timz2

http://www.snopes.com/military/lighthouse.asp

Maybe so-- I certainly don't have a better explanation of how they do it. But: KU3867 is alleged to be at 40.787750 N 73.810427 W; if you go to that lat-lon on Google/Bing you'll end up maybe 20 meters south of the church dome. I've never confirmed it, but it's a good bet it's the church that's off, not G/B; the present building isn't the one C&GS triangulated. So: how did they know to ignore this landmark? KV4906 is alleged to be 40.788858 N 74.254382 W; this time we know the present tank is not at that position. Maybe G/B saw the recovery, but looks doubtful. LX4067 was 41.100482 N 73.771478 W; the tank has been removed. G/B knew to ignore the nearby new tanks. KV4430 was 40.641898 N 74.078254 W; think the 2001 recovery is why G/B doesn't line up with the new steeple? KU3273 was 40.684800 N 73.627982 W; maybe the recovery warned G/B to ignore the new tank? HT2797 was 37.716096 N 122.395055 W-- the foundation seems to still be there. Did the recovery warn G/B to not use the new tower? HT2827 is the most recent change; the old tank was 37.723034 N 122.424366 W and some of the Bing pics show the tank there, but Google shows the new tank and ignores it.

So when you're within... say, 5 km of a surveyed landmark, you can expect Google/Bing to be correct within... how many meters?

If you want to know how accurate Google Maps (or Bing) is in your area, just go to http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl and get a bunch of known locations in your area. Water tanks, churches, radio towers-- or discs, if you can go there and see where it is in relationship to something that's visible in the aerial pic. You'll be surprised how good Google and Bing are. Naturally you use the center of the base of the water tank, not the top. You'd think we couldn't expect them to do well with landmarks on the tops of hills, but seems like they do better than you'd expect there too.

Search for HT0774 and it initially comes up as a 3rd-order horizontal control station, with the full lat-lon that NGS started including a while ago-- but when you "select" it it's just vertical control, with lat-lon given to the second. Any explanation?

Dunno how new, but it's new all right. Maybe the last month or two?

The OP sent me the designation for the cache that he's trying to calculate, but I'm not registered at geocaching.com. Has anybody got the actual problem he's working on?

Nobody knows the answer until you explain what you mean by a minute. The first guess would be 14.877 minutes of latitude and 7.137 minutes of longitude, but you're saying that won't give the "correct" results. So what are the "correct" results?

NADCON converts NAD27 to NAD83. Hopefully that's halfway close to WGS84, but not guaranteed.

All the above examples are based on good GPS stations, which means you can't see the station itself, which means you have to take my word for the location of the visible object (unless you go there and do the measurement yourself). But if we're willing to accept third-order buildings and so on, there are thousands of places we can use to check GE's accuracy. How about California's capitol dome, which is JS4124 at 38.576655 n 121.493647 w. Send GE to that lat-lon and you seem to be off a few feet-- but for all we know that could be due to GE's camera not looking vertically down on the dome. For all we know GE has homed in on the correct point on the ground, invisible under the building. A good example of that is the Washington Monument HV4442 at 38.889467 n 77.03524 w, where the GE cursor shows that position being nowhere near the top of the obelisk-- but pretty near the center of the base, which is what it should be. (Incidentally, it seems that station is supposed to be good to a couple of centimeters.) Apparently the camera was looking just about vertically down on the Transamerica Bldg HT3117 at 37.79516 n 122.402772; likewise the Daniels and Fisher tower KK1706 at 39.748098 n 104.995680 w. Just grabbing a few water tanks at random-- ME3155 is 41.796730 n 87.956935 w; the camera didn't see it vertically, but you can see how close the cursor is to the center of the base of the tank. Ditto DG2665 at 33.612797 n 84.449494 w and CS2834 at 32.956925 n 96.763673 w. Maybe GE can't really be that good-- maybe they collected all that NGS control and fitted their images to it. They knew the actual lat-lon for all those water tanks and conceivably they based their lat-lons on those known points. Can't say much about that possibility, but note that they didn't get fooled by KV4906, where the actual tank (presumably a newer replacement for the tank C&GS triangulated) is 50+ feet away from the 40.788858 n 74.254382 w position given for KV4906. And they didn't get fooled by the Ellis Island tanks: KV4307 at 40.700156 n 74.039921 w actually is the one that's been demolished; KV4310 at 40.700014 n 74.040103 w is the one that's still there.

