Re: Google Earth's geographic grid
Dear Roger, I have been following this thread for some time when questions were raised last year about finding north-south. Like John C, with my own house as the test object, I have used plumb lines, slot-in-a-card methods, and large scale plans. The latter uses our acclaimed Ordnance Survey, but even at the 1:1250 scale the house on the plan is only 12mm long. Against the grid system and correcting for geographic north (convergence) I obtained 9.5 deg west of north (bearing 350.5 deg). Google map gave me 8.5 deg with a printed image length of 25mm, and aligning against the eaves. I did another print out and used the ridge line, and got 9.0 deg. To me, this stresses the point that the way the light and shadows fall can change the appearance and choice of best fit for the protractor. Further enlargement only brings more blurring. As an aside, I am lucky with my Google map - a mere 10km away the area has yet to be photographed at the customary high resolution and one can barely make out the streets, let alone buildings. Pursuing optical methods with an old theodolite on the sun and Polaris (at its transit) I obtained 9.28 and 9.20 deg. I am still working on this method, which requires more practice to eliminate 'operator error'. I am therefore interested in the GPS method and how to obtain the extraordinary precision of 0.1 arc sec. For example, I cannot believe that a single hand held device can be pointed to this accuracy, and what I have seen of the screens of such receivers, the compass effect is crude. Similarly, what base line is used to get the high precision? If we take a building 100 feet long and the corners can only be found to within 10 feet, this is not much better than finding the moss on the north side of a tree. I jest, because if the readings at each end are taken close together in time, then the same cluster of satellites will be in view, then the RELATIVE positions should be found to, say 10, times the accuracy. Even so, this gives a bearing from one end to the other to an order a degree. Incidentally I did some experiments with a very good magnetic compass that could be read to 0.2 deg; with care and the current (website derived) deviation you can achieve better that 0.5 deg. What then, is the secret of such alignment precision, and neglecting survey GPS equipment or differential GPS against precise pre-surveyed locations? Or, am I misunderstanding the point being made, for I do assume that, at worst, the global Google 'grid' converges on the sub-north and south polar points to within a few metres. Doug On Jan 26, 2007, at 20:40, Roger W. Sinnott wrote: At 11:01 AM 1/26/2007 -0500, J. Tallman wrote: I guess I am not entirely willing to automatically accept their output as 100% perfect...and I wonder if anybody on the list has any interesting thoughts or practical experience re: Google Earth and the accuracy of their geographic grid. Jim, I've found the Google Earth grid to be incredibly accurate when compared to a GPS receiver, provided the GPS unit is set to the WGS84 datum. They agree to 0.1 arcsec (about 10 feet), the resolution of my GPS. What is amazing is to look at a Google Earth picture of hilly terrain. The grid lines would be essentially straight if you were looking straight down on them from above, but the Google Earth images were taken from either an aircraft or a satellite that generally viewed any specific spot on a slant. So, you'd expect to see minor distortions whenever a grid line crosses a hill -- and you do! For this same reason, it might not be accurate to measure bearings on a Google Earth image with a protractor. But if you figure out the bearing using the exact latitude and longitude of the end points (trigonometrically), the result should be orders of magnitude better than that measured with a magnetic compass. -- Roger --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Google Earth's geographic grid
Doug, I'm in complete agreement with all the excellent points you make! A GPS-derived bearing can't be accurate to 0.1 arcsecond -- that's absurd. I just meant that the latitudes and longitudes of an accurate GPS fix agree with Google Earth's coordinates (on the WGS84 datum) to roughly that level, in my experience. For several years I've been trying to determine azimuths of distant landmarks as seen from choice (unobstructed) observing locations. So if a neat planetary conjunction, comet, eclipse, thin crescent Moon, etc., is going to be visible low in the sky on a certain date, I can use my azimuth notebook to pick a spot, in advance of traveling there, where a certain landmark (lighthouse, distant cliff, or tall building) will be correctly located to be included in a dramatic photograph of the event. Using spherical trigonometry and the GPS coordinates of the end points, an azimuth calculated using spherical trig should be accurate to better than 0.2 degree or so, since the landmarks are usually at least a half mile away. If you are trying to get the azimuth of the wall of a building, then an astronomical method (such as measuring the Sun's shadow on the wall) should be much better than looking at a fuzzy image on Google Earth -- or the GPS coordinates of the building's corners! You're right about the danger of relying on a printed map, where grid north may differ from true north. This seems to be especially true of plot plans prepared by land surveyors of house lots, at least in the USA. -- Roger At 09:49 AM 1/27/2007 +, Douglas Bateman wrote: Dear Roger, I have been following this thread for some time when questions were raised last year about finding north-south. Like John C, with my own house as the