Brian Dunning's FileMaker Custom Functions

GeoBoxBounds ( latitude ; longitude ; radiusInMeters )

Calculates coordinates for search bounding box around a location

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Jeremy Bante   Jeremy Bante
Beezwax
http://beezwax.net/

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  Sample input:
/* Coordinates for geobox bounding neighborhood up to 1 km away from FileMaker, Inc., headquarters */

LocationBoxBounds ( 37.4064806 ; -121.984204 ; 1000 )
  Sample output:
37.3974922470474878
37.4154689529525122
-121.9955194140472811
-121.9728885859527189

  Function definition: (Copy & paste into FileMaker's Edit Custom Function window)

Calculates the approximate coordinate bounds for an approximately square region around a point on the surface of the Earth. This can be useful for finding data within a certain distance of a reference point. For example:

Set Variable [$geoBox; Value:LocationBoxBounds ( $latitude ; $longitude ; $radius)]
Enter Find Mode []
Set Field [Table::latitude; Value:GetValue ( $geoBox ; 1 ) & "..." & GetValue ( $geoBox ; 2 )]
Set Field [Table::longitude; Value:GetValue ( $geoBox ; 3 ) & "..." & GetValue ( $geoBox ; 4 )]
Perform Find []

This function is similar to the GeoBox function by Steve Murray, but it provides more helpful results for search regions that may include a pole or cross the antimeridian (180°/-180° longitude). The above script example naïvely assumes that the search region will not cross the antimeridian; a more complete application will include slightly more complicated logic.

The results of this function define an approximately square region. If your application requires search within a circular region, you can perform a find like above to start with a manageable number of records, then compare each record to the center of your search region with the GeoDistance function to omit points between the circular and square neighborhoods of the center point.

 

Comments

Carsten Warnich   Carsten Warnich, Brabrand, Denmark
Jul 29, 2014
I've found that there is an error in this function when it comes to calculating the longitude values.

I changed the script to this - and now it works:

// calculate longitude bounds
~longitudeOffset = (radiusInMeters/1000)*((1/Cos(Radians ( latitude ))) /111,32);


111,32 is the approximate distance represented by 1 degree longitude at the Equator.
 
Jeremy   Jeremy, USA
Jul 30, 2014
Thanks for catching that! I opted for a different solution by working the haversine formula backwards, which appears to work fine now.
 

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