C-Map stuff

[david_e]

I have been hanging on buying a cartridge for my plotter until the upgrades are out. Got friendly chandler to ring C-Map and they say new updates out on March 20, anyone heard any different?

Am expecting to pay sub £100 for NT+ Standard+.

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[[2574]]

Sorry, can't answer that one, but another on CMap charts - I'm sure this is a dumb question - but I'm trying to get my head around working GPS on WGS84 datum with UK and France paper charts on OSGB36 and ED50 datum. I can understand that OSGB and ED have to be adjusted on paper charts to agree with GPS positions, but what about an electrnic chart, the CMap? Are all electronic charts redrawn on WGS84 so you do not have to make adjustments on the electrnic chart? Or are you supposed to adjust the Cmap because they are simply a vectored ED50 or OSGB 36? This subject does my head in I'm afraid. Any bright sparks out there who can make it all schoolboy clear?



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[hlb]

Well. When I sit in my boat tied to the pontoon and look at the screen. The pontoon is there by the side of the boat on the telly. Now if telly dont agree with pontoon. Some one has shifted pontoon or GPS is not set right. Does this help!!

<hr width=100% size=1> <font color=blue> Haydn

 

[tome]

Geodesy is quite a subject in itself, but here goes...

The earth looks spherical until you examine it closely when it resembles a potato with lumps all over the place. This makes it difficult to model accurately as a spheroid. You need a spheroid to use any form of trig or maths.

To get round this, you take a spheroid (rugby ball) and adjust its size to fit a local area, rotating and juggling it until it's surface matches the actual earth surface in the area you are interested in. This only works for small areas, which is why there are so many around the world. Each of these is defined by the spheroid (size and proportions of the rugby ball) and a number of offsets and rotations which are the juggling parameters which make the rugby ball fit.

ED50 and OSGB36 are 2 such rugby balls and are only valid in continental Europe and UK respectively. The number denotes the year they were formulated, eg OSGB36 was cracked in 1936.

WGS84 on the other hand is a world-wide rugby ball by necessity as GPS is global. In attempting to make one ball fit the whole earth, there are some big undulations which means that the WGS84 height is often considerably different to more common MSL derived height. For us at sea this doesn't make much difference as height isn't an issue, but if you were using GPS for climbing it would be important.

To add to the complication, there is the question of projection: ie how to represent a section of a sphere on a flat piece of paper. Whatever method you choose will distort the image (just as you cannot peel an orange and lay the peel flat without tearing it). Mostly we use Mercator which is like placing a light bulb in the centre of the earth and wrapping our piece of paper around the equator to form a cylinder, then taking the surface features of the earth as projected by the light bulb onto the cylinder. There are many other projections in use - in America they use a Lambert conical projection and whilst surveying in Borneo we used a tricky little number called rectified skewed orthomorphic (RSO).

This doesn't answer your question re CMap but I'm waiting for phone calls from potential crew so thought I'd bang out a reply meantime...



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