Got an example?

Have you looked at http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl ? It seems you have three stations within four miles of you that are supposedly 95%-likely to be correct within a centimeter (at 2007.0 or whatever it was).

Another one in SF-- The flagpole 20+ meters west of AE5209 is at 37 48 20.458 N 122 28 01.597 W. The flagpole itself may not even be visible on the GE pic-- just its shadow? In NY, try 40 41 26.994 N 74 03 17.782 W which is the drain with the white arrow arcing around it (21 meters east of KV6856). 40 41 54.146 N 74 02 17.648 W is the inside corner of the wall on the SE side of Ellis Island, a meter from AB3000. 40 42 03.921 N 74 00 53.487 W is the SW corner of that rectangular Coast Guard office bldg east of Battery Park (30+ meters NE of KV0587). GE is off by 3-4 ft in this case. GE is farther off at KV4023, 40 46 12.637 N 74 01 00.652 -- the mark is in the middle of that white stone that's visible west of the east edge of that semicircle. GE is off by 6 feet; I wonder if it's a coincidence that this is the first example that's not close to sea level.

Four examples from the Bay Area-- I'll get the NY ones later. In Google Earth, go to 37 42 22.15 N 122 23 26.904 W which is the NAD83(2007) lat-lon for AB7679. The white east-west line is painted on the pavement to mark the station, which is in the curb. In the other ones, you can't see the station itself: 37 48 23.507 N 122 19 52.441 W is the actual lat-lon of the north corner of the 10-by-13-ft concrete block in which AE5211 is set. 37 46 48.171 N 122 17 52.845 W is the actual lat-lon of the intersection of the north crosswalk line and the white line between the southbound lanes on that street. HT0882 is about 50 ft away from that. 37 47 44.245 N 122 16 47.386 W is the actual lat-lon of the south edge of the seawall in line with the west edge of the walkway to that yacht (the Potomac). HT0654 is 1.4 ft north of that. As you see, GE can do astonishingly well as long as we stay close to sea level. More to follow.

Turn on the ruler-- that will give you a crosshair. Might as well zoom in to 200 ft altitude or less, so the crosshair will be precise to the hundredth of a second that GE gives you.

Everyone will agree that Google Earth can occasionally be better than your GPS-- just by coincidence, if nothing else. I think those coincidences may be more common than you think. Search for survey markers at http://www.ngs.noaa.gov and pick a few good ones that you can get to, then when you get there look around for a nearby object that will be visible in the GE pic. Measure how far away it is and determine its true direction from the known point-- a sun sight will do fine, if nothing else. Use FORWARD at the NGS site to calculate the position of the visible object and compare it to GE's position. No idea what's typical, but when I did that with a few marks in the NY area I found GE to be within a couple of feet. I also tried it in the San Francisco area, where you have to keep crustal motion in the back of your mind, but it still worked pretty well. I'll dig up a few examples in the next day or two.

USGS topos are at http://www.mytopo.com/maps.cfm After you've zoomed in in the area you want, click on "MyTopo" in the upper right corner of the map to get the topo map (instead of the schematic).

Sure-- but no reason to bookmark that calculator when the accurate calculators are just as easy to use. Maybe someday they'll want accuracy.

Looks like that uses a spherical earth.

Yes, life is simpler when you ignore the unsimple stuff. I still don't know what distance and accuracy the original poster is hoping for; is the difference between grid north and true north irrelevant to him?

What accuracy do you need, at what distance from the origin? If nothing else, the online calculator at the NGS site will give you whatever precision you need, as long as it's not better than a millimeter. (And as long as you know the direction/distance that precisely.)(And as long as your height above sea level isn't enough to throw things off.) http://www.ngs.noaa.gov/cgi-bin/Inv_Fwd/forward2.prl

Why not?

Your vertical movement, you mean? No doubt it does include that, until it removes it.

So it first determines its "3D" speed, and somehow converts that to horizontal speed before displaying it?

You know how (x,y,z) three-dimensional rectangular coodinates work in general-- you have three planes, each perpendicular to each of the other two, and a point's x-coordinate is its distance from one plane, its y-coordinate is its distance from one of the other two planes, etc. As I recall, the z-coordinate of a point on earth is its distance from the plane of the equator-- so no particular connection to its elevation.