[Therion] standard deviation and survey grade

Olly Betts olly at survex.com
Mon Dec 15 01:45:58 CET 2008


On Mon, Nov 24, 2008 at 05:18:14PM +0100, Martin Budaj wrote:
> First proposal could be:
> 
> GRADE; POSITION / HEIGHT error (95% confidence); comment
> 
> 0             -- position unknown
> 1  1000 m     -- rough estimate / memory
> 2  100 m      -- GPS in bad conditions (canyons, foliage, SA), small-scale map
> 3  15 / 45 m  -- GPS, large-scale map
> 4  5 / 15 m   -- GPS longer observation, distinct features on a large-scale map
> 5  1 / 3 m    -- GIS GPS
> 6  15 cm      -- geodetic GPS, theodolite/total station, technical nivelation
> 7  5 cm       -- geodetic GPS, precise theodolite/total station,
> precise nivelation

I don't think inventing numbered grades here is a good approach.  For
centre-line survey data, a grade summarising how it was collected makes
sense as you're trying to summarise the accuracy of a lot of
measurements at once, and there's no direct indication of individual
errors for particular readings.  It also gives people a benchmark to aim
for, which can be tested by analysing loop misclosures.

But for a GPS measurement, civilian units do give an indication of
errors per reading - e.g. EPE or FOM.  While you can't directly use
these as standard deviations or anything so convenient, at least by
allowing them to be recorded, you allow software processing the data to
make use of them.

Even where an error indication isn't directly available (e.g. reading
from a map), I'd have to look up in the table above to know that my
"distinct feature on a large-scale map" qualifies me for "grade 4".
I might as well use the table to see it qualifies me for "5m horizontal,
15m vertical" position error.

Also, the suggested grades force particular pairings of horizontal and
vertical errors.  Just because a "GIS GPS" might be 1m/3m, that doesn't
mean that every alternative technique for surveying to 1m horizontal
accuracy would give 3m vertical accuracy.

Cheers,
    Olly



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