A warning about new GPS compatable phones that may give incorrect readings when plotted on marine charts. Written by Dudley Jackson 2008

We arrived near the club mooring at Port Levy at 9.30 pm and it was quite dark. There were no navigation lights to take a bearing off only a few house lights dotted around the bay. We knew roughly where we were and the depth gauge gave us some confidence of our position but even with a powerful torch I could not locate the mooring. Not having a chartplotter on board (skipper is poor!)  “I'll get a GPS position from my new Nokia mobile phone,” I stated to my wife confidently, (after all the club handbook has the GPS coordinates listed and after entering these into my phone as a waypoint it would guide me to the exact spot just like a car navigation map). To my horror it said I was 38KM SE of the mooring! I searched frantically through the advanced options and re-checked the waypoint I had entered but could not find any reason for this huge error. I re-positioned the yacht to a clear area andI threw out the anchor. I settled down for the night disappointed with my new phone’s accuracy. When I awoke at sunrise I could see the mooring only a short distance away. What was the problem here?

172 deg 50.1 minutes E

My phone shows my location as 43.645 S 172.828 E which in fact looks like its in north Christchurch. The GPS phone was not faulty, it has no maps stored so it’s not a WGS84 mapping problem, it’s something completely different but very important to any mariner. My phone, like most phones or non marine GPS devices, displays information in Decimal Degrees NOT Degrees & Decimal Minutes, as a hydrographic chart shows.

Google Earth, to confuse you further, has 4 methods of giving you different locations

1 – Decimal Degrees (required by some land-based GPS devices)

2 – Degrees, Minutes, Seconds (for very old charts)

3 – Degrees, Decimal Minutes (modern nautical charts)

4 – Universal Transverse Mercator (another alternative system)

So if you use a GPS with Google maps on your notebook, or GPS on your phone, make sure you have the correct notation displayed!

My next challenge is to download charts & satellite photos onto my phone, then use the GPS function to plot my position. And, as an added advantage, it will not need mobile phone coverage to work since the maps will be stored on the internal 2GB card and be totally free! A cheap easy chart plotter?

Decimal degrees - (from Wikipedia)

Decimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many Geographic Information Systems (GIS), web mapping applications such as Google Maps, and GPS devices. Decimal degrees are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180° each.

Positive latitudes are north of the equator, negative latitudes are south of the equator. Positive longitudes are east of Prime Meridian, negative longitudes are west of the Prime Meridian. Latitude and longitude are usually expressed in that sequence, latitude before longitude.

Dudley Jackson.  Learn to sail on ‘Good Point’ www.learn2sail.co.nz