Google Earth

J

jhayes1068

New member
Last week I was courious about how detailed the Google Earth free download program would be, as it relates to looking at the Bellingham to Skagway route. I started by typing Bellingham in the search box and when Bellingham came up, and to my surprise, by using the controls on the right side of the program and scroll on my mouse, I was able to fly the entire route to Skagway. You can look at it from just about any altitude and as I progressed up the route past Port Hardy, I noticed in the lower right hand corner of the display that it was also giving me the GPS co-ordinates as I went along. My question is: has anyone tried to input the GPS co-ordinates from Google Earth into a nav system as way points? I know there are cards and chips available for that purpose, but, I was interested in this possibility, as well. I was impressed with all of the details of the program: showing islands etc. Jim
 
Most chartplotting software lets you enter waypoints. Fugawi, for instance, is software I've used that let's you enter waypoints, along with descriptive information like phone numbers, text descriptions of marinas, etc. I've used GoogleEarth to help calibrate rasterized charts, identify rest stops on the interstate, etc.
 
I love Google Earth. If I'm not sleeping at 2am I'm usually on this site or virtually cruising the planet. I finally got KY Fish & Wildlife to take it seriously as a means of presenting some of their map data. Google Earth can be a serious spatial tool.

All that said, I'd never consider using raw Google Earth coordinate data as a source for navigation on the water. The spatial relationship between the 3D model and their sources of satellite or aerial imagery, as well as vector data such as roads and boundaries can vary widely. I was going to show an example of Wrangle, AK where all the roads were in the water and the water was quite the roller-coaster; the 3D model and imagery were nearly a mile off - but they have done a major correction. It's still poor registration between the data layers, but it's better.

I don't know of any accuracy standard that Google Earth is using to present the data they get from some pretty diverse sources. Oregon looks real good, but that's probably due to Cy Smith's crew at the state GIS. Other states have provided ortho-rectified imagery that fits well together but I have no idea what data drives the coordinates you get when you add a Placemark.

So, I might do some testing with Google Earth 'waypoints' in areas that are visited frequently and see how they correspond with your GPS. If you develop a level of comfort after such testing, go for it. But I can't recommend using the raw coordinate data from Google Earth untested.

As a side issue, Google Earth's relatively poor address matching scheme makes land navigation a chore; in my experience MapQuest addresses are typically more spatially accurate. GE's terrestrial imagery helps out a lot in urban areas if you're looking for a business with a sign out front.

T
 
Hi Smitty and Tom. That's what I'm wondering; are the co-ordinates as accurate as a nav unit? If I get the boat and make that trip, I'm going to take my laptop just as a check against the nav unit just to see how accurate it is on water. I know it's sure nice to see the trip on Google and get some idea about the route and possible short cuts off the main shipping routes. Jim
 
jhayes1068":2im5jm7p said:
Hi Smitty and Tom. That's what I'm wondering; are the co-ordinates as accurate as a nav unit? If I get the boat and make that trip, I'm going to take my laptop just as a check against the nav unit just to see how accurate it is on water. I know it's sure nice to see the trip on Google and get some idea about the route and possible short cuts off the main shipping routes. Jim
Click to expand...

You'll find that laptop SO useful on the boat; more than just Google Earth, although that is a good usage... detailed weather, staying in touch (SKYPE is cheap if you are out of the US and can get wifi), chart downloads, photo downloads and storage, entertainment, writing, trip planning. It is in integral part of our boat equipment.

Best wishes,
Jim B.
 
At my home the Google Earth lat/long is about 100 feet off, from the observed GPS data.--not bad. I don't think that Google Earth is close enough for critical navigation, but it is a great tool. The address look up is over 900 feet off from where the house actually is.

Places like Mexico and Central America--as well as the inland passage, where photos or charts may not tell the whole story it is a great tool. Of course you have to be able to down load it when you are under way--which is a challanage. Certainlly printing out entrances to places would be helpful--as are the photos in the Garmin Vision series (but I haven't really found them useful personally).

