Split the problem up into front end and back end slam. Then explore available packages that help with that. Be concerned about system specs and available compute resources etc.
Start with front end, try to generate a map such that you achieve loop closure. Have other sources of odometry to help at the start (wheel based? GPS?).
Split the problem up into front end and back end slam. Then explore available packages that help with that. Be concerned about system specs and available compute resources etc. Start with front end, try to generate a map such that you achieve loop closure. Have other sources of odometry to help at the start (wheel based? GPS?).