Return to Depth

Its been a long three months of COVID-19 lockdown but finally things are slowly returning to normal and Vobster Quay is open once again for pre-booked diving.

During lockdown a list of ideas and things to test and try out with UWIS navigator formed. It has been time to reflect and look for ways to improve how we gather GPS data and relate the positional information to images. The little software app was one of the results and last Thursday found us back in the water.

Agisoft Metashape Pro can be very good at estimating the GPS antenna position relative to camera sensor. Previous results showing 8% variance were promising but there was room for improvement. With the diver tracker unit mounted on the tank the distance between transponder and camera was significant – could this be introducing errors?

Testing, testing

Mounting the tracker unit needed a bracket fabricating. Including a mount for the Alltab tablet seemed sensible.

The Alltab and UWIS tracker mounted on the camera housing. Image courtesy Marcus Blatchford
The Alltab and UWIS tracker mounted on the camera housing. Image courtesy Marcus Blatchford

The position is not perfect; the tracker unit remains offset from the sensor and care needs to be taken when holding the camera. With the tracker close to the diver’s body keeping the transponder in sight of the buoys needs a little care to maintain communication.

Back in the Water

The tablet mounting plate adds buoyancy keeping the tablet neutral so the extra weight was hardly noticeable. The scuttled safety boat was the target and with such a small subject didn’t take long. The boat sits on top of some of very fine silt and care was needed laying out the scale bars.

Laying out the scale bars with care. Image courtesy Marcus Blatchford
Laying out the scale bars with care. Image courtesy Marcus Blatchford
Conditions in Vobster on the day were ideal for underwater photogrammetry. Image courtesy Marcus Blatchford
Conditions in Vobster on the day were ideal for underwater photogrammetry. Image courtesy Marcus Blatchford

Post Processing

With images aligned and markers detected the check bars were added. The variation within local scaling was now 2% – a big improvement over 8% from previous tests. One standalone example represents a very small sample but we are confident the change was worth implementing. Having the tablet for navigation, comms and marking/recording points is a bonus too.

The DEM revealing the depth of water over the site of the old safety boat.
The DEM revealing the depth of water over the site.

Conclusions

The initial findings give us confidence and we will be testing again soon. Its not a massive target but after a long gap between dives returning to the water should always be a gradual workup and keeping task loading and expectations low helps.

As ever we would like to say a big “Thank you” to everyone at Vobster Quay and UWIS as none of this happens without their support, for which we are grateful.

After a long time out of the water it was great to be back. The positive results were the icing on the cake. The smile behind the mask says it all:

Smile behind the mask - happy diver! A big thank you to Marcus Blatchford for buddying up and shooting some behind the scenes images of the day.
Smile behind the mask – happy diver! A big thank you to Marcus Blatchford for buddying up and shooting some behind the scenes images of the day.