> ----- > From: stephen white > Sent: Sunday, August 05, 2007 4:26 PM > > On 06/08/2007, at 11:13 AM, Ben Discoe wrote: > > 5. Using the timecode to correlate, subtract the second unit's > > drift from the first unit's coordinates. > > I can't see how you would do this, because you wouldn't be able to > tell the difference between movement and drift. The second unit would > be drifting around at random, rather than having a fixed offset that > you can add or subtract from the first unit.
Because the second unit is stationary, we would know that all of its variation is due to signal problems, correct? (Ionosphere, Ephemeris, Troposphere, etc.) Hence it's not movement, it's drift. The assumption is that the first unit has both movement and drift, both units will suffer around the same drift, so we can subtract the drift and get just movement. >From what another poster says, i guess the problem is assuming that the units >will suffer the same drift. I suppose they need to lock onto the same >satellites for that to happen. Since "professional" DGPS does actually work, >i can only assume it does something fancier in the stationary unit, like lock >onto all possible satellites (superset of what the mobile unit can see) and >subtract each satellite's drift independently, which consumer units obviously >can't do. > I would need to know the situation where you need that level of > accuracy before I could recommend a range of possible solutions. Great! I've got several parcels of land, each several acres, with lots of features in them (fences, trees, buildings, etc.) I need to digitize the location of the boundaries and features, and some sloppiness is acceptable (1-2m) as this isn't legal or proper survey work. 3D surface height would be super, but i know that's impossible, so 2D points is fine. I'm happy to pay prosumer prices (<$1000) but the typical cost of pro gear (>$8000) is right out. Is there a range of possible solutions? > For example, I use orthorectified aerial maps along with photographs > to get about 5cm accuracy at selected points. The only cloud-free aerials available for the area (which are murky with strong shadows) have been exclusively licensed from DigitalGlobe by Google for GMaps/GEarth. Nobody else can license them. We can all look at them (e.g. http://maps.google.com/?ie=UTF8&ll=20.06097,-155.50036&spn=0.015742,0.014741&t=k&z=16&om=1) and in theory digitize features off them, but they are not well geocorrected, many features are hidden by trees, etc. I can tell the registration is bad because the DigitalGlobe imagery, USGS topos, NAVTEQ roads, and County Tax Maps for this area all vary from each other by up to 30m in places, i.e. they are all useless for something aiming for 1-2m accuracy. -Ben _______________________________________________ Geowanking mailing list [email protected] http://lists.burri.to/mailman/listinfo/geowanking
