Alain MORY wrote: > I did'nt clearly expose my problem : A wall must be built above a > foundation that has to be exactly oriented face to south. This wall will > contain an iron armature, so that a magnetic compass will not work > exactly near the wall.
I don't see this as a problem. First, because since the wall is not built yet, its armature is not a problem. You can locate the wall using the magnetic compass. You put a line in place and you are done. The only problem I see here is how precise this measure can be. You would have to use a very good compass. And you would have to use it in a very professional way. You should take care both of magnetic declination and problems with the compass itself. You could easily end up with an error of 3 or 4 degrees, or even more. Now, even if you already have some magnetic influence on the place, this still should not be a problem. You can go as further away as you want (and can). Than you take a sight (for instance, using a engineering or militar compass) as you ask someone to put in place two stakes in the East/West direction. > So I thought to give an azimuth to the workers, for example by saying : > the wall must be perfectly aligned with this house angle an with this > treee. If the workers can follow this kind of instruction, ok. But it certainly would be easier to put the stakes in the right place and tie a line between them so the wall can just be built along the line. > I thought that it was possible to point with the GPS particular points > around the wanted site. > I will take relatively far points to give a better accurate guideline > (e.g. some mountains twenty km far from my wall) > Is it playable ? For this the compass maybe more precise than the GPS. The GPS only shows the direction when you are moving. In this case you should be moving in a straight line. Maybe repeating the movement back and forth, until you find the azimuth you want. Does not seem an easy task. Or even desirable. Now, you could use the GPS to take several fixes in far away points (5, 10 or even 20 km). The further away, the less important the error the GPS will be presenting (you should expect up to 100 m). Then, after taking the proper fixes you could connect them visually and from that line derive the azimuth you want. Finally, I don't think neither the compass nor the GPS is a good solution for the problem you have. I think for this particular case a time-proven solution of using the sun shadow is still easier, more precise and less cumbersome. Just put a vertical stake on the. You can also use a plumb line with a node somewhere at a suitable hight. Now, at chosen intervals, like 9 o'clock, 10 o'clock... until noon, mark in the ground where the stake shadow ends. For each mark, draw a circle having the stake as the center. In the afternoon, mark where the shadow is at each corresponding hour. That is, 1 pm, 2 pm and 3 pm corresponding, respectively to 11 am, 10 am and 9 am. Now draw a line connecting each pair of hour. Now pick one of the lines and bisect it and draw a perpendicular line. This line should coincide with the noon line. Also, if everything is ok, this perpendicular should bisect all the other lines. Two marks should suffice. Say, 11 am and 1 pm. Nevertheless, it is always good to have several marks. On one hand, because you may have clouds at the right time, making it impossible to draw the second mark when the time comes. If you have several marks then you will have as many opportunities to find the second mark. On the other hand, if you have several marks you can always check one against the others. This means more certainty. If the stake if correctly placed in the vertical you will end up with a very precise North/South line from which you can find any azimuth you may need. (By the way, the closer the solstice on the "other" hemisphere the more price the measuring will be). I do think this method is easier and more precise than the compass and the GPS (at least in "normal" usage). Regards - fernando > > > > Regards > > Alain MORY > > 47°N 7°E > 500 m > > Alsace, the country where the Sun shines in the gold wine glasses ! :-) -- 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 47º 49' 58.6" W 19º 37' 57.0" S 45º 17' 13.6" W