Here is another fun link, using street level photos, and Google maps:
http://www.vpike.com/ Again these are based on addresses, so may be off, but is a great time waster seeing where you have lived in the past--or places where you want to visit---if they have been photographed.
 
my house you have to turn 180 to see my house.

vpike is just a link thru google earth. they drove behind my house but not around the front and it looks like a day we were on the boat. its not in front of the camper
 
The literate cat burglar is going to find Google Earth a major business tool...
And car repo guys...
And the neighborhood busybody...
And zoning inspectors...
A fun tool...

denny-o
 
Thanks for the v-pike link Bob, it looks like it can be a lot of fun to mess with. I don't think I'd use Earth for navigation, however, it might be fun to check it against the nav system from time to time to see if it was reliable to the point that you could use it to get in the ball park if the nav unit quit and the charts blew away. :crook Thanks, Jim
 
Google earth has a useful guide for importing your waypoints from your GPS. The link can be found under the Google Earth Help guide at
http://earth.google.com/support/bin/sta ... ide_toc.cs
Look under "Importing your data".

Note that Google uses the WGS84 datum. If your GPS allows different datum then you should make sure that you have it set to match WGS84. If your GPS datum differs from WGS84 you will see offsets.

regards,

Karl
 
Good note on the Datum used. Make sure that matches any map or device you reference using Google Earth coordinates. The shift can be a few feet or a few hundred miles away if you are not careful.

There are a number topics to cover here and I will try to clarify a few things.

Resolution
is a measure how small a reading is subdivided. If a digital reading has 5 digits, then it has a resolution of one part in 100,000 (00000 to 99999). A reading that has a resolution of 5 digits does not mean that it is precise or accurate to that value. In fact, all it means is that the display you are reading has 5 decimal numbers showing, nothing else. Precision can never be better than resolution but it is usually a lot worse.

If you set Google Earth to Decimal Degrees it will give you 6 decimal places for a resolution of .36 feet at 50 degrees latitude north which is being conservative.


Precision
is the measure of the repeatability of your reading. If you take 100 readings of the same measurement, there will be a variation in the readings. The smaller the variation or range of readings, the better the precision. Accuracy is not related to precision except that precision will put an upper limit on accuracy.

This is an unknown factor with Google Earth but is likely close to basic industry standards at about 2 centimeters.


Accuracy
is the variation between the measured readings and the true value of the measurement. For example, I can place a location pin on a map and measure the Lat/Long coordinates 10 times. The readings could be within a thousandth of a degree. But the accuracy of the reading will be dependant on the interpretation of where the pin should be placed, the accuracy of the reference point to which the reading is compared and the accuracy of the measuring device. I may place the pin in a spot and be able to take readings that are precise to 10 metres but maybe I placed the pin 1km from the true location of the feature or maybe the measuring device is not calibrated and is 10% in error. The accuracy in that case would only be at best +/-1km. Accuracy can be no better than precision but it can often be a lot worse. Human and interface factors also play a large role in accuracy interpretation. Unless you understand the underlying errors in the measurements, your reading can often be misleading.


There is variability in the horizontal control of Image data in all map viewers including Google earth. This variability is also a moving target as you pan your way across the map using different source data and image sets. How accurately geo-referenced was the image set you are viewing as you move the cursor to that point of interest and read the coordinates displayed? A highly accurate local image set for government use will have a horizontal accuracy of about 6 to 18 inches but for regional sets you could expect as much as 15-30ft of error to start with. couple that with errors in interpretation and interaction you could easily see movements in the 50ft range. It depends greatly on the source.

