I'm Arthur Endlein (endarthur at mostly everywhere, like gmail and mastodon), and I'm a geologist. My main research areas are applied structural geology, applied geostatistics, GIS, remote sensing, and software development for geosciences. If you are using an Android or iOS phone with compass and accelerometer, this page currently works as an experimental geological compass.
The lower left icon (compass trigger) on the projection locks the measurement when held. This behavior can be inverted by clicking on the upper left icon (open padlock), then the measurement will be locked by default, and released while the trigger icon is pressed. To measure lines instead of planes, click on the upper right icon (planar attitude map symbol).
As said before, this compass is highly experimental, and as so should probably not be relied upon for actual field work. Might be useful as a teaching aid, though. Any questions or suggestions are more than welcome.