Fernando, I am not quite sure of exactly what you were trying to determine! You need have no worries re the accuracy of your GPS.
By "old GPS", I assume that you mean an 8-satellite one (like my 8 year old brick!). Have no worries about the accuracy of an 8-satellite GPS. I bought my son (an avid explorer of rugged bushland near Sydney, that involved him in a couple of epics when he went into the wrong creek, necessitating nearly 10 abseils (US = rappel) including two of 75 m on 70 m ropes!). I set it up for him before I gave it to him, and out of curiosity, I checked my old 8-satellite Garmin against the new 12-satellite Garmin. The only difference in multiple readings was in the last digit (i.e. the metres digit). This convinces me that now that the CIA (sorry, US government) has removed selective availability, you can use the old GPS with no worries at all. BTW, my staff have also tested 12-channel GPSs on surveyed points in the landscape. Again, the only difference was in the metres digit. Given that the new Garmin Etrex costs AUD $380 (i.e. probably around $US 150), this is pretty acceptable accuracy. In fact, it is damn fine accuracy. Having said all that, the little yellow thing of electronics in your hand doesn't have the same aesthetic attraction as a theodolite! Cheers, John "Far better an approximate answer to the right question which may be difficult to frame, than an exact answer to the wrong question which is always easy to ask" John W Tukey, statistician ----- Original Message ----- From: "Fernando Cabral" <[EMAIL PROTECTED]> To: "sundial mailing list" <[email protected]> Sent: Tuesday, January 08, 2002 11:29 PM Subject: Finding the meridian with a GPS After buying an old theodolite I merrily travelled to my little farm with the firm purpose of finding the local meridian with high precision. And, as an extra, I would survey the region and have a very precise map. For no purpose at all, but what is a theodolite good for if you don't find ways to use it? They say that if you give a child a hammer she will try to fix the whole world using it. So, if an adult gives himself a theodolite, he MUST find what to do with it, right? Well, theodolite, almanac, calculator, everything in place except... the moon, the sun, the stars. It rained cats and dogs for several days. Starting on December 20 until January 5, we had plenty of water from the skies. As chance uses to have it, if I had to go to main street to buy some stuff or perhaps visit a friend, that's when we had a brief period of clear sky. But experience had taught me: if I decided to rush back home clouds would immediately fly in and start pouring the water god wanted to send us... After several days I picked up my old GPS and decided to use it to replace the stars. I chose a far away point. A tree about 1000 m from the place I set up the theodolite. I walked to it and took a reading (in fact, several readings). Then I walked back to where the theodolite was and took some readings. According to the GPS distance was a little bellow 1000m. I used the GPS to find the bearing from one point to the other. I set the limbo to 0 at that tree. From there I could find any other bearing. My reasoning was that even if the GPS would give me an error of, say, 20 meters to one side (worst case) I'd still be very close to the real azimuth. In fact, let's suppose (this was what I reasoned) I have a right triangle with two legs of 707,1 m each. The third side would be 999,99 (that is, 1000 m). Now, the angle each of the two other angles would be 45 degrees. Now, if, instead of having a leg of 707,1m I had it with 727,1, the difference in the angles would be arc tan (727,1 / 707,1) = 45,7 so, an error of less than 1 degree. Close enough for most purposes, I guess, And probabily better than the other simple methods. Of course, if I wanted to secure higher precision I could still try to go further away (terrain permiting). With a tower (or tree) 5 km away my error would be of less than 10 minutes for a GPS error of 20 m in the worst direction. I still expect to refine my findings when I have a good star in the proper position and a cloudless sky. But for the time being I am happy that I could put my o'theodolite to use. Now I am a happy (ancient) kid! - fernando -- REDUZIR, REUSAR, RECICLAR -- Dever de todos, amor aos que virão REDUCE, REUSE, RECYCLE -- Everybody's duty, love to those who are to come Fernando Cabral Padrao iX Sistemas Abertos mailto:[EMAIL PROTECTED] http://www.pix.com.br Fone Direto: +55 61 329-0206 mailto:[EMAIL PROTECTED] PABX: +55 61 329-0202 Fax: +55 61 326-3082 15º 45' 04.9" S (23 L 0196446/8256520) 47º 49' 58.6" W 19º 37' 57.0" S (23 K 0469898/7829161) 45º 17' 13.6" W
