关于sfm.txt的问题

[hai-bo1017]

//  保存地图点
void Map::SaveMapPoint(ofstream &f, MapPoint *mp)
{
    //保存当前MapPoint世界坐标值
    cv::Mat mpWorldPos = mp->GetWorldPos();
    f <<" " <<mpWorldPos.at<float>(0)<<" " << mpWorldPos.at<float>(1)<<" " << mpWorldPos.at<float>(2) << " ";
    f << (mp->nObs)/2<< " ";

    std::map<KeyFrame*,size_t> mapObservation = mp->GetObservations();
    for(auto mit = mapObservation.begin(); mit != mapObservation.end(); mit++)
    {
        int Frameid;
        Frameid = mit->first->mnId;
        auto keyid = find(KeyId.begin(),KeyId.end(),Frameid) - KeyId.begin();
        f << keyid << " ";
    }
    f << "\n";
}

请问大佬往sfm.txt当中写这个点被哪些关键字观测到时，为什么要除2呀