calcAreaFromLatLngPolygon ( polygonJson ; outputUnit )

calculate area in different units of a polygon based on a lat/lng coordinate array (json)

Average rating: 4.0 (1 vote) Log in to vote

Alexander Baier - Show more from this author
datamat Software Development
http://fm-tutorial.de

Sample input:

calcAreaFromLatLngPolygon( "[{\"lat\":52.5220503095481,\"lng\":13.3857699952148},{\"lat\":52.5230947811845,\"lng\":13.3692905030273},{\"lat\":52.5120221182497,\"lng\":13.3644839844726},{\"lat\":52.506589225608,\"lng\":13.3871432862305},{\"lat\":52.5151561739,\"lng\":13.377873571875},{\"lat\":52.5178721746602,\"lng\":13.3895465455078}]" ; "" )

Sample output:

{
"acre" : 426.560354699059,
"feet" : 18581782.9278478,
"hectar" : 172.628975546709,
"inch" : 2675749120.97399,
"kilometre" : 1.72628975546709,
"metre" : 1726289.75546709,
"yard" : 1932754.01022096
}

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

Let([

	polygonLength = ValueCount( JSONListKeys ( polygonJson ; "" ) ) ;

	//copy first point at last, closing polygon
	polygonJson = JSONSetElement( polygonJson ; ["[" & polygonLength & "]" ; JSONGetElement( polygonJson ; "[0]" ) ; JSONRaw] );

	earthRadius = 6378137;

	radianPart = (Pi / 180);

	area = While( [ i = 0 ; area = 0 ; p1Lat = 0 ; p1Lng = 0 ; p2Lat = 0 ; p2Lng = 0] ;

			i  < polygonLength ;

			[ 
				p1Lat = JSONGetElement( polygonJson ; "[" & i & "].lat");
				p1Lng = JSONGetElement( polygonJson ; "[" & i & "].lng");
				p2Lat = JSONGetElement( polygonJson ; "[" & i + 1 & "].lat");
				p2Lng = JSONGetElement( polygonJson ; "[" & i + 1 & "].lng");

				area = area  +  ( radianPart * (p2Lng - p1Lng) )  * (2 + Sin( radianPart * p1Lat )  + Sin( radianPart * p2Lat ));  
				
				i = i + 1 
			 ]; 

			 area
		);

	area = Abs((area * earthRadius * earthRadius) / 2);

	json = JSONSetElement( "" ; 
			["metre" ; area ; JSONNumber];			
			["kilometre" ; area / (10 ^ 6) ; JSONNumber];			
			["yard" ; area * 1,1196 ; JSONNumber];	
			["feet" ; area * 10,764 ; JSONNumber];	
			["inch" ; area * 1550 ; JSONNumber];	
			["hectar" ; area / 10000 ; JSONNumber];	
			["acre" ; area / 4047 ; JSONNumber]
		)
	];

	JSONFormatElements( JSONGetElement( json ; outputUnit ) )
)

Calculates the area of an n-sided polygon based on Lat and Lng.

Create a JSON array [{"lat": xxx, "lng": yyyy}]

Returns the area of the polygon in various units of measure as the result. Set "meter" as "outputUnit" to get the result in meters. If the value is empty, a JSON with all units is returned.

Comments

		Nick Gaunt Aug 21, 2021
This is a useful function but the commas in "yard" and "feet" need to be changed to periods before it will compile.

Note: these functions are not guaranteed or supported by BrianDunning.com. Please contact the individual developer with any questions or problems.

calcAreaFromLatLngPolygon ( polygonJson ; outputUnit )

Comments

Support this website.

Support this website.