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If there are no obstacles in the world, the laser will not reflect off of anything and give no response. This is an error condition which may have several causes (such as absorptive bodies) and the data cannot be treated as having been cleared to max range. Thus without any obstacles the laser is always reporting no data and gmapping cannot build a map. You can update the configuration to make this work, but it's a very specific edge case which is rarely encountered in the real world and can lead to false negatives on obstacles.

I expect the reason the map is jumping around is that it's trying to optimize based on close to zero data. Almost any point is equally likely so as gmapping samples possible locations different ones win out due to noise in the system.