1、LEGO-LOAM编译出错 ：fatal error: cloud_msgs/cloud_info.h: No such file or directory
解决方法：
catkin_make -j1
或者：
catkin_make -DCATKIN_WHITELIST_PACKAGES="cloud_msgs"
2、 LEGO-LOAM编译出错 ：Project ‘cv_bridge‘ specifies ‘/usr/include/opencv‘ as an include dir, which is not found
CMake Error at /opt/ros/melodic/share/cv_bridge/cmake/cv_bridgeConfig.cmake:113 (message): Project 'cv_bridge' specifies '/usr/include/opencv' as an include dir, which is not found. It does neither exist as an absolute directory nor in '/opt/ros/melodic//usr/include/opencv'. Check the issue tracker 'https://github.com/ros-perception/vision_opencv/issues' and consider creating a ticket if the problem has not been reported yet. Call Stack (most recent call first): /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:76 (find_package) open_quadtree_mapping/CMakeLists.txt:24 (find_package)
解决办法：
ubuntu18.04上面安装的是opencv4，位置命名成了/usr/include/opencv4
sudo vim /opt/ros/melodic/share/cv_bridge/cmake/cv_bridgeConfig.cmake
第96行改为opencv4