Add Haversine formula to a Magento resource collection. Expects the collection to have 'lat' and 'lng' attributes in the main FROM table.

haversine.php

<?php
// Just add the following methods inside your existing resource collection class.
class ModuleNamespace_ModuleName_Model_Resource_EntityName_Collection
    extends Mage_Core_Model_Resource_Db_Collection_Abstract
{
    /**
     * Filters the collection to only include records within X miles of a specified
     * latitude and longitude coordinates.
     *
     * Example (radius unit: mailes):
     * $params = array(
     *     'lat'    =>  44.9764839
     *     'lng'    => -93.4546814
     *     'radius' => 25
     * );
     *
     * @param array $params
     * @return ModuleNamespace_ModuleName_Model_Resource_EntityName_Collection
     */
    public function applyMapSearchFilter($params)
    {
        if (!$this->hasFlag('_appliedHaversine')) {
            $this->setFlag('_appliedHaversine', true);
            // Sanitize input
            $params['lat'] = floatval($params['lat']);
            $params['lng'] = floatval($params['lng']);
            $sql = $this->getSelect();
            $sql->columns("(3959 *
                ACOS(
                    COS( RADIANS( {$params['lat']} ) ) *
                    COS( RADIANS( main_table.lat ) ) *
                    COS(
                        RADIANS( main_table.lng ) -
                        RADIANS( {$params['lng']} ) ) +
                        SIN( RADIANS( {$params['lat']} ) ) *
                        SIN( RADIANS( main_table.lat ) )
                    )
                ) AS distance_from_search")
            ;
            $sql->where('main_table.lat IS NOT NULL');
            $sql->where('main_table.lng IS NOT NULL');
            $sql->having('`distance_from_search` <= ?', $params['radius']);
            $this->setFlag('search_radius', $params['radius']); // used again in count select
        }
        return $this;
    }

    /**
     * Get SQL for get record count.
     * Extra HAVING, and (conditionally) Haversine formula added.
     *
     * @return Varien_Db_Select
     */
    public function getSelectCountSql()
    {
        $countSelect = parent::getSelectCountSql();
        $countSelect->reset(Zend_Db_Select::HAVING);
        $this->_applyHaversineToCount($countSelect);
        return $countSelect;
    }
    
    /**
     * Filters the select count sql to records within range of the search radius.
     *
     * @param Varien_Db_Select $countSelect
     */
    protected function _applyHaversineToCount($countSelect)
    {
        /* @var $resource Mage_Core_Model_Resource */
        /* @var $read     Magento_Db_Adapter_Pdo_Mysql */

        if ($this->getFlag('_appliedHaversine')) {
            $parts = $this->getSelect()->getPart(Zend_Db_Select::COLUMNS);
            foreach ($parts as $part) {
                $expr = is_array($part) && isset($part[1]) ? $part[1] : null;

                if (   $expr instanceOf Zend_Db_Expr
                    && 'distance_from_search' == substr($expr->__toString(), -20)
                ) {
                    $resource = Mage::getSingleton('core/resource');
                    $read     = $resource->getConnection('core_read');
                    $sql      = new Varien_Db_Select($read);
                    $sql
                        ->from(
                            array(
                                'main_table' => $resource->getTableName($this->getModelName())
                            ),
                            array(
                                'entity_id',
                                $expr->__toString()
                            )
                        )
                        ->where('lat IS NOT NULL')
                        ->where('lng IS NOT NULL')
                        ->having('distance_from_search <= ?', $this->getFlag('search_radius'))
                    ;
                    $entityIds = $read->fetchCol($sql);
                    $countSelect->where('entity_id IN (?)', $entityIds ? $entityIds : '0');
                }
            }
        }
    }

}