Dec 8, 2012 at 4:56 pm #1296881
@scfhomeLocale: Chocorua NH, USA
Here's a pickle that some might know how to address:
I marked and recorded on a Fortrex 301 a number of UTM coordinates, unfortunately without first checking to see what datum the new GPS was set at. On checking, discovered that it was set to a datum different than the USGS Colorado maps, and the coordinates would not work on those maps.
After resetting the GPS to the needed NAD27 datum, the coordinates appeared to be correct in locating points on the USGS maps; but I'm not sure. Is it necessary to re-mark and record the coordinates with the GPS datum set to the maps? Or can I rely on the new coordinates regenerated by the GPS when the GPS was reset to the map datum?
Any helpful replies will be much appreciated. Thanks.Dec 8, 2012 at 5:26 pm #1934169
@sschloss1Locale: New England
I frequently switch between UTM and Lat/Lon, and my GPS automatically translates my saved points between units. Basically, the GPS knows the "true" location for your points and can display those locations in whatever coordinate system you choose.
That said, there's a simple way to check: jot down the coordinates for one point in NAD27. Switch the GPS to the new datum, and see if it's changed. Also, check to see if the new UTM coordinates make sense on your topo maps. I think it should work fine.Dec 8, 2012 at 8:24 pm #1934197
@scfhomeLocale: Chocorua NH, USA
Thanks very much. That is what I was hoping. Did not want to have to repeat several days hiking just to re-mark some coordinates, although the area is in the Never Summer range, which is strikingly beautiful.
FYI – the datum the Foretrex came with, as you would expect, did not yield coordinates that made any sense on a USGS map. They were way off. The coordinates after the datum was reset are right on in most cases, but in some cases were a few hundred feet from where they ought to be. That could just be because the location of the old track or trail on the map is not quite correct, but that does not explain variations where peaks and highpoints are marked on the USGS maps. In general the trail locations on the USGS maps coincide with GPS readings, unlike the National Forest and National Geographic Trails Illustrated maps, where the trails are often depicted nowhere near where they actually are.
I had a conversation with Raymond Ave, who has written several northern Colorado maps and guides, about this, and his feelings about the TI maps are about the same as mine. They have only gotten much worse since NG took them over. Ray's maps cover the Rawah range, but for the Never Summer, I've had to cut up USGS 7.5 minute quads, make strip maps of them, and laminate them in ultra-thin plastic. If they are copied on the computer with Windows bit map, some of the important details, like shading up to timberline, just don't come through. Learned that the hard way. So for areas further south, will use the original maps rather than TI or USGS copies. Thanks again.Dec 8, 2012 at 9:08 pm #1934203
@skopeoLocale: British Columbia
Scott is correct that you should be able to flip between projections and datums without any loss of ground accuracy as long as your projection and datum agree with the map you are referencing.
I will point out however, that a datum and a projection are not the same thing. I think you will find that most GPS receivers will use the WGS84 datum as the default as it is the current (most accurate) datum for world wide coverage. NAD83 is also used and is almost identical to WGS84 but is considered a local datum which covers North America. NAD27 is the reference system you will find on many US Gov maps and is an older datum that is less accurate (if your a Geodesis) but fine for our small scale mapping applications (as long as you set your GPS to agree with the map).Dec 9, 2012 at 7:04 pm #1934383
I think you guys are conflating a number of concepts here. A coordinate system, a projection, and a datum are all different things. I bet Bob Gross can help here.
To answer the OP's question: for almost all GPS units regardless of which datum you have your GPS set to display coordinates, it stores them internally in the WGS84 datum. If you select to display them in any other datum, the unit does a local conversion and displays them as you request. However internally they are WGS84. When you download data from a GPS, it is almost always formatted in WGS84.
You are quite right though: if your map is based on the NAD27 datum (as most USGS maps are) then you need to set NAD27 on your GPS. But your 'recording' of a waypoint won't be in error if you have to switch.Dec 9, 2012 at 7:45 pm #1934389
@b-g-2-2Locale: Silicon Valley
The statements by James are true. However, different GPS receivers handle things a tiny bit differently and I have never used a Fortrex 301.
When most GPS receivers have to store a user waypoint, they do it with their own internal machine numbers, and that may not have any similarity to the user's coordinate system or datum. By the same token, when that user waypoint is called up again, it gets converted into whatever coordinate system and datum is currently selected by the user. So, you can store in one datum and read in another datum without loss of all accuracy. Keep in mind that there is not a perfect correlator between one datum and another, but they are pretty close. The word "close" is hereby undefined.
One problem that I've seen with this over the last seventeen years or so has to do with the underlying map. The oldest GPS receivers did not have any internal map data, so they didn't have this problem. Once they started holding the internal map database, many had problems of error in that database. Maybe one area would be correct and the next area would have an error of two hundred meters. Especially with the sloppy map database in the era while Selective Availability was in effect, the user never got enough absolute accuracy to know whether it was GPS accuracy or map accuracy that made things confusing. Lots of map databases simply had no indicated or implied map datum at all (like WGS84 or NAD27). Just to make things additionally sloppy, the Earth's surface is sliding a bit. This is especially evident in places along the San Andreas Fault in California, so error is creeping in all the time.
Anytime that I start to wonder how accurately any of my own GPS receivers are working, I go to one benchmark that is within one mile of my home. There is an online listing of these reference points by USGS. I have three receivers, so I take turns placing them down on top of the benchmark, letting them "soak" for five minutes or so, and then I compare the numbers. If I repeat the same steps a year later, I get slightly different numbers, although not by much.
–B.G.–Dec 9, 2012 at 11:55 pm #1934429
@skopeoLocale: British Columbia
>> A coordinate system, a projection, and a datum are all different things <<
Actually, a datum and a map projection are coordinate systems.
None-the-less, I think we are all saying the same thing… the GPS can handle the coordinate conversion.
… and it's good to see you back in the forums Bob!
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