On Saturday 28 Jun 2008 12:11, Kenneth Gonsalves wrote: > On 28-Jun-08, at 11:37 AM, jtd wrote: > > Also by fixing two points accurately on the periphery by non gps > > means,
http://www.iers.org/ Essentially use star data to locate a point on earth, then reference that to WGS84. About Indian geodectic state of affairs http://www.mycoordinates.org/geodetic-infrastructure.php Might be good idea to contact the Survey of India, Hyderabad. But if you do not need the absolute coordinates. Just mark two points 1Km apart accurately aligned NS. This is your baseline. Note a few coordinates with a gps placed flat on the ground. Calculate the distance between the coords. The error is your correction factor. Also available on the net is rinex data which gives you the ionospheric variation for a particular area, from which one can estimate error. > > how is this done? I am told that in America there are markers > everywhere giving reference points. The only one in India I have > seen is at St Thomas Mount where they started the survey of India. Yes. A steel rod fixed in concrete in the ground. also known as a geodectic reference point. > > > you could get a deviation figure and correct your readings. > > You will have to take the correction every few hrs as ionospheric > > delays vary with sun activity. http://www.precision-gps.org./Mathematics.htm > I tried marking the main fountain in Ooty - walked around it 3 > times and marked the waypoints. They were all over the place. I was > using a garmin device and it was showing accuracy of 30-35 metres > on that day. That is because it's proly 12 channel, has an internal antenna and no waas / sbas. Since India has launched GIS sats. I am inclined towards the fact that geodectic info must be available either at isro or Survey of India. Perhaps an approach by some institution (HBCSE, AU-KBC) may prise it out. -- Rgds JTD -- http://mm.glug-bom.org/mailman/listinfo/linuxers