test object, I have used plumb lines, slot-in-a-card methods, and large scale plans. The latter uses our acclaimed Ordnance Survey, but even at the 1:1250 scale the house on the plan is only 12mm long. Against the grid system and correcting for geographic north (convergence) I obtained 9.5 deg west of north (bearing 350.5 deg). Google map gave me 8.5 deg with a printed image length of 25mm, and aligning against the eaves. I did another print out and used the ridge line, and got 9.0 deg. To me, this stresses the point that the way the light and shadows fall can change the appearance and choice of best fit for the protractor. Further enlargement only brings more blurring. As an aside, I am lucky with my Google map - a mere 10km away the area has yet to be photographed at the customary high resolution and one can barely make out the streets, let alone buildings. Pursuing optical methods with an old theodolite on the sun and Polaris (at its transit) I obtained 9.28 and 9.20 deg. I am still working on this method, which requires more practice to eliminate 'operator error'. I am therefore interested in the GPS method and how to obtain the extraordinary precision of 0.1 arc sec. For example, I cannot believe that a single hand held device can be pointed to this accuracy, and what I have seen of the screens of such receivers, the compass effect is crude. Similarly, what base line is used to get the high precision? If we take a building 100 feet long and the corners can only be found to within 10 feet, this is not much better than finding the moss on the north side of a tree. I jest, because if the readings at each end are taken close together in time, then the same cluster of satellites will be in view, then the RELATIVE positions should be found to, say 10, times the accuracy. Even so, this gives a bearing from one end to the other to an order a degree. Incidentally I did some experiments with a very good magnetic compass that could be read to 0.2 deg; with care and the current (website derived) deviation you can achieve better that 0.5 deg. What then, is the secret of such alignment precision, and neglecting survey GPS equipment or differential GPS against precise pre-surveyed locations? Or, am I misunderstanding the point being made, for I do assume that, at worst, the global Google 'grid' converges on the sub-north and south polar points to within a few metres. Doug --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Google Earth's geographic grid
To Jim and anyone who measures wall declinations: Ever since Roger Bailey told us about his idea for getting an approximate wall declination measurement by using a Google satellite image, I have been using a variation of it for measuring walls in distant locations that I can't measure physically. It is also a great way to double check a traditional physical measurement. Here is the technique that I use. I think is more precise and easier: Instead of using the Google geographic grid and a protractor, I can let Delta Cad give me an exact measurement without using paper printouts and a protractor with their inherent lack of precision. Here is the technique that I use. I think is more precise and easier: 1) Import a jpg. picture of the Google satellite image into Delta Cad. 2) Then draw a line along the wall I wish to measure. 3) Click the Edit button then click on the line. 4) Read the angle value of the line on the Edit popup window. 5) Determine the wall's declination from the angle value. Done! I have compared the results of this technique with actual physical measurements of my house and the results were within just one degree of the actual measurements. I think this is pretty good and is probably good enough to design most declining wall sundials using this technique. This can really simplify your life if you are designing a wall dial for a far off location that you can't measure yourself using traditional techniques. I still don't trust my clients to make this measurement themselves using traditional declination measuring techniques! John Jim Tallman wrote: Hello All, I periodically use Google Earth to check rough declination at remote locations. When a Spectra sundial customer contacts me and is unsure about the direction/orientation of their home, I can get a really good idea by looking at their location with Google Earth. By turning on the geographic grid, making a printout of their neighborhood/house, and using a protractor I can usually get pretty close...close enough for what I am doing, since the Spectra will not be physically mounted to a wall, etc...it can simply be rotated a bit to compensate for any small amount of declination error. Sometimes I wonder about just how accurate Google Earth is, especially their mapping of the satellite photos and maps to the coordinate grid. I do not need to split hairs and be perfect - when precision is needed I have remote customers drop a plumb line, mark a shadow, and send me the appropriate time/solar measurement at their location, should they wish to do so. I guess I am not entirely willing to automatically accept their output as 100% perfect...and I wonder if anybody on the list has any interesting thoughts or practical experience re: Google Earth and the accuracy of their geographic grid. Best, Jim Tallman http://www.artisanindustrials.com http://www.spectrasundial.com [EMAIL PROTECTED] --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Google Earth's geographic grid
