The jury is out a little on some of these, simply on the practical side I wonder whether you are safer with a pencil and paper however, if you can guarantee the iPad against weather and accident then there are some very useful apps.
Decibel meter – something that could be useful in environmental quality surveys, giving objective data to a subjective method.
Light Meter – a basic assessment of light, just like an SLR / DSLR camera. Could be used in ecological studies as long as the requirement for precise data was not there. More of an estimate, but I have known these being used for fieldwork even for Environmental Systems and Societies internal assessments in the IB Diploma.
Camera – not an app but an in-built tool of immense value, particularly given the integration that is possible for processing and annotating the image or video later.
360 degree panoramas – wonderful for aiding recollection of a location and giving the student a sense of really ‘being there’. You can get the same result using various other apps. Consider using this in combination with the excellent Sphere app.
Clinometer – yes it works and I have tried it out, but trusting a load of iPads to students on a sand dune transect seems risky to me!
Compass – as above – it works just fine, but to trust it in the field is a probable, ‘no’. Some work around a school site is ideal though.
Don’t forget to use the video function and also any simple voice recorder for those passing notes – ideal for when you are driving past something and can’t stop.