Re: [time-nuts] question about Thunderbolt geo acuracy
On 11/05/2012 00:44, b...@lysator.liu.se wrote: Hi Tim, The answer is NO. Even though decent accuracy can be had with long averaging. It was discussed a few years ago on this list. -- Björn Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. Thanks Tim ___ 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. ___ 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. Well that's disappointing! I need to measure the height of my house floor to be above the flood plane contour. I might have a look at some dted from work. Might have to pay a real surveyor to measure the height datum. Thanks for all the info though guys Tim ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
Go to your local building and planning commission, and get yourself a copy of the topographical map for your address. They are cheap, and are the standard by which everyone (insurance, zoning, ...) determines your flood plane exposure. -Chuck Harris ... Well that's disappointing! I need to measure the height of my house floor to be above the flood plane contour. I might have a look at some dted from work. Might have to pay a real surveyor to measure the height datum. Thanks for all the info though guys Tim ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On 5/11/12 5:23 AM, swingbyte wrote: s disappointing! I need to measure the height of my house floor to be above the flood plane contour. I might have a look at some dted from work. Might have to pay a real surveyor to measure the height datum. Thanks for all the info though guys for that, you need a real surveyor who can provide a legally accepted measurement. Someone who can a) know from the flood level definition what vertical datum they are using (probably NOT something normal in the geodesy world) b) knows the legalities of establishing the difference The mechanics of surveying (leveling in this case) are straightforward to learn. The legalities and local practices in documentation are not. This is what getting a Land Surveyor's license is all about. There's also a question of what the legal height of your house is, relative to the property (from a flood insurance standpoint). They might have some arbitrary offset in the rules. Sort of like how baseline electrical power consumption is actually about 2/3 of the expected minimum consumption in the area for a given size house and appliances (e.g. nobody is likely to consume less than baseline) There are some mortgage servicers, by the way, who take property addresses that have been geolocated and FEMA flood plain definition maps to determine whether you definitely don't, definitely do, or just might need flood insurance. The maps change (as does the geolocation). From what I understand, about 3-5% of the properties scanned require some sort of manual intervention (maybe the address doesn't geolocate, or it's right on the line, or) ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On 5/11/12 5:54 AM, Chuck Harris wrote: Go to your local building and planning commission, and get yourself a copy of the topographical map for your address. They are cheap, and are the standard by which everyone (insurance, zoning, ...) determines your flood plane exposure. I have been informed (in the last 5 minutes) that whether you are in a flood plain, these days, are determined almost entirely by the geographic position of your property on the FEMA flood plain map. (at least as far as lenders and HO insurance goes) FEMAs maps may or may not align with USGS maps. They almost certainly do NOT align with the county recorder's maps. If you're in an area where FEMA doesn't issue maps then it's something else, and USGS or local maps may determine. But I notice from the FEMA Flood Map server that they cover even things up in the mountains (e.g. Alpine county in California) ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
The FEMA maps didn't exist the last time I did this. I would think it likely that the building and planning commission office for his area would have the appropriate maps, as establishing that the proposed house's location is outside of the the 100 year flood plane, is a necessary check mark in getting a building permit. -Chuck Harris Jim Lux wrote: On 5/11/12 5:54 AM, Chuck Harris wrote: Go to your local building and planning commission, and get yourself a copy of the topographical map for your address. They are cheap, and are the standard by which everyone (insurance, zoning, ...) determines your flood plane exposure. I have been informed (in the last 5 minutes) that whether you are in a flood plain, these days, are determined almost entirely by the geographic position of your property on the FEMA flood plain map. (at least as far as lenders and HO insurance goes) FEMAs maps may or may not align with USGS maps. They almost certainly do NOT align with the county recorder's maps. If you're in an area where FEMA doesn't issue maps then it's something else, and USGS or local maps may determine. But I notice from the FEMA Flood Map server that they cover even things up in the mountains (e.g. Alpine county in California) ___ 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. ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On 5/11/2012 6:46 AM, Jim Lux wrote: On 5/11/12 5:23 AM, swingbyte wrote: s disappointing! I need to measure the height of my house floor to be above the flood plane contour. I might have a look at some dted from work. Might have to pay a real surveyor to measure the height datum. Thanks for all the info though guys for that, you need a real surveyor who can provide a legally accepted measurement. Someone who can a) know from the flood level definition what vertical datum they are using (probably NOT something normal in the geodesy world) b) knows the legalities of establishing the difference The mechanics of surveying (leveling in this case) are straightforward to learn. The legalities and local practices in documentation are not. This is what getting a Land Surveyor's license is all about. There's also a question of what the legal height of your house is, relative to the property (from a flood insurance standpoint). They might have some arbitrary offset in the rules. Sort of like how baseline electrical power consumption is actually about 2/3 of the expected minimum consumption in the area for a given size house and appliances (e.g. nobody is likely to consume less than baseline) There are some mortgage servicers, by the way, who take property addresses that have been geolocated and FEMA flood plain definition maps to determine whether you definitely don't, definitely do, or just might need flood insurance. The maps change (as does the geolocation). From what I understand, about 3-5% of the properties scanned require some sort of manual intervention (maybe the address doesn't geolocate, or it's right on the line, or) ___ 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. Actually, The percentage can be higher. The scale of the FEMA flood panels are usually around 1=2000. Some of the older panels were 1=4000. The newest panels can be around 1=1000 (approx 5 to the section). Horizontal scale is not the problem, it's the vertical scale. Also how the stream bed profile was established (surveyed). There can be a lot of change in the real world compared to was gets plotted on the panel and in the profile. When there is an obvious discrepancy between the two (mapped profile and real world) a registered surveyor or engineer must be called in to reconcile the difference. The cost for doing this might seem high, but when compared to the cost of flood insurance paid over the life of a mortgage, it's very cheep. Just my 2 cents worth. . . Randy Hunt, retired Engineering Technician, Flood Plain Administrator (32years) ___ 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.
[time-nuts] question about Thunderbolt geo acuracy
Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. Thanks Tim ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
How accurate do you need your height? Remember that height is the least accurate of GPS parameters due to the fact that you rarely have a GPS satellite directly overhead. Rob Kimberley -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of swingbyte Sent: 10 May 2012 13:50 To: Discussion of precise time and frequency measurement Subject: [time-nuts] question about Thunderbolt geo acuracy Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. Thanks Tim ___ 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. ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On 05/10/2012 02:50 PM, swingbyte wrote: Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. There are many sides to this issue. You will most definitely be best served by a choke-ring or similar antenna that suppresses multi-path reflections. In addition to that, you would want Lady Heather to do a 24 hour position averaging. This should give you an OK solution, but really not the best achievable. Accurate height data is complex, since besides the receiver and antenna issues, height data has more uncertainty than longitude and latitude measures, and also since even if precise WGS84 height is achieved, you would need to correct it to your datum, your sea-level etc. You would also like to have better ionspheric correction than a plain GPS solution gives you, but the Thunderbolt does not give you direct support for such corrections. Exactly how much effort you need to do depends on how accurate you need it, +/- 10 m, 1 m, 1 dm, 1 cm or 1 mm. Cheers, Magnus ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
