GPS System

Hi

I'm just trying out new dive sites, using admiralty maps etc i have coordinates to find sites which mught be interesting to our divers.

But....

Does anyone now or have experince with a GPS system that is fairly good or have suggestions that will point me in the right direction.

Many Thanks
Mike Minton.

As a club we did a GPS review for Dive magazine last year.
As a result, we purchased the GPSMAP176 from Garmin.
We also purchased the charts so they can be displayed as we go along.
I have set up a number of routes and waypoints on the PC then downloaded them to the GPS. This is a lot easier than doing it on the GPS it self.

Most of the marks from the Admiralty charts are approximations.
And one wreck in Poole bay is a rock :(

Using the chart display on a RIB whilst moving is a bit hard. So for driving the RIB we use the rolling road display, with things like the current speed to keep us honest in the harbour.

I would say the GPS with charts is briliant for finding sites for drifts. But you do need better marks than provided from the charts for finding wrecks.
3D sonar is next on the wish list:)

Andy

Hi there Andy, just a point regarding GPS and Charts. Please forgive me if you have taken account of the differing datums between charts and GPS, you didn't make it clear in your posting that you did so here goes.

The majority of charts used by divers tend to be old or at least a couple of years old so will have a datum referred to as OSGB (Ordinance survey of Great Britain) whilst only the very latest chart updates have a datum of WGS84 (World Geodetic System 84) which is the universal reference for Global Positioning Systems. To use a GPS successfully when referencing a chart that has a datum of OSGB, you will need to alter the datum within your GPS system to match that of the chart you are using. Don't worry, it's a simple task which is explained in the user handbook. The error created by not changing your GPS datum settiings are in the order of 150 mtrs, so it's hardly surprising you state that you had difficulty in locating marks from the charts you are using. Try it again using the revised settings and your hit rate will improve dramatically. Best of luck

Geoff Oldfield

Hi Geoff,

I was actually talking about the charts that load directly from PC into the GPS plotter which are all WGS84; C-Charts, Garmin BlueChart, etc.
The underlying Admiralty charts are just not accurate enough to put you on top a wreck.

Hi Geoff,

I was actually talking about the charts that load directly from PC into the GPS plotter which are all WGS84; C-Charts, Garmin BlueChart, etc.
The underlying Admiralty charts are just not accurate enough to put you on top a wreck.

The problem with paper charts is measuring off them. You would be doing very well to estimate a position from a paper chart to within 50m of its real position. Even if the wwreck is PD, the chart does not mention that position.

I cannot tell from their website if the Garmin Bluechart data (which is proprietary) is vector or raster data. The difference could be important. If raster, then this is just an electronic picture, lots of dots. This means that even if you zoomed in on the picture, you get no more 'accuracy'.

If the data is vector, it is made up of lines and points. This could be more accurate. Ie. the data could store the actual position of the wreck, and display an image based on this position. This will not be the case if the vector data has been derived from tracing over a raster image.

The UK Hydrographic Office has some info on the charts, but I don't think they come in a format for Garmins et al.

Adrian

:=Hi Geoff,
:=
:=I was actually talking about the charts that load directly from PC into the GPS plotter which are all WGS84; C-Charts, Garmin BlueChart, etc.
:=The underlying Admiralty charts are just not accurate enough to put you on top a wreck.

The problem with paper charts is measuring off them. You would be doing very well to estimate a position from a paper chart to within 50m of its real position. Even if the wwreck is PD, the chart does not mention that position.

I cannot tell from their website if the Garmin Bluechart data (which is proprietary) is vector or raster data. The difference could be important. If raster, then this is just an electronic picture, lots of dots. This means that even if you zoomed in on the picture, you get no more 'accuracy'.

If the data is vector, it is made up of lines and points. This could be more accurate. Ie. the data could store the actual position of the wreck, and display an image based on this position. This will not be the case if the vector data has been derived from tracing over a raster image.

The UK Hydrographic Office has some info on the charts, but I don't think they come in a format for Garmins et al.

The Garmin Bluecharts are vector based. The data appears to be stored as layered db tables similar to other GIS systems. The charts appear visually as very similar to the paper Admiralty charts. The wreck marks appear no more accurate than you can derrive from a paper chart. I guess they plot the original reported position, which was inaccurate, very accurately. Great for yachts, almost useless for divers.

The UKHO do have accurate survey information for some areas which is useful for divers. IIRC They charge about 3 quid a mark. So you could get around 50 accurate numbers for the cost of a set of Bluecharts.

IMVHO

Electronic charts are of limited use to divers. It takes an expensive GPS to update the chart at the 20 knots a RHIB drives at. For journey planning electronic charts have one major flaw, you can't draw on them.

Electronic charts can be useful for teaching purposes as it is easier to relate numbers to landmarks. But you pay a lot for the priveledge.

In a car it is a different story. My GPSMap176C has saved me enough time and money to not worry how much I wasted on the Bluecharts. Now if only people would put Lat and Long next to their post code...!

The Garmin Bluecharts are vector based. The data appears to be stored as layered db tables similar to other GIS systems.

While most GISs use databases, they each have their own spin on internal data model - a nightmare. I have spent far too much time migrating data from one to another. I wonder if OpenGIS will really deliver. And as for those abortions of systems based around CAD...

The charts appear visually as very similar to the paper Admiralty charts. The wreck marks appear no more accurate than you can derrive from a paper chart. I guess they plot the original reported position, which was inaccurate, very accurately. Great for yachts, almost useless for divers.

Agreed, and very few wrecks are PD'ed

The UKHO do have accurate survey information for some areas which is useful for divers. IIRC They charge about 3 quid a mark. So you could get around 50 accurate numbers for the cost of a set of Bluecharts.

IMVHO

Electronic charts are of limited use to divers. It takes an expensive GPS to update the chart at the 20 knots a RHIB drives at. For journey planning electronic charts have one major flaw, you can't draw on them.

Electronic charts can be useful for teaching purposes as it is easier to relate numbers to landmarks. But you pay a lot for the priveledge.

In a car it is a different story. My GPSMap176C has saved me enough time and money to not worry how much I wasted on the Bluecharts. Now if only people would put Lat and Long next to their post code...!

You could buy OS AddressPoint and convert the OS Coords ;-)

I think it is the common use of GPS that is forcing the OS to re-jig the positional accuracy of their data, much to the dismay of organisations who plot their own assets relative to the OS.

Adrian

Now if only people would put Lat and Long next to their post code...!

streetmap.co.uk will convert post codes into lat/longs and national grid refs.

Philip Smith