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GPS While Diving

Coming Soon  

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.

underwater operations

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. Click Here 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.

GPS Units.

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.

The Housing.

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.

Flag and Housing

Closeup photo of the flag
and housing shows GPS inside

Predive Operations

  • 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.

Underwater Operations

  • 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.

Materials List


Housed GPS, diveflag, and line.


  • Battery powered GPS capable of
    storing and downloading tracks.
  • Walmart dry goods container with
    rubber sealing clamp lid.
  • Dive flag with styrofoam float.
  • Flag line and takeup reel.
  • Dive watch with a seconds indicator.
  • Depth indicator and a compass.
  • Dive slate or underwater notebook.
  • GPS download cable and a PC computer.
  • Download software for the PC computer.
  • Spreadsheet software (.XLS supplied here)

Download the Track Data

The next step, usually shoreside, is to download the track data from the GPS, then correct the offset errors between where the flag/GPS was and the diver's actual location.

Downloading requires a cable, bundled with the Garmin 76 or sold as an accessory to the other GPS units, and a software package to communicate with the GPS.

There are several of these s/w programs around. I use the shareware generated by A.S. Murphy in the UK. It handles many GPS units, and among other things, dumps the track data to a text file which can be viewed by many PC programs. Click the icon to visit his website to learn details or purchase.

Resolving the Offset

GPS system

Data collected on the slate is used
to compute lat/long corrections.


The underwater data from the slate (depth, line length, compass heading, and time) can now be used to find the flag/GPS location and it's offsets from the underwater site. A corrected Lat/Long will result.

The steps taken are listed here, and are made easier by using the .XLS spreadsheet "divesite_corrections.xls", downloadable below.

  • From the downloaded GPS data, select the
    lat/longs corresponding to the time on site.
  • Correct the depth data to FSW.
  • Using depth and line length, compute
    the surface offset (in feet) from the site.
  • Using compass heading, resolve
    the surface offset to N/S and E/W parts.
  • Convert N/W and E/W offsets in feet
    to minutes of latitude or longitude
  • Correct the GPS track with
    these offsets, to locate the site
  • OR
  • Key the data into the spreadsheet
    below and let it do it!

Click to Download
Microsoft Excel file   (8K)

For Best Results

  • Keep the flagline close to the depth.
    No more than 15% works best.
  • Read compass heading to 5 degrees.
  • Know depth guage units. FFW or FSW?
  • Dwell on the site as long as possible.
    Multiple datapoints help eliminate errors.
  • Take photos of the site and keep a log.
  • Make a sketch of surroundings.
    Which way do the edges drop?
    General heading of the features?

Other than bottom time, there is no limit to the number of sites that can be mapped during a dive. It usually takes around a minute to arrive at a site and write legible data onto the slate.