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Where to get accurate Longitude and Latitude?

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#26 Per Frejvall

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Posted 06 December 2012 - 02:30 PM

Folks,

Do not forget that GPS uses an spheroid model of the earth curvature (WGS84) that bears more resemblance to reality in some areas and less in others. Wehere I live (east coast of Sweden outside of Stockholm) the formal GPS altitude - the correct one - is more than 30 meters off the real world altitude.

I know Garmin, in some models, attempt to correct this with rough elevation models, but most GPS receivers will report "correct" WGS84 spheroid altitude as it is (which is most likely wrong in terms of the real world).

If elevation is of importance, I would download elevation data from the internet and see what it says. Google Earth data seems to be derived from an interpolated elevation grid. This most clearly demonstrated by hovering the cursor over a lake surface... At my place, the water surface is lowest in the middle of our lagoon and slightly higher (a few meters) closer to the shore...

I'm ranting here, but point being: the only good elevation data that you can get is either a local survey from your county land office or a dense elevation grid.

Another point worth noting about GPS accuracy is that the figure reported by Garmin's units is an estimate, nothing else. It does not reflect the actual accuracy.

Survey grade equipment is usually accurate down to two millimeters (phase detection) while normal GPS can achieve 0.3 meters. If you want the really cool stuff, you go for a Real Time Kinematic solution with roving units and local reference stations and do post processing. Then you're well below one millimeter ;)

/per

#27 Raginar

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Posted 07 December 2012 - 12:42 AM

Per,

You can usully change your reference model. For instance, HAE is what we commonly use when we want to be accurate in the military.

He's right about the accuracy of a Garmin; though if you read the article above from the WI DNR, you'll see that it's very accurate. The question is, of course, cost. What are you willing to pay for accuracy? If everyone on your online research team has an iphone, it's probably good enough.

GPS is amazing.

#28 Per Frejvall

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Posted 07 December 2012 - 01:13 AM

Yes Chris, you can change it to whatever is available. I am not familiar with HAE (could be because I'm not in the US).

Garmin's assessment of accuracy is surprisingly accurate but some of their receivers (most notably GPS-17 and anything that has the same chip/firmware combination) can take long hikes at times, sometimes up to 150m, before slowly going back to the correct position. I am coastal so I always "diff" my boat systems with IALA corrections via a separate receiver. That in combination with a Hemisphere GPS module that has a really good atmospheric model gets me to a cool and verified +/- 15cm at 95% of the time.

And on topic, my recommendation for most would be to use the iPhone or other smart phone, and if you need high precision altitude, never trust a GPS; contact your county land office and ask the for a gridded or surveyed altitude for your GPS coordinates.

As was correctly stated above, the position is usually less important than the time. Remember that a one second time error in your mount will set you off by 15 arcseconds on the sky!

Oh, I agree; GPS is amazing, especially if you take into account that it is a 70's design!

All the best from a snowy Sweden,

Per

#29 Raginar

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Posted 07 December 2012 - 08:01 AM

Here is a like to what Height above Elipsoid means:
http://www.esri.com/.../geoid1of3.html

Essentially, it's the native plane of GPS from which it converts to WGS84 or whatever reference system you're using.

#30 Calypte

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Posted 07 December 2012 - 11:34 PM

I think a lot of people are missing the point of the original question...

You are correct. I did! :( I was very surprised one time to compare a position from my GPS receiver with the alleged same position in Google Earth. They differed by maybe 100 meters. It was my first confrontation with the concept of "datums." But, oddly, usually the positions are identical.

#31 Trombone

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Posted 08 December 2012 - 12:32 AM

Raginar: Great link. Fascinating read.

#32 Christopher Erickson

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Posted 08 December 2012 - 12:56 PM

This is a picture of the custom GPS-based timers I have used a number of times for recent Pluto occultation timings. It triggers the frame grabber on the high quantum-efficiency video camera.

Everyone uses the same timer with the same configuration and the operator manually-logs the GPS coordinates displayed by their timer for later data reduction.

Smartphones with GPS chips will simply not work for this application.

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#33 Christopher Erickson

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Posted 08 December 2012 - 01:25 PM

Here is a picture of a different, less-expensive type of GPS-based occultation/transit timer. This one has analog video in/out goes in between a low-lux video camera (Mallincam, Stellacam, Orion Starshoot, Watec WAT-120N, etc.) and an analog video recorder. It adds a time stamp overlay to the bottom of each video frame. It can't work with the special, high quantum-efficiency video cameras because they don't have analog video outputs.

Once again, smartphones with GPS chips can't be used for this application.

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#34 SkipW

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Posted 08 December 2012 - 11:37 PM

Do not forget that GPS uses an spheroid model of the earth curvature (WGS84) that bears more resemblance to reality in some areas and less in others.

Just to clarify one point, WGS84 is an ellipsoidal model, not spherical.

From Wikipedia: "The WGS 84 datum surface is an oblate spheroid (ellipsoid) with major (transverse) radius a = 6378137 m at the equator and flattening f = 1/298.257223563. The polar semi-minor (conjugate) radius b then equals a times (1 - f), or 6356752.3142 m." We use this ellipsoid model daily at work. I haven't checked these numbers for a and b against what we use, from reliable sources, but that .3142 looks familiar.

WGS 84 ellipsoidal height (elevation) does differ in places from Mean Sea Level by up to several dozen meters, so for precision work, especially in elevation, it's important to know what datum you're referencing. If you need centimeter accuracy, from whatever datum, you're probably going to have to hire a surveyor.






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