Finding an old PoV for a new photo

I recently brought a print of an old mage to try and take a new photo from the same point of view.

That worked out OK but far from perfect. I should have been further to the right. (Maybe the old PoV is blocked by the wall of a more recent building.)  See Flickr.

What I need, is a camera that shows the old and not yet taken images overlaid, so I can move my PoV until they fit. There is probably an app for it out there, but I can't find it.

Integration between versions of the same book.

I want to start reading a hardcover book, scan the last page read with my phone, and continue from the same page with an e-book or audio reader.

I also want to scan a paragraph in a translated paper book, and get the original version of that paragraph. Or a translation, if I am struggling with an original version.

For language training, it would be useful to have an audio book that can repeat the previous sentence. Or start from a point I click in the text. (The latter is available in the edX videos: I can have a transcript rolling beside the video, and I can position the video by clicking in the transcript.)

Sea cave length measurement.

Caver Bob maintains a list of long sea caves. I'm looking for instruments to measure the length of sea caves.

So far, I've used a laser to measure two small caves, 33 and 55 m long.

But I think the best solution might be sonar with an active transponder. The master sends a ping, the transponder answers pong - on another frequency, to avoid confusion with all the echoes in a cave.

The master then divides the delay by two, and multiplies with the speed of sound.

This solution should work, even around corners.

A simpler one-frequency set-up could be a conventional depth sounder turned horizontally, and a submerged radar reflector. But I think false echoes would be a problem.

Also see Faroese caves on Facebook and Google Maps.

Google update: JW Fishers make this stuff.

Automatic kayak photography

When the kayaking gets real interesting, I'm busy with my paddle, unable to handle the camera.

I'd like a camera that automatically takes photos when the going gets interesting. In a sea kayak, the triggers might be vertical acceleration, sudden listing, and being upside down. (While when skiing, I guess free fall is interesting...)

Phone with GPS and barometer - I want a Kalman filter

My personal elevation, and air pressure corrected for elevation both change rather slowly.

With a suitable Kalman filter, my phone should be able to use the barometer to cancel some of the GPS elevation noise. I don't know how much noise the barometer has, but it seems low, compared with pressure change when I walk up a hill.

Most important, I want the corrected elevation estimate used to compute my local sea level pressure.

The pressure curve doesn't tell much about the weather, when I suddenly walk home from the office.

Screenshot from Barometer Monitor.

USB memory stick where anyone can add files, but only I can read, modify or delete.

I.e. a stick where adding a file is equivalent to encrypting the file with my public key and mailing it to me.

PFD for Vacuum Flask

I want my thermos to float when I lose it from a kayak. So it needs a Personal Floating Device. Most likely solution is a neoprene cover. That would also reduce the noise when it's rolling around in an otherwise empty cargo compartment.