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Old 19-01-2009, 04:55   #1
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Google Earth ?

In prepartation for a cruise to Central America I have been checking the few electronic charts that I have against Google Earth to see how accurate they are.

Also, if there isn't a good chart I will make one from Google Earth and mark it up with placemarks and a path using a cruising guide like Rauscher as a reference. I then calibrate and add it to my electronic chart collection.

Often I find discrepencies with the charts and the guide book. When there is a difference, I have been assuming that Google Earth can't be wrong. Is this a good assumption?
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Old 19-01-2009, 04:59   #2
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How exactly do you "use" G/E for piloting? What do you do for depth? Are you comparing the GPS lat lon with the G/E position? Or are you plotting courses through reefs?

Please explain.

This raises the concept of using GE as a basis for chart plotters. No?
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Old 19-01-2009, 05:08   #3
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Most of the time you can see the deep water from GE.

To compare the chart with GE I use Fugawi to overlay the chart onto GE and compare them. To check bearings mentioned in the guide book I create a path and check the bearing with a compass tool on GE.

Yes I use GE to create charts in difficult situations. The pass at Porto Stuck and the entrance to San Pedro are good examples.

I would be nice if there were guide books that used GE pictures rather than drawing their own. They could easily mark them up.
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Old 19-01-2009, 05:16   #4
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Cartographers use aerial photos all the time to create maps and charts, but they always have a step in the process called "ground truthing". This ensures that the physical features being mapped are accurately depicted within the spatial grid of the chart. I don't know if Google Earth has done any ground truthing, so I wouldn't trust it's lat/lons, if I were you. On the other hand, the folks that write the cruising guides usually create their sketch maps and waypoints based on actually being there. I've found Chris Doyle's guides to be more accurate than my CMAP NT digital charts, for example.

In any case, my recommendation is to use GE, and paper or electronic charts in the Caribbean as guides only. Use your eyeballs when entering unfamiliar territory where there's any danger of running aground. I can't tell you how many times I would have come to grief if I'd blindly followed the charts.
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Old 19-01-2009, 05:35   #5
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Quote:
In preparation for a cruise to Central America I have been checking the few electronic charts that I have against Goggle Earth to see how accurate they are.
You will find they add the most value if you just leave them the way they are. Having had training in aerial photography the photo's used have all been "adjusted". They are adjusted to make them "edge match". So like a great many of used cars on the lot they are made to look better than they really are. At this point you can't correct them to any degree. Your attempts to do so will be unyielding.

They have error in scale made worse by the tilt in the camera, the differences in the elevation of the airplane, and the distortion of the lens. For map making the process is called ratioed and rectified. It's expensive and these images are not corrected that way. They use optical adjustments to force fit them together so they look right. This would be the "bigger hammer" approach. There are software tools that can help them or they wouldn't have enough images. Contrary to the brand name they don't have the whole earth. The detail level in the far away and unfrequented places we all seek are the worst coverage. It may be a good thing since it might keep the crowds away.

What you do have that has value is a picture of the land. You can see what is beyond the beach. If you try to use a GPS with them you will be in serious trouble. Having them to look at can add things to your ability. Much as the map on a bus schedule is not a chart it does convey some information. These are not charts and pretending won't make it so.
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Old 19-01-2009, 05:47   #6
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The charts I have used in the Caribbean and the GPS back then was a Trimble and it put me on top of a mountain in Moustique if I recall correctly.

So this means that the charts were off or the GPS was off or both. And then there's the datum which all charts GPS and paper have and GE doesn't

I would think this would introduce small error positions in charts vs G/E?

I have wondered why they don't use satellite data to "draw charts" ie... land /sea boundaries. Aren't we ready for this level of accuracy?
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Old 19-01-2009, 05:58   #7
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So this means that the charts were off or the GPS was off or both. And then there's the datum which all charts GPS and paper have and GE doesn't
According to the GE user's guide it uses WGS-84 datum.

Comparing GE to a "bus schedule" is a bit unfair. I've used GE charts in my local area (Lake Ontario) and found the lat/lon to be exactly the same as any official chart.

As with any chart I would use my eyes when in tight situations but I still believe using GE and a GPS to get to approximately the right lat/lon would be as accurate as any chart.
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Old 19-01-2009, 06:00   #8
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Originally Posted by Pblais View Post
You will find they add the most value if you just leave them the way they are. Having had training in aerial photography the photo's used have all been "adjusted". They are adjusted to make them "edge match". So like a great many of used cars on the lot they are made to look better than they really are. At this point you can't correct them to any degree. Your attempts to do so will be unyielding.

They have error in scale made worse by the tilt in the camera, the differences in the elevation of the airplane, and the distortion of the lens. For map making the process is called ratioed and rectified. It's expensive and these images are not corrected that way. They use optical adjustments to force fit them together so they look right. This would be the "bigger hammer" approach. There are software tools that can help them or they wouldn't have enough images. Contrary to the brand name they don't have the whole earth. The detail level in the far away and unfrequented places we all seek are the worst coverage. It may be a good thing since it might keep the crowds away.

What you do have that has value is a picture of the land. You can see what is beyond the beach. If you try to use a GPS with them you will be in serious trouble. Having them to look at can add things to your ability. Much as the map on a bus schedule is not a chart it does convey some information. These are not charts and pretending won't make it so.
Google Earth is done from a fixed altitude with no angle.
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Old 19-01-2009, 06:11   #9
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QUOTE]The charts I have used in the Caribbean and the GPS back then was a Trimble and it put me on top of a mountain in Moustique if I recall correctly.[/QUOTE]

So how did you get off the mountain top? Must have been a great view though.

