Converting different chart data ? [Archive]

norman lomax

28-05-2003, 14:19

Hi

Can somebody explain issues regarding different datum for GPS marks ?

For instance if I'm given a mark based on OSGB 36(1), how do I convert it to WGS (may have this wrong) 84 ?

How many different datum's are there ?

What about marks that have not been converted to any chart datum ?

Many thanks

Norman

Adrian Kelland

28-05-2003, 15:14

Hi

Can somebody explain issues regarding different datum for GPS marks ?

A big issue with GPS numbers is were they produced before Selective Availability was turned off. This was a system that degraded the positional accuracy of any numbers. It was tuned off in May 2000 IIRC. The effect was an error of upto around 100m in x and y directions. Z is not usually an issue at sea level ;-)

For instance if I'm given a mark based on OSGB 36(1), how do I convert it to WGS (may have this wrong) 84 ?

WGS is right, World Geodetic System. <a href="http://www.wgs84.com" >http://www.wgs84.com</a>

Have a look here. It is a good start.

<a href="http://www.gps.gov.uk/additionalInfo/images/A_guide_to_coord.pdf" >http://www.gps.gov.uk/additionalInfo/images/A_guide_to_coord.pdf</a>

You want Appendix C, a lot of fun. Should not be hard to put into Excel or similar though.

How many different datum's are there ?

Probably several for each country...

What about marks that have not been converted to any chart datum ?

How were they recorded in the first place?

Many thanks

Norman

Hope this helps,

Adrian