Jim and John, You know, I have questions about the orientation of the sat maps also. The other day my wife was asking where something was on a hillside in a global sense so she'd recognize that same hillside when we drive the curvy road in a nearby park. So I got out my trusty Leica (nee Leitz) laser rangefinder and found a white rock on that hill lies 650 yards away. Then I shot the rock with my magnetic compass. I'd dialed in the mag dec and checked it against the gnomon on my south-facing wall dial and also against the north star with compensation for the clocking of the star about the pole, and so I know the compass isn't bad. Then I went to google earth and drew a line from where I was on our property to the white rock. I captured that image and imported it into Ashlar-Vellum's Graphite and measured the angle. It was almost exactly 2 degrees in error. I guess the next step is to walk over to the rock with my Garmin and see where it is. Unfortunately, the land is posted and where I'd! be walking would be in clear view of half the neighborhood. So there is another layer of effort before I can walk out there. Or a stealthy midnight sneak? 8-) Oh, wait... Why not just take my trusty Garmin to two different _accessible_ locations and follow the same procedure? Almost the same thing. John -Original Message- From: [EMAIL PROTECTED] To: [EMAIL PROTECTED]; sundial@rrz.uni-koeln.de Sent: Fri, 26 Jan 2007 9:52 AM Subject: RE: Google Earth's geographic grid To Jim and anyone who measures wall declinations: Ever since Roger Bailey told us about his idea for getting an approximate wall declination measurement by using a Google satellite image, I have been using a variation of it for measuring walls in distant locations that I can't measure physically. It is also a great way to double check a traditional physical measurement. Here is the technique that I use. I think is more precise and easier: Instead of using the Google geographic grid and a protractor, I can let Delta Cad give me an exact measurement without using paper printouts and a protractor with their inherent lack of precision. Here is the technique that I use. I think is more precise and easier: 1) Import a jpg. picture of the Google satellite image into Delta Cad. 2) Then draw a line along the wall I wish to measure. 3) Click the Edit button then click on the line. 4) Read the angle value of the line on the Edit popup window. 5) Determine the wall's declination fro! m the angle value. Done! I have compared the results of this technique with actual physical measurements of my house and the results were within just one degree of the actual measurements. I think this is pretty good and is probably good enough to design most declining wall sundials using this technique. This can really simplify your life if you are designing a wall dial for a far off location that you can't measure yourself using traditional techniques. I still don't trust my clients to make this measurement themselves using traditional declination measuring techniques! John Jim Tallman wrote: Hello All, I periodically use Google Earth to check rough declination at remote locations. When a Spectra sundial customer contacts me and is unsure about the direction/orientation of their home, I can get a really good idea by looking at their location with Google Earth. By turning on the geographic grid, making a printout of their neighborhood/house, and using a protractor! I can usually get pretty close...close enough for what I am doing, s ince the Spectra will not be physically mounted to a wall, etc...it can simply be rotated a bit to compensate for any small amount of declination error. Sometimes I wonder about just how accurate Google Earth is, especially their mapping of the satellite photos and maps to the coordinate grid. I do not need to split hairs and be perfect - when precision is needed I have remote customers drop a plumb line, mark a shadow, and send me the appropriate time/solar measurement at their location, should they wish to do so. I guess I am not entirely willing to automatically accept their output as 100% perfect...and I wonder if anybody on the list has any interesting thoughts or practical experience re: Google Earth and the accuracy of their geographic grid. Best, Jim Tallman http://www.artisanindustrials.com http://www.spectrasundial.com [EMAIL PROTECTED] --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial Check out the new AOL. Most comprehensive set of free safety and security tools, free access to millions of high-quality videos from across the web, free AOL Mail and more
Re: Google Earth's geographic grid
At 11:01 AM 1/26/2007 -0500, J. Tallman wrote: I guess I am not entirely willing to automatically accept their output as 100% perfect...and I wonder if anybody on the list has any interesting thoughts or practical experience re: Google Earth and the accuracy of their geographic grid. Jim, I've found the Google Earth grid to be incredibly accurate when compared to a GPS receiver, provided the GPS unit is set to the WGS84 datum. They agree to 0.1 arcsec (about 10 feet), the resolution of my GPS. What is amazing is to look at a Google Earth picture of hilly terrain. The grid lines would be essentially straight if you were looking straight down on them from above, but the Google Earth images were taken from either an aircraft or a satellite that generally viewed any specific spot on a slant. So, you'd expect to see minor distortions whenever a grid line crosses a hill -- and you do! For this same reason, it might not be accurate to measure bearings on a Google Earth image with a protractor. But if you figure out the bearing using the exact latitude and longitude of the end points (trigonometrically), the result should be orders of magnitude better than that measured with a magnetic compass. -- Roger --- https://lists.uni-koeln.de/mailman/listinfo/sundial
