Equipment Review:
Dr Depth Sea Bottom Mapping Software

A GPS provides highly accurate position information. Your fishfinder provides highly accurate depth information. Put those together and you have the data needed to produce a highly accurate chart, with as much detail as you care to create, as long as you can go and collect enough data! Dr Depth is a simple but clever piece of software to turn this data into a chart, developed by Per Pelin and sold as an on-line download from his web site www.drdepth.se .

There are two ways of using this software: either load it on a laptop and connect the laptop to your GPS system to collect data directly as you move around, or alternatively collect the data on an independent data logger and load the data file onto your laptop and Dr Depth software back home. The data logger can be a simple PDA device or, if you have a suitable GPS/Chartplotter that stores Track data, it can be the GPS itself.

One pre-condition to all this is that you have to have a means of collecting both position and depth data at the same time. If you have a separate fishfinder and chartplotter, it will be more complicated - but not impossible. To collect data from which you can create a two- or three-dimensional bottom map, all you have to do is motor over your selected area in a grid pattern, back and forth, and side to side until you have covered the area to be mapped with your GPS track. You will need to read the Dr Depth manual carefully, and understand the settings and mapping tools available. There are demonstrations and examples provided on the web site and with the software, so you can practice.

I have a Garmin GPSMap 4008 chartplotter, to which I have connected the black box fishfinder module so all the data required is in the one unit. The 4008 collects Track data (speed, time, position and depth) which can be transferred to a SD card and taken away. The frequency of data points can be set on the menu to be from every second, upwards. The capacity of the Track data memory is the limitation - at one second intervals, a 30 minute track used about 20% of the memory so it would have to be a large area of mapping to fill it to the limit.

So far, I have only carried out one trial and that has proved very successful, and actually very simple. Here is what I did to create a detailed chart of Hayling Shoal:

• Set the Garmin menu option to "Fill" (not "Wrap") the data memory
• Set the track time interval to 1 second
• Covered the area to be mapped with my boat track - this can be a little tedious!
• Transferred the User Data to a suitable SD card.
• Converted the Garmin User Data to a format recognised by Dr Depth. In my case I have Garmin HomePort, so I read the User Data into HomePort, then exported Track data in .gpx (GPS eXchange) format.
• Opened Dr Depth, and imported the file.
• Created the map, and played with the options.
• Deleted data where the fishfinder had lost bottom contact momentarily, this created a bizarre spike on the bottom, as Dr Depth interpreted this as zero depth.

Here is a detailed chart of Hayling Shoal. The 3D visualisation is not very exciting, as the bottom variation is only about 6 feet. A map of Utopia with be better!

The wreck of the Flag Theofano at Dean Tail is better, this clearly show the deep scour to the north of the wreck. The southern Cardinal Buoy is positioned just south of the end of the western end of the wreck.

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