At 06:06 PM 10/9/2007, you wrote: >Rick Collins wrote: > > So how does this mean that GPS signals are more accurate? The last > > time I checked, GPS calculations were done by explicitly calculating > > both the position and the speed of multiple satellites in a very high > > orbit, which means a very, very high speed compared to an aircraft or > > low earth orbit. > >Indeed. But this particular calculation _is_ in fact something that >someone has spent billions of dollars on, if you will permit me to use >your argument, as the US Government uses it to pass Tomahawks through >windows. > >I know we don't necessarily get their accuracy, but my point is that the >original calculation is going to be very, very good.
Yes, the calculations can be superb! But as with any processing, garbage in, garbage out. If your signals are not perfect, you don't get the accuracy claimed. BTW, the military uses a completely separate system at this point. Foot soldiers and the like still use the original system, but advanced weapons use newer, improved signals, not to mention processing; so there is very little comparison. > >> My GPS on the other hand, is an OpenStreetMap recommended model, and I > >> was walking up and down the centre of the roads (not cycling or driving) > >> with a clear view of the sky. > > > > I have no idea what is meant by "OpenStreetMap recommended > > model". Are you trying to say that your model of GPS receiver is > > more accurate and consistent than others? > >No, I mean that the openstreetmap.org wiki recommends it on their >hardware reviews page, and has even set up a special deal so they get >some money when you buy one. It's a NaviGPS (BGT-11 model). So how does that affect the accuracy of the unit? The data sheet claims 25 meter CEP. That is actually not very good compared to some, but I don't know that any of the "specs" relate to real world conditions. > > How was that > > determined? What model is it? How about checking your route by > > comparing to some other data rather than just making an assumption > > about which is more accurate and which is less accurate? If you > > don't want to use the USGS images, why not try measuring the same > > route on multiple days? > >What are these USGS images you speak of? I haven't seen them as an >option or an add-in for JOSM. Are they as high-res as the Yahoo images >in the area in question? I have posted twice about them. I don't know much about JOSM really. I am not finding it to be a very facile tool. If JOSM is to be used for this sort of work, it might be a good idea to add in capability of viewing the USGS data. Of course this is only useful within the US. USGS is a department of the US government (US Geodetic Survey, IIRC). They produce maps and such. They have made their topo maps and aerial images available to companies and in turn Terraserver (Microsoft) provides them in consumer usable image form. A program I us to view these maps and images allows you to overlay GPX tracks and graphically edit the GPX data. This is not a perfect editor either, but it is very good for viewing and not bad for moving points around (culling the data points is not so facile). Once you have a culled data set, USAPM is not a bad way to edit the file for accuracy (assuming that you trust the accuracy of the maps and images). As others have said, it is not unlikely that all of these data sets have some inaccuracies. This will provide another reference for you to use before you decide to keep or move your data. The few runs I have made show very good alignment to the USGS data. You can download USAPM at http://jdmcox.com/. The program will download the image data as needed when you refresh a screen. There is a companion program to download arbitrary regions. This is not an open source program, but he does provide the c source. He does not provide the other source files such as the .h files or the .rc files. So you can view what is happening, but you can't do anything to fix problems. The USGS data comes in a variety of resolutions. The topo maps reach 4 meter per pixel (don't ask me what that is in terms of x:1). The aerial data is available in 1 meter per pixel or for areas around the major cities 0.25 meter per pixel in color. _______________________________________________ newbies mailing list [email protected] http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/newbies