Not being able to receive signals from GPS satellites anywhere below the horizon is an even larger problem for vertical accuracy. On Thu, 10 May 2012 13:59:51 +0100, Rob Kimberley robkimber...@btinternet.com wrote: How accurate do you need your height? Remember that height is the least accurate of GPS parameters due to the fact that you rarely have a GPS satellite directly overhead. Rob Kimberley -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of swingbyte Sent: 10 May 2012 13:50 To: Discussion of precise time and frequency measurement Subject: [time-nuts] question about Thunderbolt geo acuracy Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. Tim ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On Thu, 10 May 2012 22:50:15 +1000 swingbyte swingb...@exemail.com.au wrote: Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. How fast do you need it? One project i'm involved with uses a LEA6-T with its phase data output and averaging over several hours to get x/y resolutions in the 2-4mm range. I'm quite sure you can do something similar with altitude as well. Attila Kinali -- The trouble with you, Shev, is you don't say anything until you've saved up a whole truckload of damned heavy brick arguments and then you dump them all out and never look at the bleeding body mangled beneath the heap -- Tirin, The Dispossessed, U. Le Guin ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On 5/10/12 6:08 AM, Magnus Danielson wrote: On 05/10/2012 02:50 PM, swingbyte wrote: Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. There are many sides to this issue. You will most definitely be best served by a choke-ring or similar antenna that suppresses multi-path reflections. In addition to that, you would want Lady Heather to do a 24 hour position averaging. This should give you an OK solution, but really not the best achievable. Accurate height data is complex, since besides the receiver and antenna issues, height data has more uncertainty than longitude and latitude measures, and also since even if precise WGS84 height is achieved, you would need to correct it to your datum, your sea-level etc. You would also like to have better ionspheric correction than a plain GPS solution gives you, but the Thunderbolt does not give you direct support for such corrections. Exactly how much effort you need to do depends on how accurate you need it, +/- 10 m, 1 m, 1 dm, 1 cm or 1 mm. If you can get RINEX format files, you can post process them through GIPSY at JPL and get higher precision, using post determined ionospheric and other corrections. My friends in the GPS world say that getting to 1 meter absolute position is fairly straightforward but once you start getting finer than that, all the various factors start ganging up on you: ionosphere, solid earth tides, multipath, phase center shifts, etc.etc. Likewise, getting 1mm + 1 ppm of separation distance sorts of uncertainty in a differential measurement is fairly straightforward. ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
A man with only one GPS Surveys from different receivers I have. All taken at the same height from prolonged surveys. WGS84 datum. Oncore UT+ A 207,62m Oncore UT+ B 209,24m Z3801A 180,72m Oncore VP A 229,95m TBolt 207.00m Le 10/05/2012 14:50, swingbyte a écrit : Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. Thanks Tim ___ 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. ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On 5/10/12 6:42 AM, mike cook wrote: A man with only one GPS Surveys from different receivers I have. All taken at the same height from prolonged surveys. WGS84 datum. Oncore UT+ A 207,62m Oncore UT+ B 209,24m Z3801A 180,72m Oncore VP A 229,95m TBolt 207.00m That's a pretty big variation (10s of meters), a lot more than I'd expect (I'd expect variations more like the difference between the two UT+s and the Tbolt). I wonder what about the VP and Z3801 fixes pushes them so far away. ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
Le 10/05/2012 15:51, Jim Lux a écrit : On 5/10/12 6:42 AM, mike cook wrote: A man with only one GPS Surveys from different receivers I have. All taken at the same height from prolonged surveys. WGS84 datum. Oncore UT+ A 207,62m Oncore UT+ B 209,24m Z3801A 180,72m Oncore VP A 229,95m TBolt 207.00m That's a pretty big variation (10s of meters), a lot more than I'd expect (I'd expect variations more like the difference between the two UT+s and the Tbolt). I wonder what about the VP and Z3801 fixes pushes them so far away. May have an explanation for the Z3801A fix. I have just seen in my notes that the Z3801A was displaying MSL and not WGS84 . ___ 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. ___ 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.
