Convert GeoPHP getArea() values into km2 or Acres

user_888894 0
Viewed 1393

I am using GeoPHP to get the area of my GeoJson polygons.

Here is my function below.

/**
 * gets the areas in acres for field geojson polygons
 * @return string
 */
public function polygon_area($json)
{

    // get the geoPHP library
    require_once(__DIR__ . '/vendor/GeoPHP/geoPHP.inc');

    // create the json array as string
    $geoJson = '{"type":"Polygon","coordinates":['.$json.']}';

    // create the polygon geoPHP object
    $polygon = geoPHP::load($geoJson,'json');

    // the area of the polygon
    $area = $polygon->getArea();

    // convert area into acres
    $acre = $area * 247.105;

    // return the geophp area
    return $area;

}

This is assuming that the areas returned from the polygon json, is square kilometres. But I don't think it is.

See below the values returned using the GeoPHP $polygon->getArea() function from the above code.

Area: 1.21816181142E-5
Area: 4.06926334051E-6
Area: 5.59853614135E-6                  
Area: 3.74998247921E-6         
Area: 5.91964048269E-6
Area: 2.50475049057E-6            
Area: 5.84341879772E-7
Area: 7.7207101441E-6
Area: 4.27298301986E-6
Area: 2.99669066806E-6
Area: 1.76724087666E-5

What I can't understand, is what unit of measurement are these areas calculated in using GeoPHP. There documentation does any unit at all?

When I convert to acres, and round($acre, 2) the values come out at zero. These polygon areas should be ranging from 5-15 acres.

Does anyone know how convert the areas to acres or km2?

https://geophp.net/api.html

Thanks in advance.

geojson php
1 Answers

GeoPHP returns square degrees in the area calculations. Take a look at https://github.com/spinen/laravel-geometry. It build upon phayes/geoPHP and adds getSquareMeters() and getAcres() methods.

In your case, the area can be calculated as:

// create the json array as string
$geoJson = '{"type":"Polygon","coordinates":['.$json.']}';

// create the polygon geoPHP object
$polygon = geoPHP::load($geoJson,'json');

// get the area in square meters
$areaSquareM = $polygon->getSquareMeters();

// convert to square kilometers
$areaSquareKm = $areaSquareM / 1000000;

// area in acres
$areaAcres = $polygon->getAcres();

Note: the library does not support MultiPolygon types, so you'll have to loop the geometries and get the area for each Polygon individually.

user_85839470
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