<?
 foreach($arResult["YANDEX_COORD"]["PLACEMARKS"] as $arItemKey=>$arItem)
 {
  $min_x = ($arItem["LAT"]<$min_x || !$min_x)? $arItem["LAT"] : $min_x;
  $max_x = ($arItem["LAT"]>$max_x || !$max_x)? $arItem["LAT"] : $max_x;
  $min_y = ($arItem["LON"]<$min_y || !$min_y)? $arItem["LON"] : $min_y;
  $max_y = ($arItem["LON"]>$max_y || !$max_y)? $arItem["LON"] : $max_y;
 }

 $x = round((($max_x - $min_x)/2 + $min_x), 6);
 $y = round((($max_y - $min_y)/2 + $min_y), 6);

 $arResult["YANDEX_COORD"]['yandex_lat'] = $x;
 $arResult["YANDEX_COORD"]['yandex_lon'] = $y;
 ?>