Drupal\geolocation\Plugin\Field\FieldType\GeolocationItem.php
public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
// Guadalajara.
$longitude = (float) -103.3496;
$latitude = (float) 20.6597;
$radius = rand(1,5); // in miles
$lng_min = $longitude - $radius / abs(cos(deg2rad($latitude)) * 69);
$lng_max = $longitude + $radius / abs(cos(deg2rad($latitude)) * 69);
$lat_min = $latitude - ($radius / 69);
$lat_max = $latitude + ($radius / 69);
$values['lat'] = rand($lat_min, $lat_max);
$values['lng'] = rand($lng_min, $lng_max);
return $values;
}