DIY: a surfaceable GPS
How to assemble the GPS data logger that has been used for most of the maps
Examples of GPS traces
SS Centennial wreck
Please note: this GPS is a data logger, it records the position every second for subsequent analysis and it cannot provide any real time guidance lacking a display. Be aware that, like any other GPS, it cannot receive satellite data when submerged even by a few cm., hence the importance of being able to surface it on demand, without the burden of dragging a float for the full dive. As matter of fact, its presence is barely noticeable when not in use (it floats above my shoulder) so I carry it with me in every dive just in case I need to record a specific point of interest or simply review my path afterwards. The straight lines in the above pictures represent segments where I was keeping the GPS underwater.
If there is interest in starting the dive at a specific coordinates, such as for dropping onto a wreck, a bigger housing box can be purchased from the same brand to host a smartphone with prerecorded coordinates as described here, then carrying the housing along underwater. The same phone in a box solution can be used for recording points, but if you need to record lines such as a sand line, fferently from the device described on this page, the phone in the box, with its limited buoyancy, tends to immerse as soon as you move.
Things I have learned by using it:
improved range control: I now know that my cruise speed is 25 m/minute (35 when surface swimming), this allows me to plan in advance after how many minutes of swim I should find a wreck and avoid overshooting it if I haven't found it. Along the same empiric line, I can also relate fin kick counting and distance.
navigation error feedback by reviewing the actual route
improved safety: by knowing the range of distances swum in dives, I can plan knowing the level of effort involved and my limits.
better knowledge of the layout of the dive sites by correlating visual memory and position on the map. In the case of Cabbage Tree Bay, by recording the position of multiple sightings of Weedy Seadragons allowed to identify the area where they can be seen. By making use of their map, freedivers are now given a chance to spot them for the first time.
Key element for navigation is the availability of a compass you can always keep the eyes on while swimming, ideally coupled with a depth meter and a watch (for measuring the minutes of swim as described above). You simply cannot keep a precise direction by checking a wrist compass every few seconds.
I have solved by attaching my dive computer and the compass to the hand torch (Tovatec 3600, pretty bulky but it's a good base for this setting). The compass gets some side light from the torch at night and with my thumb I can press the light button on the dive computer, so in one sight I have the visual of the space in front of me, the heading, the depth and the time.