+Tubby Rower Posted February 7, 2003 Share Posted February 7, 2003 Does anyone know how far off the NAD87 is from the WGS84 map datum. I went looking for a benchmark yesterday and I could not find it. I realized today that the map datum on my gps was not the same as what was reported on the website. I'm going back today to use the right datum and dig through the snow. kc row, row, row your boat Quote Link to comment
Wild T2 Posted February 7, 2003 Share Posted February 7, 2003 The two datums in common use by surveyors are NAD'27 and NAD'83. Most GPS receivers new, out-of-the-box are factory set to WGS'84. There is only a one to two meter difference between NAD'83 and WGS'84. Too small to worry about. The old NAD'27 datum can be significantly different from NAD'83 depending on where you are in the US. There is a GOOD FAQ on the NGS web page explaining NAD'27 vs NAD'83. See link on main bench mark page. Quote Link to comment
+beejay&esskay Posted February 7, 2003 Share Posted February 7, 2003 In my area, there are a significant number of benchmarks with "scaled" coordinates and an estimated error of +/- 6 seconds. That's a huge error if you are trying to find something in the snow. Trust the description of where it is. (Hope I'm not telling you something you already know. ) Quote Link to comment
+Poindexter Posted February 7, 2003 Share Posted February 7, 2003 quote:Originally posted by Tubby Rower:Does anyone know how far off the NAD87 is from the WGS84 map datum. That depends on what coordinate system you are using. Here's an example. The Lat/Lon coordinates for the washington monument are approx. 38 53.3484N 77 02.1186W WGS84, and 38 53.3464N 77 02.138W NAD27. So if I plotted the NAD27 coordinates on a WGS84 map, it would put me off by 93 feet. UTM coordinates are easting of 323476 northing of 4306443 WGS84, and easting of 323444 northing of 4306233 NAD27. That's a difference of over 700 feet. Most of the coordinates for survey markers are not very accurate unless they have been verified by GPS. The data sheet for the markers should tell you that information. I assume you meant NAD27, not NAD87. Quote Link to comment
+Tubby Rower Posted February 8, 2003 Author Share Posted February 8, 2003 I think that I'll wait on the snow to melt before I go looking for benchmarks. Although, we're supposed to get some more snow tomorrow night. Thanks for the help. quote:Originally posted by beejay&esskay: In my area, there are a significant number of benchmarks with "scaled" coordinates and an estimated error of +/- 6 seconds. That's a huge error if you are trying to find something in the snow. Trust the description of where it is. I noticed the descriptions but I just didn't want to print out all of those pages!! kc row, row, row your boat Quote Link to comment
+beejay&esskay Posted February 8, 2003 Share Posted February 8, 2003 So we can play along at home, which benchmark are you looking for? And I'd be interested how this search turns out using coordinates rather than description. I think a 6 minute error is 600 feet. Even without snow that is a substantial area to search...unless the benchmark is a smokestack. Quote Link to comment
survey tech Posted February 8, 2003 Share Posted February 8, 2003 If you really want to go hunting with GPS only and skip the descriptions, you need to distinguish between horizontal control points, all of which have precise coordinates, whether they were set using GPS or not, and vertical control points, all of which will be off by widely varying amounts because their coordinates were never intended to be precise. Since there are more verticals than horizontals, you will probably soon find that you have to travel a considerable distance to find new horizontals to look for. Quote Link to comment
+Tubby Rower Posted February 11, 2003 Author Share Posted February 11, 2003 http://www.geocaching.com/mark/details.asp?PID=GW2666 I've tried to use the directions on this one too. State road 855 doesn't exist in this location. I've looked at a 1989 map of the area and no 855. There is a State Road 898 where 855 should be using the co-ords as well as the distance from Lynchburg College. This one just has me stumped. But the snow is melting now and maybe I can get a good look at the area before the snow comes this Friday! Thanks everyone for any help. kc row, row, row your boat Quote Link to comment
+embra Posted February 11, 2003 Share Posted February 11, 2003 The coords on this one are adjusted, so they should be pretty good. As for the highways, numbers sometimes change...US 221 should be the same, but I would regard whatever road is in the position suggested by the description of 855 as the described road, regardless of name. This is a recently recovered disk, so there's a good chance you'll be able to find it. (Still--it could be gone). Max Often wrong but seldom in doubt Quote Link to comment
+Tubby Rower Posted February 11, 2003 Author Share Posted February 11, 2003 Does recovered mean "found" or "picked up or collected"? I meant to ask this earlier but I forgot. kc row, row, row your boat Quote Link to comment
+beejay&esskay Posted February 11, 2003 Share Posted February 11, 2003 "Recovered" means found. Quote Link to comment
+embra Posted February 11, 2003 Share Posted February 11, 2003 quote:Originally posted by embra:This is a recently recovered disk, so there's a good chance you'll be able to find it. I'm a little loose on my terminology here...actually, it's recently monumented (placed). No recovery note has been recorded since then. Max Often wrong but seldom in doubt Quote Link to comment
JonSL Posted February 14, 2003 Share Posted February 14, 2003 how can the different dataums be converted? Hard to have different data in the gps with different dataums? Quote Link to comment
+parkrrrr Posted February 14, 2003 Share Posted February 14, 2003 My favorite datum and coordinate system transformation software is GeoTrans. Quote Link to comment
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.