Quote:
I have wondered why they don't use satellite data to "draw charts" ir... land /sea boundaries. Aren't we ready for this level of accuracy?
This level of imagery is fine for the CIA watching what you are eating for a picnic lunch but not very good at capturing the entire park and the surrounding villages at the same time. Aiming the camera is only one problem out of many.

Just because they use digital images does not mean they are unlike optical images. They point to one spot with accuracy but they don't collect all the spots around them to the same accuracy. The worse part is the accuracy as you radiate from the center is not consistent.

The process for putting them together is really the hard part. That is not the goal of Google. They really are not going to work that hard. The zooming into a point from space is not really the same as charting a traverse between two points. It is pretty slick to use though. The illusion of zooming is what blows your mind and the accuracy does not need to be very accurate at all because your focus is on the center point. The software has decided up front where the point you want is. The rest is just the show to get you there. The reality is the camera is moving violently in position as you zoom but the optics of the show hide that from you.
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Old 19-01-2009, 06:31   #10
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I disagree, but I don't "know" for sure. We have mapped the moon and mars and can map the earth with radar scanning...I am not talking picture taking and stitching them together. In fact, I suspect that it's all done by the DOD but has not been passed to the private sector.
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Old 19-01-2009, 07:55   #11
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We have mapped the moon and mars and can map the earth with radar scanning...I am not talking picture taking and stitching them together. In fact, I suspect that it's all done by the DOD but has not been passed to the private sector.
I believe the current maps of the world are accurate beyond the level possible from space image sources. We already measure plate tektonics to the sub millimeter level. You can't do that from space and it's never been done on another body in space. We have an istalled GPS system as well. Maps of Mars and the Moon are not even close to the level of accuracy we already have.

The map of the earth is not uniformly accurate. It is too big to measure everything all at the same instant in time. At some level you do like Magellan and just deal with it. He did have the finest navigation charts and tools known to the modern world acquired at extreme expense.

What the map of the earth lacks is all the detail you can't see from the images. It's what makes a nautical chart nautical. You can high level accuracy digital shore outlines on line from USGS.
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Old 19-01-2009, 13:54   #12
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I have been assuming that Google Earth can't be wrong. Is this a good assumption?


Google Earth is the most wonderful bit of software on this computer!

I have the recently outdated Google Earth Plus which allows real time GPS input. This will be made available in the Free version very soon.

Yes, depth can be assessed from Google Earth.

Yes, use it in conjunction with electronic charts and pilot guides. But Google Earth is the accurate one. Just use the data on your charts.

In the Pacific some islands, reefs etc (should I just say ALL islands, reefs etc) on every chart were woefully inaccurate.

It saved my bacon once when going into a reef entrance when the weather was closing out onto a lee shore and night was falling.

When I had a 'disbeliever' on board I was on a mooring in a bay in Tonga where all our charts said all our boats were on a beach. Google Earth showed a boat opn the mooring I was on. I fired up the GPS and the position was exactly on that boat on that mooring! EXACTLY!

If Google wants to take over marine navigation charts they are in a position to do so.

Before leaving port go close up on Google over your whole trip. This saves the photos in the computers cache (up to 2 GB) then when you dont have an internet connection Google Earth will start and use those cached photos. When it asks you if something is wrong and do you want to report it say "No".

Mark

**PS Where your plotter has accurate charts you will use that
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Old 19-01-2009, 14:05   #13
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<snip>I have the recently outdated Google Earth Plus which allows real time GPS input. This will be made available in the Free version very soon.
I think you meant updated, Mark, but given the pace at which all things computer-related evolve, outdated will be the correct word all too soon!

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Old 19-01-2009, 14:10   #14
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I think you meant updated, Mark, but given the pace at which all things computer-related evolve, outdated will be the correct word all too soon!

TaoJones
No, I mean Outdated. Its now Obsolete (like all you older guys!)
Its being disbanded, kaputted, blown up, shoved out to door to the crook pastures etc.

Google Earth Plus which cost $20 per year now goes to some very complicated thingo for commercial users and folks like me get put back to the normal Google Earth.

However they are bringing the GPS facility to the free google earth when that is UPdated


Quote:
----- Original Message ----- From: "Google Help" <
To: <
Sent: Friday, December 05, 2008 9:21 AM
Subject: Re: [#372318141] Other_problem



>
> Specifically, one of the most important features for users of Google Earth Plus is GPS tracking, and we want to assure you that we're working on making this a feature of the Google Earth free version in the near future.
>
>
> Regards,
> The Google Team
>
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Old 19-01-2009, 15:15   #15
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No, I mean Outdated. Its now Obsolete (like all you older guys!)
I'm happy to report that it's actually true - you're only as old as you feel. It's all a state-of-mind thing, I think, and since I've always been athletic, never smoked anything legal (and nothing in more than twenty years), never drank to excess, always used my seat belt, and never gotten involved with women of "questionable character," I don't feel a day over 30! (Well, most of the time, anyway. ) Actuarially, the statistics indicate that I could easily live well into my nineties, like my Pa (91), and his Pa (97).

My goal has become that expressed in The Tibetan Book of the Dead, a classic; i.e. A GD WNR. I pronounce that "A good winner," and those letters stand for A Graceful Death with No Regrets.

Of course, when I was younger and much more cynical, I went with the always reliable LABATYD. I pronounced that one "Lab-a-tide," and those letters stand for Life's a Bitch, and Then You Die. Heh!

TaoJones

PS: Sorry for my misunderstanding of your out- vs up- intent, Mark.
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