[time-nuts] question about Thunderbolt geo acuracy
I've found significant altitude errors using a GPS and the following quotes found on the internet will explain why. From my experience of hiking in the mountains of New Hampshire an aneroid altimeter will vary with atmospheric pressure about 200 feet for a change of 0.2 of mercury so you have to continually set it at known waypoints, just like setting a frequency standard against a known reference, and then it will be 'accurate' for some length of time, and then you set it again. GPS altitude will be off but it should be fairly consistent in spite of the changing atmospheric pressure. The earth neither spins at a constant rate nor is it a perfect sphere. Maybe we need to trade it in for a newer model. ;-) GPS altitude measures the users' distance from the center of the SVs orbits. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent. What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned. First, the geodetic model of the earth can have much more than this amount of error at any specific point and second, you have the GPS error itself to add in. As a result of this combined error, I am not surprised to be at the seashore and see -40 meter errors in some spots. We have to make some assumptions about the shape of the earth. WGS84 has defined that shape to be an ellipsoid, with a major and minor axis. The particular dimensions chosen are only an approximation to the real shape. Ideally, such an ellipsoid would correspond precisely to sealevel everywhere in the world. As it turns out, there are very few places where the WGS84 ellipsoid definition coincides with sealevel. On average, the discrepancy is zero, but that doesn't help much when you're standing at the water's edge of an ocean beach and your GPS is reading -100ft below sealevel. The deviation can be as large as 300ft in some isolated locations. When the National Marine Electronics Association came up with the NMEA standard, they decreed that altitudes reported via NMEA protocol, shall be relative to mean (average) sea level. This posed a problem for GPS manufacturers. How to report altitudes relative to mean sea level, when they were only calculating altitude relative to the WGS84 ellipsoid. Ignoring the discrepancy wasn't likely to make GPS users very happy. As it happens, there is actually a model of the difference between the WGS84 ellipsoid and mean sea level. This involves harmonic expansions at the 360th order. It's a very good model, but rather unusable in a handheld device. It was determined that this model could be made into a fairly simple lookup table included in the GPS receiver. The table is usually fairly coarse lat/lon wise, but the ellipsoid to mean sea level variation, known as geoidal separation, varies slowly as you move in lat/lon. -Arthur ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
Hi Tim, The answer is NO. Even though decent accuracy can be had with long averaging. It was discussed a few years ago on this list. -- Björn Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. Thanks Tim ___ 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. ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
Not for survey type accuracy (sub-meter, short measurement time). The average (over a 48 hour period) was pretty good (about 1.5 meters, RMS), but the reading over any 1 minute period can be off as much as 3-5 meters, satellite geometry dependent. I Have two units with good antennas, mounted roughly 40 meters apart, and after locating one of the antennas I use the second TBolt in a differential mode and get the 1-2 meter accuracy all the time. Michael / K7HIL On Thu, May 10, 2012 at 5:50 AM, swingbyte swingb...@exemail.com.au wrote: Hi all, Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. Thanks Tim __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
Attilla, On Thu, 10 May 2012 22:50:15 +1000 swingbyte swingb...@exemail.com.au wrote: Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. How fast do you need it? One project i'm involved with uses a LEA6-T with its phase data output and averaging over several hours to get x/y resolutions in the 2-4mm range. I'm quite sure you can do something similar with altitude as well. Attila Kinali That is relative positions over a baseline of ca 100m. Not absolute positions. --- Björn ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
