$spot->field_coordinates[LANGUAGE_NONE][0] = geofield_compute_values(
          array(
              'lat' => $venue->location->lat,
              'lon' => $venue->location->lng,
      ), GEOFIELD_INPUT_LAT_LON);

$lat = 42.281646;
$lon = -83.744222;
$geofield = array(
  'geom' => "POINT ($lon $lat)",
  'geo_type' => 'point',
  'lat' => $lat . "000000",
  'lon' => $lon . "000000",
  'left' => $lon . "000000",
  'top' => $lat . "000000",
  'right' => $lon . "000000",
  'bottom' => $lat . "000000"
);
$lang = $node->language;
$node->field_position[$lang][0] = $geofield;

{
  "nid":123,
  "field_position":{"und":[
    {"geom":{"lat":"42.2808256","lon":"-83.7430378"}}
  ]}
}

$test = node_load($entity->nid);
module_load_include('inc', 'node', 'node.pages');
$test_state['values']['op'] = t('Save');
drupal_form_submit('NODENAME_node_form', $test_state, $test);