To add to this there are comerical services that appear to claim 50 cm accuracy from their own imaging process.    http://www.photosat.ca/   My understanding is that getting this level of accuracy also requires having pre surved targets near the items you wish to locate.      http://www.photosat.ca/pdf/photosat_ground_control_laying_targets.pdf   For the effort involved in pre surveying your targets you could just survey the location of your GPS antenna.    That being said I suppose if you had a clearly identifiable pre surveyed target near your GPS antenna that was visible on google earth you might be able to figure the position of your GPS antenna.         Message: 4 Date: Thu, 2 May 2013 08:19:17 -0700 From: Chris Albertson <albertson.chris@gmail.com> To: Discussion of precise time and frequency measurement     <time-nuts@febo.com> Subject: Re: [time-nuts] Precise positions for GPSDOs Message-ID:     <CABbxVHuE1zSzkdVHkk+VjXMsGzS-VvkvYHw1CsNuhnfXYkedxw@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Google maps is NOT that good, it can be off by a lot, tens of meters. I had to have my property line surveyed some years ago to get a city building permit. So now I have two brass markers at know position. The survey crew used traditional transits from a brass benchmark. Google Earth thinks these brass markers are a few meters from here the survey crew said. (Yes I know about WGS84, we are all working in that system) I think the problem is that the lland is not flat here.   If I lived in Kanas the Google system might work.   But I don't think Google warps the images to account for hills and even slopes.  I don't know the source of Google's error.  The 1 Sigma on the self survey is about .5 meters more or less. I think the best why to measure is to let the self survey run for a full 24  hours so you get two full orbital periods of each satellite. And also to  make sure you have 360 degree view of the sky.    I think a view in only one direction might be biased. But yu can check Google.  Find a few brass government benchmarks near your house and have Google locate them and if you got a match go with Google On Thu, May 2, 2013 at 2:29 AM, Stewart Cobb <stewart.cobb@gmail.com> wrote: > A GPSDO typically makes the assumption that the position of its antenna is > fixed and well-known. That removes position uncertainty from the navigation > equations, and allows all the "information" from the satellite measurements > to be used to improve the time estimate. Errors in this position create > errors in timing, with a magnitude scaled by the speed of light (one ns per > foot, three ns per meter). > > Most GPSDOs do some sort of position averaging when they are first turned > on, to come up with a good-enough estimate of antenna position. For a true > time-nut, that might not be good enough. > > GPS surveying equipment can easily determine the position of your antenna > to within a few centimeters (~20 ps). Unfortunately, such equipment is > expensive and difficult to borrow. > > A high-end GPSDO designed today should have the ability to record phase > data into RINEX files, which could be sent to a service like OPUS to find > the antenna position. > > <http://www.ngs.noaa.gov/opus/> > > But few do, so far. > > The next best idea is to locate your antenna on Google Maps. Type in the > self-surveyed position to the Google search box, either as decimal degrees > or as DMS, formatted like this but without the quote marks: > > "37.384542, -122.005526" > > "37 23 4.35, -122 0 19.89" > > Click on the map and zoom in. Click on the "Map" box in the upper right and > uncheck the "45 degree view" icon. Then right-click on the spot on the > picture where your antenna is actually located, and select "What's here?" > from the pop-up menu. A green arrow marker will appear, pointing to your > antenna. Left-click on the arrow, and read your latitude and longitude in > both formats. Enter one of them into your GPSDO, replacing the self-survey, > and enjoy increased accuracy. > > A true time-nut will take one more step to improve accuracy. (Sorry, but > the rest of this is specific to North America. Similar details apply to > other parts of the world, but I only know the recipe for the place I live.) > > Google Maps photos are registered (quite accurately) to the North American > Datum "NAD83". Unfortunately, your GPSDO operates in a different datum > known variously as WGS84, ITRF, or IGS (these are all essentially the > same). The difference between these two datums can be a couple of meters, > easily visible on the map photos and worth 5 ns or more of time error. > Fortunately, you can convert NAD83 to ITRF2008 at this website: > > <http://www.geod.nrcan.gc.ca/apps/tmobs/tmobs_e.php> > > For "ITRF epoch", just enter today's date. For "ellipsoidal height", use > the value from your self-survey if you don't have a better one. You might > be able to get a better one from Google Earth, or by finding a nearby > benchmark from this site (US only) and extrapolating to your antenna > location. > > <http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl> > > Note that the WGS84 ellipsoid is tens of meters higher than sea level > through most of North America, so if you live near the ocean, your > "ellipsoidal height" will probably be negative. > > Hope someone find this useful. > > Cheers! > --Stu > _______________________________________________ > 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. -- Chris Albertson Redondo Beach, California ------------------------------ _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts End of time-nuts Digest, Vol 106, Issue 10 ******************************************