As for address searches and matching, there are many factors at play. Addresses are located using a process called geocoding. This is an address matching method using roadway addressing directions and ranges that are highly variable in currency and accuracy. When you search for an address, it uses each portion of the address piece by piece to narrow the result until it lands on your street. Once on the street, it interprets the range on that street and divides the road segment into equal segments to give each number in the range an equal length. If the range is perfectly accurate from start to finish and all properties have the same amount of street frontage, the match will look perfect and place a dot right in front of your property. In real life, ranges have an element of "fluff" at the ends to allow matching for emergency services with less demand for constant maintenance. They would rather get to the right street that not have any match at all. That is why you normally have a match to the right street but the dot placement on that street may fall down at one end or another. In a county of about 750,000 it takes about 3 full time staff to simply maintain addressing for reasonably accurate match results.

The data used in free online search tools is the cheapest, oldest, and least precise available. When you buy a new GPS unit that includes mapping from a company like Garmin, you are paying also for an upgrade to a more current set of road addressing data that has been more carefully collected/assembled and updated for better results. There is a slowly growing trend to move away from road centerline addressing to a point-on-building approach but it will take years to see the benefits on a wide scale.

Google Earth is a great tool and fun to use as well. The coordinates pulled from it could be used for reference to find a neat spot with your GPS but of course are no substitue for proper charts. I would say it's largest strength is providing perspective for planning and trip review purposes.

Greg
 
Excellent points Greg. Extremely important to use the correct Datum (which I did in checking the actual GPS vs the Google Earth. Also GPS still has some error--If I keep a unit running for 24 hours, logging the positons occasionally there will be a reading which is 50 or more feet off.

Our county is on GIS maping protocol, which is very accurate, as they have overlaid aerial photos--and this is is more accurate than the Google Eath--at least in our neighborhood.

Interesting point about the Garmin--but even our latest Nuvi (late 2009) is the same distance off as Google earth's address. (and the same point).

Another issue, which I am not sure of is the projection. There is a difference between projection and datum. "While a projection is used in mapping to define the earth on a flat surface, a datum is used to describe the actual shape of the earth in mathematical terms." This is from Google's web site on using google earth for mapping.

Google Earth uses: Simple Cylindrical projection, where as NOAA charts use Mercator Projection.

I also think of resolution in two ways--one as Greg mentioned, the other is the actual photographic resolution. Some areas are very good--for example in the one over my house, I can determine what season, where the sun is, what vehicles are on the street, and in my drive, as well as the boats at the dock. Other areas of the world have very poor photographic resolution--some may be dumbed down intentinally (military bases)--others because a photo is used which covers many hundreds of acres or even square miles. There may also be some "stitching" errors.
 
You will nearly always get a few outlying points with poor accuracy using GPS. There are many sources of interference and blockage but thankfully the water is a great place to distance yourself from most of them. Second by second they are often sketchy tools but when you string a few positions together it starts to be useful.

You will find errors in the best mapping available because nobody can keep up with reality. It is likely that Google and Garmin are both using the addressing data from your County so the results are basically limited by the accuracy of that data. Your County would be the actual source of the error. For small local governments with even less maintenance of addressing data, a company like Garmin will do field checking to improve the data but in a more devoloped area, they mostly use what is provided assuming it is relatively accurate.

The difference in projections used between Google and Charts is for representation and of no matter for our purposes for coordinate reference as long as you have the position format and datum matching.

The resolution you speak of goes from 2inches to 10 meters or more depending on where you look. More people usually equals more money and better imagery in developed countries. Detailed images are flown by aircraft but many of the more coarse images are actually high resolution satellite images.
 
Karl and Greg, thanks for the link to Google support and also the discussion about the different methods of obtaining data by Google, Garmin and others. It's interesting reading and makes me wonder how we ever found our way before the invention of GPS and electronics. :D Regards, Jim
 
Thanks for the Active Captain link. It looks like it will also be interesting. I have more material to look at and read, than I can get to in a couple of weeks. But when you're trying to learn about a new segment of boating, it's a wonderful thing!! :D Jim
 
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