[Manifold-l] Custom datums

[F Whiteley]

>> centimetre resolution... 
>> Is there a wider need in the UK community for this?

Amongst surveyors yes, but amongst GIS community unlikely.

I certainly have had no use so far, however, in some arguments over
coordinate transformation this could become an issue of client confidence
rather that any necessity. 

Our objects are between 100m and 10km apart but quoted in GPS to various
decimal places, UTM or GK generally rounded to 1m but sometimes 1cm.
Accuracy has been varied and unverifiable.  We generally only re-project to
put things into context.

So far Manifold has always won any transformation arguments, but we
generally have worked at 25m resolution which matches sat images we have
used in the past.  

Now that 1m resolution sat and aerial images appear to be available via
Google and other servers, clients may expect their data to match exactly and
sub 10m accuracy may become important.

I doubt that the error that Adam estimates is as big as this - I am sure
someone working with CAD/GPS/OSGB/UTM/GK and differing agencies would have
noticed earlier, but then again if you work with only one datum then you
won't be aware of the problem.  

There are free transformation programs available so you can check (see other
threads).

I have assumed that height above datum is not important at 1m accuracy.

Francis