From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Brooke Clarke Sent: Monday, July 27, 2009 7:04 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Position accuracy to improve Thunderbolt performance
Hi Warren: I've found Google Earth to be very accurate, but Google maps not so accurate. You can see the elevation by moving the cursor over the spot. Have Fun, Brooke Clarke http://www.prc68.com WarrenS wrote: > Does anyone know how accurate Google map is for Lat & Lon in California? --- Anytime you're talking a few meters or finer resolution, you need to start asking about things like which geoid model and datum are they using. Your GPS uses WGS84, but google whatever may use something else. (I'm sure it's documented somewhere) There's also the age of the photo/map they're using. My house moves northwest at a few cm a year (being on the pacific plate side of the San Andreas fault), so if the image that Google is using is 5 years old, you're already getting up towards that 1 foot precision in the original post. And, what's the original source map that's used.. For instance, a lot of global or national atlas style online maps are based on the 1:250k USGS maps which according to "National Map Accuracy Standards" are accurate to about 100 meters (that is, a feature on the map is within "one pencil point" of where it should be). I'm pretty sure Google is better than that, but if they're using 1:24000 quads as the base map, for instance, that's still only good to 10 meter-ish (after you've dealt with datums, etc.). So, when they take the imagery and "adjust" it to fit the map, there could be meter scale distortions, and it would still be considered "good enough". This is particularly an issue when there is significant vertical relief, because you've got a flat image that has to be "mapped" onto a 3D surface. You can see this when you span a photo boundary on Google maps.. things like sidewalks don't always perfectly line up. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
