Anyone know of a site like heavensabove for astronomical predictions including Iridium flares. So you input a latitude and longitude and the application returns a date and time when 3 or more GPS satellites are within (selectable) say 30, degrees of the zenith. Wish to determine the height of a patch of ground . It is away from roads, so daftlogic.com etc and old OS maps (before they got precious with their data) and proper spot heights, is no use.
Google Earth?
This doesn't let you select the degrees-from-zenith, but you can get a good idea by looking at the 2D or 3D map.
Pretty much anywhere in the UK should have decent GPS coverage - take a GPS receiver to the spot and let it sit there for a while. GPS altitude is not as accurate as latitude and longitude, but the longer you can keep the receiver still, the better it will get.
If you have really a lot of time, let it sit there for many hours logging data, and plot the results... there will be a definite distribution and you can pick the number closest to the center. If you dig into the settings enough, some receivers have options for stationary vs pedestrian vs vehicle - this tweaks the averaging/filtering that they do on the raw data, and may help you get a reliable answer sooner.
Google Maps has a "terrain" feature. You can only see the contour lines in a specific zoom range - at roughly 1.5" to 1000' scale, I get 100 m major contours and 20 m minor contours. Zooming out one mouse-wheel to roughly 1.5" to 2000' scale, I get 200 m major and 40 m minor. Two more mouse-wheels out and no contours at all.
Oh, this one is better.
Matt Roberds
I'll try again later today. ISTR that whan it first came out, I tried it on a piece of ground with known survey-level and it came back rubbish. Like anything based on google maps, (just linear interpolation between road junctions so doesn't catch humps and dips )
I've tried that, in effect. Known survey-level piece of ground and for ten minute sessions on 3 days. However I averaged the results, the answer was no better than .5m or so of the known surveyed level. I wanted to repeat this on some day and time with known good zenithal satellites, get a good agrreing result, and then move to the unknown patch of ground on another day, but again zenithal for that spot, it is not nearby to me
If you really need this data to less than 0.5m, then try asking a surveyor to come by with a proffessional gps, which have millimeter-accuracy.
Leif
-- beslutning at undlade det.
Just take your personal GPS receiver out there and read it. Now that the whole constellation of 24 satellites is up, with many in orbit spares also up there are very few places that do not see 4 to 8 satellites at any time. Also at this point the number of satellites visible can be solved analytically of with any number of computer algebra and computer geometry programs or even an spreadsheet.
?-)
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No. It does not mm grade accuracy, never did. I have discussed this with my professional surveyor friends. It is good to about 1 to 2 cm in horizontal and 5 cm in vertical with survey grade equipment. Moreover, to get that you must use known equipment in the differential mode. Handheld/automobile units are within 10 m moving, but with an averaging centroid of maybe 2 m. Pretty much the same for all non-survey grade equipment. It is just the physics of the system.
?-)
So not much different to what anyone can do with a calibrated sighting telescope on a camera tripod and a 20m long polythene pipe manometer tube and homemade staffs , if there is a convenient circa 1910 OS map with decimal-foot spot height nearby or ,very rare these days, a still extant benchmark. Its intenely annoying that later series publically available OS maps have no bench mark heights at all, less spot heights, and when they are there, much less quoted accuracy resolution.
I'm only assuming that if you have 3 GPS satellites near the zenith then the altitude would be a more accurate determination. But as we would not know the bias of useage in the algorithm then only 1 additional satellite near the horizon may well c*ck it all up anyway
Well, perhaps I wasn't accurate enough :-)
Leif
-- beslutning at undlade det.
N_Cook wrote in news:m4hla9$ruc$ snipped-for-privacy@dont-email.me:
Tidal forces cause semidiurnal elevation changes of the Earth's crust with an ampltude up to 55cm at the equator. Unless you subtract the current GPS elevation of a nearby benchmark, averaging over an arbitary period will be fairly meaningless. -- Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED) ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL
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