[Manifold-l] Custom datums

[Linus F Upson III]

    Height above datum at 1M and less.
    Depending on the uses intended, the Z is either much, much more 
important or much, much less, in my experience.  The first question: 
"Exactly how accurate IS the datum?"  You might be surprized how rough the 
landscape is at these resolutions and how much change takes place in a 
(very) short period of time.  One busy back hoe ...
    The second question: "Exactly how important is it to understand (not 
know) the image characteristics vs the cultural & floral aspects of the 
area?"  In an urban space, how tall are the buildings?  Do you count towers? 
What if  antennas do not appear on the imagery?  In a forested place, how 
high are the trees and does that mean anything if the best DEM data is at 
100M or larger?  In a lightly forested area, is it important to know the 
hight AGL of a single tree top?   Sorting out these issues can be huge 
contractual issues.


----- Original Message ----- 
From: "F Whiteley" <franciswhiteley at btconnect.com>
To: "'Martin Roseveare'" <m.roseveare at archaeophysica.co.uk>; 
<adamw at manifold.net>; <manifold-l at lists.directionsmag.com>
Sent: Thursday, 21 September, 2006 6:49 AM
Subject: RE: [Manifold-l] Custom datums


>>> 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
>
>
>
>
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