There is an error in your quoted text. The author must have though there was a difference between WGS84 and true sea level. No that is not true. If you paper map that you bought from US Gological Survey says WGS84 on it then THAT is the definition of sea level on that map. The altitudes of contour lines and peaks will be in WGS84 and should match what the GPS says. Many older maps use a different system so their saw level is defined differently. Almost all GPSes have away to select the elipoid. It defauls to WGS84 but you need to set it to match your paper map One problem is the geometry of the satellites in view. Unless the antenna can see to the horizon the sight lines up to the sats make a deep V and if you can see to the horizon there is 3X or 4X more atmosphere along the horizontal path On Thu, May 10, 2012 at 7:40 AM, Arthur Dent golgarfrinc...@yahoo.com wrote: I've found significant altitude errors using a GPS and the following quotes found on the internet will explain why. From my experience of hiking in the mountains of New Hampshire an aneroid altimeter will vary with atmospheric pressure about 200 feet for a change of 0.2 of mercury so you have to continually set it at known waypoints, just like setting a frequency standard against a known reference, and then it will be 'accurate' for some length of time, and then you set it again. GPS altitude will be off but it should be fairly consistent in spite of the changing atmospheric pressure. The earth neither spins at a constant rate nor is it a perfect sphere. Maybe we need to trade it in for a newer model. ;-) GPS altitude measures the users' distance from the center of the SVs orbits. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent. What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned. First, the geodetic model of the earth can have much more than this amount of error at any specific point and second, you have the GPS error itself to add in. As a result of this combined error, I am not surprised to be at the seashore and see -40 meter errors in some spots. We have to make some assumptions about the shape of the earth. WGS84 has defined that shape to be an ellipsoid, with a major and minor axis. The particular dimensions chosen are only an approximation to the real shape. Ideally, such an ellipsoid would correspond precisely to sealevel everywhere in the world. As it turns out, there are very few places where the WGS84 ellipsoid definition coincides with sealevel. On average, the discrepancy is zero, but that doesn't help much when you're standing at the water's edge of an ocean beach and your GPS is reading -100ft below sealevel. The deviation can be as large as 300ft in some isolated locations. When the National Marine Electronics Association came up with the NMEA standard, they decreed that altitudes reported via NMEA protocol, shall be relative to mean (average) sea level. This posed a problem for GPS manufacturers. How to report altitudes relative to mean sea level, when they were only calculating altitude relative to the WGS84 ellipsoid. Ignoring the discrepancy wasn't likely to make GPS users very happy. As it happens, there is actually a model of the difference between the WGS84 ellipsoid and mean sea level. This involves harmonic expansions at the 360th order. It's a very good model, but rather unusable in a handheld device. It was determined that this model could be made into a fairly simple lookup table included in the GPS receiver. The table is usually fairly coarse lat/lon wise, but the ellipsoid to mean sea level variation, known as geoidal separation, varies slowly as you move in lat/lon. -Arthur ___ 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 To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] question about Thunderbolt geo acuracy
On Thu, 10 May 2012 17:01:48 +0200 b...@lysator.liu.se wrote: On Thu, 10 May 2012 22:50:15 +1000 swingbyte swingb...@exemail.com.au wrote: Hope this isn't too chat roomy, however, I have need of a survey precise geolocation type gps. I was wondering if the precise timing abilities extend to its precision in position output? I have a thunderbolt and one of those conical white aerials from china and would like to know if this combination will give me accurate height data. How fast do you need it? One project i'm involved with uses a LEA6-T with its phase data output and averaging over several hours to get x/y resolutions in the 2-4mm range. I'm quite sure you can do something similar with altitude as well. That is relative positions over a baseline of ca 100m. Not absolute positions. The Baseline is definitly larger than just 100m. the current testing field is spread over the side of a mountain... I haven't looked at the scale of the map, but i'd say it was somewhere in the range of 2-5km. I do not know whether they use fixed reference stations. I am not aware of any. Attila Kinali -- Why does it take years to find the answers to the questions one should have asked long ago? ___ 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.
