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I'm soon going on a canal boat trip and I wanted to create a timelapse of the boats travel.

I've obviously only got one shot at this, so I wanted to know what is the best method for calculating the right interval to take each frame at. I know the speed limit of the canal and want to create a video that is comfortable to watch (I've seen examples on youtube that are either too fast or too slow).

If you play the video at 30 frames/s, taking one picture/second is a 30x speedup. If your boat sails at 3mph, the time lapse at one picture/second will be a simulated 90mph. How much of a road can you see driving a car at that speed?

Taking the problem by the other end, figure out your "cruise" speed when in a car, divide by the boat speed. This is your "acceptable speedup". This speedup is your timelapse image interval multiplied by the framerate of the playback (20-30fps).

Of course if you want to shoot the whole cruise (say 3 hours=10000s) and make that a 10mn video (600 seconds) then you have to take 12000 pictures, roughly one picture/second but it can be a bit fast.

Keep in mind that the fjord cruise is taken from high above water in a wide fjord, so the feel of the speed is much less and you can use greater speedup factors. This would also be true if you drive your car in a desert (vs. in a forest or downtown). But picturesque waterways are seldom in a very open landscape.

Last, shooting too fast may induce more wear in the camera, and require more storage(*) and/or battery capacity but you can drop frames to slow down the final video. If you are too slow when taking the pictures, you can't fix it later.

(*) Of course for a time lapse, you may set your camera to a lower resolution or lesser JPEG quality.

A 'storyboard' approach will be best to understand the desired end result. Not only the speed of the boat and the distance of the subjects are key, but also your planned camera movements. Items on the foreground (e.g. people in the boat) can be allowed to move fast. The background needs to be perfectly stable, so you will need to carefully align your images. If you use a tripod and do not move the camera you only will need to adjust for the movement of the boat. If you shoot without tripod and/or plan camera movements you need to have sufficient shots to create a smooth transition (using e.g. After Effects).

The time between shots needs to be identical, so you may need additional timing interval tools for this if your camera does not support it. The great aspect of time/hyperlapse movies is that they can easily be 8k or better because you make use of high resolution pictures. If you need to align images due to movement you may lose a significant part of the images (cropping) which will result in lower overall resolution.

You mention a canal boat trip. I live in downtown Amsterdam and see these trips every day. Distance to the canal houses will typically be 100m/300ft or less, the boats will rock and there are lots or moving items nearby (other boats, bikes on the streets etc.). It will be useless to have a tripod on a rocking canal boat, so for best results you may opt for a gyroscope-driven 3-axis gimbal stabilizer (around $100 starting price). The canals in Venice are much smaller so more complex than Amsterdam.

I've created several hyperlapse videos and decided I needed al least 4 shots per second. If there is little movement you simply use half or one-fourth of your shots. If there is significant movement you can use the extra shots to create a smooth path by interpolation. There's no way you can create a smooth time/hyperlapse movie on a canal boat with less shots per second.

To avoid having to buy and carry extra camera gear on vacation trips I've used the continuous shooting option of my camera. This resulted in about 6 shots per second without any camera problems over periods of up to 5 minutes. Just press and hold. By eliminating half or more of the shots you can decide afterwards how fast/smooth you'd like to see your timelapse movie.
But you still need to align these images, which may be time consuming for a rocky boat and fast moving subjects. The aligning and stabilization can be near-perfect but you probably will lose 3/4 of the image. A 20mpix shot sequence will therefore result in not much more than a 4k movie.

To avoid the hassle of selecting and aligning images from multiple thousand shots you may opt to use a 4k video camera for the 'master' and use video software to eliminate as many frames to create a timelapse video.

A professional timelapse/hyperlapse photographer will laugh at this solution, but it may provide exactly what you had in mind without spending several hundred hours to create a perfect video. Use multiple takes of few minutes each, eliminate frames to get the desired speed and combine them in video editing software. Better: use two 4k video cameras: one for the general view and the other to create few-minute close-ups.