How accurate is Google Earth?

I have a Magellan GPS but, am not using the most accurate datum for my area. At the moment I'm using WGS84 while I need the naparima BWI datum. The BWI datum isn't on the GPS and cannot be downloaded. (I spoke to Magellan already.)

I've been using google earth as a reference to see how far my GPS readings are from the 'actual spot'. Is this a wise move?

Have anyone else compared their GPS readings against google earth? Should I assume google earth is the ultimate in accuracy even with respect to altitude? I see threads in here which uses the ruler function to estimate distance of a flahlight's throw and I guess that's accurate enough since they're measuring distance in an actual picture but, with respect to coordinates what should I think?

Thanks for any help. :thumbsup:

BTW: I have a USER 'datum' on the unit in which I can change values. Is there anyone in here who can help me put in the appropriate parameters for 'USER' to be equivalent to the Naparima BWI datum?

Thank you.

eebowler.

Re: How accurage is Google Earth?

Commenting here without any actual facts...

It would seem to me that Google Earth would not be the ultimate in accuracy.

I base this solely on the following:

* My guess is that the primary purpose of Google Earth is to look good. Absolute accuracy may be secondary when the difference between looking right and actually being right is computationally expensive.

* There is an enormous amount of data involved in digitizing the entire planet, and Google is feeding it to you over the internet so that you can appear to fly from point A to point B in real time. There must be simplifications made to make the amount of data involved managable. I would think that Google wouldn't waste a lot of bandwidth sending around lots of fine-grained altitude information, for example.

I could be wrong. There could be a couple of dedicated folks at Google who sweated all the details just to make sure that when you plopped a marker down on Google Earth that you got the absolute best GPS coordinates possible just so folks like you would be happy. That wouldn't be out of character.

Re: How accurage is Google Earth?

google earth has 2 versions: personal and professional. The professional cost quite a bit of money and it is absolutely accurate.

The personal version is also accurate but it doesn't measure the point of interest down to exact feet as the professional edition. I know this fact from playing with the pro version of a co-worker who works in real estate. He has to measure the exact dimension of a building so he can write up a contract.

For non-business purposes, the personal edition is suffice for most people.

Re: How accurage is Google Earth?

Don't they now have Google Moon and Google Sky?

Re: How accurage is Google Earth?

So I guess if I adjust the parameters in the USER datum to approximate that of specific locations in google earth then, I'll have a relatively accurate datum? (At least more accurate than what I'm using now. According to the coordinates in google earth, my coordinates are approximately 210m off.)

Re: How accurage is Google Earth?

picard said:

google earth has 2 versions: personal and professional. The professional cost quite a bit of money and it is absolutely accurate.

The personal version is also accurate but it doesn't measure the point of interest down to exact feet as the professional edition. I know this fact from playing with the pro version of a co-worker who works in real estate. He has to measure the exact dimension of a building so he can write up a contract.

Click to expand...

How do the measurements from GE pro compare to the actual buliding dimensions? Exactly accurate?
I am asking cuz this is a case where we can compare like wiht like directly.

Re: How accurage is Google Earth?

We were just looking at google earth on my daughters iphone this morning. We looked up our home by address and found it in the photo but the addresses were off by two doors.

Re: How accurage is Google Earth?

Addresses seem to be estimates based on a range and the length of the block... And the start/end don't always seem to be 100% correct in all map data bases...

Regarding the specific datum, have you looked at places like this or that for offset information?

-Bill

I don't have anything to add to this, but just wanted to fix the title spelling of "accurate"...which ironically, was spelled inaccurately.

Edit: Dang it...it didn't fix it...

Re: How accurage is Google Earth?

BB: thank you I came across the first page already and input the dX, dY and dZ values and changed the sign from +ve to -ve since the talbe is for converting local to WGS84. This still gives me 200+ meters off readings from google.

I don't know what to do with the eX, eY and eZ values since the page says 'e' is error estimate in meters while my meter has four other parameters labled Rotation X (sec), Rotation Y (sec), Rotation Z (sec) and Scale factor (ppm).

LOL ttran97

I would believe that Google Earth could be off by 200m pretty easily... GPS technology has quickly eclipsed the accuracy of maps for the general public (typical road/hiking maps, etc.).

And with maps being printed from surveys that can be dozens or even a century old--different maps and sections may show different results. So--it is entirely possible that your "by the numbers" datum conversion is much more accurate than any run of the mill maps you may be able to find.

From what I have read elsewhere, having accurate digital maps available for free (or near free) as done by the US Government is quite rare in the world at large. Even if there are very accurate maps available, and not covered by national security concerns, the government can still charge hefty fees for the accurate data.

Your best bet would be to find a local benchmark (survey point) with the local origin lat/lon stamped on it--or if you can find that data at "city hall" or from a local survey company. Then confirm/adjust your datum to the surveyed point.

While trying to look around for an answer, found that people have setup a moving map display for Google Earth tied with a GPS. Looks pretty neat, and once you have a few "confirmed" waypoints--you will be set to go.

If you are really into photographic and mapping, you might take a look at this all things GPS website. There are a lot of articles/links on how to scan and adjust scanned maps/photos so that they can be accurately used with GPS points on a computer (GPS's generally have proprietary data files and it is difficult to impossible to put your own maps on a hand held GPS).

-Bill

Yep, on my county roads where I live, their street lines don't even fit on the actual photos correctly. They are sometimes off by a block.

Yesterday I tested 3 GPS units, sitting side by side, to see how much variation there was between them. I was pleased to see that they were all very close. When I plugged the coordinates from one of the units into Google Maps the result was approx 4 metres from where they were sitting, pretty good considering we have no WAAS or similar here.

I was using the WGS84 datum, which is very similar (within 10cm) to the current Australian datum GDA.

I've been playing with the personal version and have scanned the area around where I live, accuracy seems pretty good. However it hasn't been updated for 3-4 years or so. I base that on the fact that my dad's pickup is in the driveway and he sold it 3 years ago. Below 1500 feet everything gets very fuzzy, all the detail washes out and when you adjust the horizon level the buildings all look flat and there is no 3D effect there. Probably too high up still. I imagine the pay version is much better though but it's fun to play with.

MARNAV1, that could be your firewall. I use the computer at work. Last year, when I discovered google earth, the mountains were 3D then it stopped and became flat. Now, it's back to 3D. IT is really inconsistent here.

atm: 4 meters! I dream of that!!! I'll make checks with different datums tomorrow and let you guys know what I get.