[time-nuts] question about Thunderbolt geo acuracy
Chris Albertson albertson.chris at gmail.com There is an error in your quoted text. The author must have though there was a difference between WGS84 and true sea level. No that is not true. If you paper map that you bought from US Gological Survey says WGS84 on it then THAT is the definition of sea level on that map. The altitudes of contour lines and peaks will be in WGS84 and should match what the GPS says. Many older maps use a different system so their saw level is defined differently. Almost all GPSes have away to select the elipoid. It defauls to WGS84 but you need to set it to match your paper map. I'm not sure what point you're trying to make but it is a fact, as the OP pointed out, that there are differences between the empirical data of 'true elevation' and what various GPS receivers will indicate based on whatever model they are using. Elevation errors associated with GPS are well known and I have observed these errors and the 2 sources I quoted pointed them out as well. I've known the error between well-established landmarks and the GPS to be significantly off (over 200') and the error will vary from location to location but the error will be essentially constant for any single location. If you're saying that a GPS and a map based on the same model would agree and give the same incorrect elevation, then that is entirely possible but it would simply make both of them incorrect by the same amount. The example given in both references I quoted of having a GPS tell you are say 100 feet below the sea surface when you are standing on the beach high and dry is a situation where I'd believe my observations and doubt the model the GPS is using. The point of the 2 references is to explain why there could be a difference between two GPS receivers reporting elevation and not to get us mired in the weeds near MSL at the sea shore. -Arthur ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On Thu, May 10, 2012 at 10:23 AM, Arthur Dent golgarfrinc...@yahoo.com wrote: Chris Albertson albertson.chris at gmail.com I'm not sure what point you're trying to make but it is a fact, as the OP pointed out, that there are differences between the empirical data of 'true elevation' and what various GPS receivers will indicate based on whatever model they are using. Sorry if not clear. My point was that (1) the wgs84 reference his GPS uses is not wrong. It can't be. It is the definition. It may not be the definition he wants and (2) GPS just is not good at altitude and he'd be better off using a paper map, they are free now so why not. mean sea level is not meaningful any more. What shape is the ocean and what if you live in Kanas? How to extrapolate the ocean level to Kanas? The answer is to use a model of some kind Here where I live I can walk down to the beach and pound a stake in the sand and mark the water level. People actually do that (in a more sophisticated way with tide misting stations up and down the coast) But in Kanas you need some kind of model that tell you what the ocean level would be if there were an ocean in Kanas. But BIG PROBLEM. No one knows how to do make such a model. So they simply DEFINE the height of the ocean in Kanas. One definition is wgs84. The trouble with a defining it is that it will not match what you measure with your stick in the sand.So there are any number of local definitions that are closer matches to measured heights The root of the problem is that the earch has a very complex shape. It is lumpy in random ways and you can't model this, you have to measure it and then look it up. Chris Albertson Redondo Beach, California ___ 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.
Re: [time-nuts] question about Thunderbolt geo acuracy
On 5/10/12 7:40 AM, Arthur Dent wrote: I've found significant altitude errors using a GPS and the following quotes found on the internet will explain why. From my experience of hiking in the mountains of New Hampshire an aneroid altimeter will vary with atmospheric pressure about 200 feet for a change of 0.2 of mercury so you have to continually set it at known waypoints, just like setting a frequency standard against a known reference, and then it will be 'accurate' for some length of time, and then you set it again. GPS altitude will be off but it should be fairly consistent in spite of the changing atmospheric pressure. The earth neither spins at a constant rate nor is it a perfect sphere. Maybe we need to trade it in for a newer model. ;-) GPS altitude measures the users' distance from the center of the SVs orbits. Not precisely true. The SV orbits follow the usual Keplerian things so the focus of the ellipse is close to the barycenter of Earth, but of course, the moon and non uniform gravity of the Earth affect it too. GPS fixes are relative to WGS84 coordinate system (x,y,z) 0,0,0 in WGS84 is within a few cm of the center of mass of the Earth. WGS 84 also defines a datum for the surface (which is not, generally, the geoid) as an ellipsoid of revolution. (compare to the Clarke 1866 ellipsoid) These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. This is a bit fuzzy... there are differences between geodetic and geocentric altitude for