|Re: Surveying nowadays...|
Message #12 Posted by bill platt on 29 June 2003, 12:48 p.m.,
in response to message #9 by db(martinez,california)
Well, that is interesting. I guess the techno revolution proceeds unstoppable.
My use and understanding of GPS comes from a different place---marine navigation--where centimeter accuracy was never an issue, but being off a hundred yards in fog on a narrow fairway is.
So, the USCG (Coast Guard) not the USGS (though maybe they were involved too?) essentially got in a quarrel with the Dept of Defense early on regarding the "selective availability" which rendered GPS considerably less accurate than LORAN C (using TD's) and in fact unacceptably so in the aforementioned situations. Even Lobstermen found early on that their LORAN sets were far superior for briging them back to their pots in thick fog.
But I have not been out navigating in over five years (amazing how time goes by) and so have lost track of what is going on. For instance, I am pretty sure SA is turned off now?
Another interesting isssue which came up in the early nineties when I was using GPS was that of DATUM discrepancies. It was not unusual to find that your GPS would on average put you, say, 100 yards to the east of the chart, for some place overseas. So,it turned out that you had to adjust the chart datum to match the datum of the satellite constellation.
Another interesting thing was that the whole system was in fact only experiemtal, and the satellites were in fact primary for other missions. If you listened to WWV broadcasts, you would hear, "users of GPS are warned that the system is not yet fully operational...". I know that the plan was to go into the next phase by putting up birds whose primary mission would be GPS. I wonder how that process is going?
And then there is GLONASS. Our Magnavox GPS receiver in 1993 was capable of operating with GLONASS, but we never tried it. GLONASS has a web-site, but I don't really know what the status is---I don't hear about it anymore.
-regards and thanks for your post!