on the users machine.

Usage: Paste the snippet below into your VB code, and call CalcPos with a Map object, your pushpin's location, and two doubles that will receive the lat and lon respectively.

The routine accepts a map object (MapPoint.Map) and a location (MapPoint.Location) as input parameters, and will return the corresponding lat/lon in two doubles passed by reference.

Here's how it works in principle:

  • In an initialization step performed only once, it first creates a location object at the north pole and one at (0, 90W). During this step, it also computes the half circumference of the earth (distance between north and south pole) and the quarter circumference - we need that later. All this is done assuming the earth is a perfect sphere - just as the MapPoint Distance function does.
  • The latitude (angle in degrees) of locX (the location of unknown latitude and longitude) is easily computed like so:

    ' Compute latitude from distance to north pole
    ' dblLat = 90 - 180 * objMap.Distance(locNorthPole, locX) / dblHalfEarth

  • The "absolute longitude" is computed in two steps - first, compute the great circle distance to the Greenwich meridian by creating a location with the same latitude but longitude zero, then convert this resulting distance to a longitude. (the distance on a circle parallel to the equator)
  • Finally, if the distance of our unknown location to (objMap.GetLocation(0, -90) is less than a quarter of the Earth circumference, we're in the western hemisphere, so the sign of the longitude value is negative, otherwise, leave it as it is.

Discuss this story in the forum.

Author: Gilles Kohl
Email: gilles(AT)_deletethisincludingunderlines_compuserve.com
URL: http://www.procad.de
Gilles Kohl, a native of Luxembourg living in Germany, is a software development lead with PROCAD GmbH of Karlsruhe, Germany.

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.



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