instance. And then there's the reference ellipsoid (e.g. Clarke 1866), or more generally the geoid Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. Mmm.. I don't know that it roughly approximates.. WGS84 is precisely defined (that's the coordinate system). The geoid (in terms of the sea surface is defined in terms of spherical harmonics and varies some 100m or thereabouts from the reference datum. WHether your GPS uses the WGS84 datum (simple ellipsoid) or the fancier geoid, is something you'd have to look up. As with any model, there will be errors as the earth is not a simple mathematical shape to represent. What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned. First, the geodetic model of the earth can have much more than this amount of error at any specific point and second, you have the GPS error itself to add in. As a result of this combined error, I am not surprised to be at the seashore and see -40 meter errors in some spots. Actually, no.. the geodetic model (e.g. the EGM96) should be VERY close to the actual sea surface (barring tides and local geographic effects.. the Gulf Stream sits several meters higher because it's warmer and less dense) The ellipsoid could easily be off by tens of meters. We have to make some assumptions about the shape of the earth. WGS84 has defined that shape to be an ellipsoid, with a major and minor axis. The particular dimensions chosen are only an approximation to the real shape. Ideally, such an ellipsoid would correspond precisely to sealevel everywhere in the world. As it turns out, there are very few places where the WGS84 ellipsoid definition coincides with sealevel. On average, the discrepancy is zero, but that doesn't help much when you're standing at the water's edge of an ocean beach and your GPS is reading -100ft below sealevel. The deviation can be as large as 300ft in some isolated locations. When the National Marine Electronics Association came up with the NMEA standard, they decreed that altitudes reported via NMEA protocol, shall be relative to mean (average) sea level. This posed a problem for GPS manufacturers. How to report altitudes relative to mean sea level, when they were only calculating altitude relative to the WGS84 ellipsoid. Ignoring the discrepancy wasn't likely to make GPS users very happy. As it happens, there is actually a model of the difference between the WGS84 ellipsoid and mean sea level. This involves harmonic expansions at the 360th order. It's a very good model, but rather unusable in a handheld device. It was determined that this model could be made into a fairly simple lookup table included in the GPS receiver. The table is usually fairly coarse lat/lon wise, but the ellipsoid to mean sea level variation, known as geoidal separation, varies slowly as you move in lat/lon. And that is a more accurate description.. The question really is what does YOUR receiver report.. if it's MSL in NMEA strings then I would imagine all modern receivers use some form of geoid model with error probably 1 meter. If it's WGS84, then it ignores the geoid. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
Re: [time-nuts] question about Thunderbolt geo acuracy
On 5/10/12 10:46 AM, Chris Albertson wrote: mean sea level is not meaningful any more. What shape is the ocean and what if you live in Kanas? How to extrapolate the ocean level to Kanas? The answer is to use a model of some kind mean sea level, these days, is a name for a particular height that matches the long term average of the ocean, where there is ocean to be measured, and which smoothly varies in between those points. The trouble with a defining it is that it will not match what you measure with your stick in the sand.So there are any number of local definitions that are closer matches to measured heights WGS84 won't match the stick in the sand, but one of the modern reference geoids most certainly will match it, within a few cm. It's important these days, where there are property boundaries referenced to things like mean high tide line. Measuring sea height (the actual height) to an accuracy of cm over a global scale is pretty straightforward these days (that's what TOPEX/JASON is all about). After that, it's a matter of choosing an appropriate averaging technique to remove the effect of tides (which you need to do on solid land, as well) WGS84, is pretty much the simplest model, and is more about defining the directions of X,Y, and Z (or lat/lon) than where the surface of the earth is. The root of the problem is that the earch has a very complex shape. It is lumpy in random ways and you can't model this, you have to measure it and then look it up. You CAN model it.. and that's what the EGM model is.. using multihundred order spherical harmonics. The model isn't simple, but neither is it just a table lookup of measured data. And, as mentioned in an earlier set of posts.. since the bumps aren't huge, if you're only interested in meter scale uncertainties, a fairly small table will give you the local variation between WGS84 ellipsoid (no bumps at all) and EGM. Chris Albertson Redondo Beach, California ___ 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. ___ 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.
