[Niume]

Hello I have a question. I was watching hell under the sea its a show on national geograchic about uboats. So episode was about operation drumbeat manly the U-boat U-123. U-123 was chased by norvegian ship trying to ram him. But U-123 could'nt dive because he din't knew the depth under keel? because of poor naval maps. But could'nt uboat just send a ping under the keel and measure it?
I am confused because in sh 3 you can check depth.
Anyone please tell me