GPS for Underwater Sites
With a little bit of effort
and readily available materials, you
can build a system that will take GPS measurments of
underwater sites as accurately as if you took the unit down
and placed it on the site itself, and recorded it's location.
Of course a GPS doesn't work underwater,
since water prevents communication from
the unit's antenna to the GPS satellites. So, the GPS floats on the surface,
in a housing attached to the dive flag, as shown to the right and further below.
The diver records sufficient data to effect the translation of the surface flag
position to the underwater site itself.
Before the dive it is turned on, set to record tracks at 10 second increments, then sealed in the housing.
The flag line has been calibrated with 10 foot marks. When the diver reaches
a point he wishes to map, he allows the flag/GPS time to settle out from surface
conditions, then records depth, deployed line length, time of day, and compass bearing of the flag line with respect to north on his
underwater notebook or slate.
After returning to
land, the GPS track file is dumped to a PC computer using it's interface cable.
The flag/GPS location near the underwater site is determined by
corresponding the diver's time mark with the GPS track of the same time.
to view sample track data, which will show how this works.
Then, using the depth, line length,
compass data, and a little trigonometry, the offset between the GPS system and the diver can be calculated, and
the lat/long of the underwater object determined.
With care, the offset can be
determined to well within the GPS error itself, so the underwater object is mapped
to the accuracy of the GPS.
I've used both the Garmin 48 and the newer Garmin 76 in the system, but any of several units will work
as long as they can store tracks and later download them to a PC.
I prefer to use 10 second intervals,
so the G48 provides 2.8 hours of data collection and the G76 provides 4.4 hours. Track recording can be set
to other increments if desired, but 10 seconds suits the dive profiles, and provides up to
6 data points per site, to average if the seastate is bad and the flag bounces around.
Walmart sells a plastic housing for dry goods such as flour, sugar, and cookies, for about $6.
It's equipped with a rubber lid seal and an over-center lid clamp. Once clamped
shut, it's watertight.
With a volume of about two quarts, it provides additional lift to the flag float. Foam inserts to keep
the GPS from rattling around inside it, and a notch cut into the styofoam float of the
diveflag keeps the housing squared up to the flagpole, so the antenna faces skyward.
A safety lanyard top and bottom keep it attached to the flagpole. Various cave and flag reels
are sold to provide line and takeup for the system, and any will work. The line itself
is marked in 10' segments. We deploy it to the segment length required for the operating depth
and current conditions, stopping on an exact mark so the line length is known. Occasionally we find
we need to put an intermediate mark between two of the 10 foot marks.
Closeup photo of the flag
and housing shows GPS inside
- Pre write the 5 headings on the slate:
Site name Depth Line Angle Time
- Sychronize dive watch to GPS time.
- Turn on GPS, verify it's logging tracks.
- Verify battery life of the GPS.
- Seal the GPS into the housing.
- (optional) Predeploy the flag line.
- Deploy the line to the mark closest to
115% of the depth.
Extreme current may dictate more line, but do not exceed 150%.
- Record depth and line length to nearest foot.
- Record compass heading of the flagline with respect to north to nearest 5 degrees.
- Record time of day to nearest second.