# Geodetic precision

From: Chris Olsson (olsson@globalnet.co.uk)
Date: Thu Aug 16 2001 - 11:18:33 PDT

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```As a newcomer to the List, from the background of being a Geodesist, I've been
wondering about the apparent precision of some List members' declared
co-ordinates and to what geodetic framework they refer.

Let's take three specific positions, in SouthEastern London, England, as a
working example to illustrate what I'm trying to say:

Position #1:
51° 28' 40.125" N   0° 00' 05.310" W    Height 92.603m

Position #2:
51° 28' 38.265" N   0° 00' 00.418" E    Height 47.155m

Position #3:
51° 28' 40.028" N   0° 00' 05.864" E    Height 89.563m

At first glance, it might appear that the three positions each describe three
different places, with Pos#1 being 124.6m from Posn#2.  The two places do not
even appear to be in the same hemisphere!

In fact, all three positions actually describe a single point in space, but
only if I mention which spheroid and datum they refer to.

Position #1 is expressed with reference to WGS84.

Position #2 is expressed with reference to the Airy spheroid and Britain's
national mapping datum OSGB36 in the horizontal plane and the height refers to
Britain's Ordnance Survey datum which is actually referred to the mean sea
level in 1921 at a small fishing town in the South West of England called
Newlyn.

Position #3 is with respect to WGS72.

Note that the actual physical location of these three positions is in fact, by
definition, exactly located on the 0° Prime Meridian at Greenwich. All three
sets of co-ordinates describe the location of Airy's Transit, which was adopted
by International Convention in 1884 to be the zero degree meridian for

Without saying which horizontal and vertical datum(s) a particular reported
position refers to, the stated position is quite useless for any but the most
vague description of geographical location.

Might I respectfully suggest to members of SeeSat-L that whenever they list
their observational location co-ordinates, they also mention to which geodetic
datum those co-ordinates refer?

It may be impressive to list position to an apparent precision of five decimals
of a degree or to a tenth of an arc-second of Lat/Long, but unless the geodetic
datum to which those co-ords are referred is associated with such apparent
precision, then any accuracy which might be associated with such co-ords is
quite wasted.

The same applies to vertical datums.  Spheroidal height and ellipsoidal height
are the same thing, but they are quite different to Geoidal height and are not
necessarily co-incident with "sea level".  In any case, it is essential to
indicate exactly what a vertical "height" refers to. That might be "sea level"
or it might be an ellipsoidal height.  Some GPS units, for example,  output
height in one form, others in another.

There are several very high quality software packages out there which appear to
be able to list the orbital height of any satellite (given good enough elsets)
to an apparent precision of a metre.   Dr TS Kelso's TrakStar, for example,
uses WGS72 rather than WGS84 as a geoidal model and geodetic framework,
apparently because the TLEs which most users have access to are still based
upon that otherwise obsolete reference frame.

Of course for most casual observers of satellites, using binoculars and a
stopwatch, a positional discrepancy in an observational location of a hundred
metres or so is of no practical consequence whatsoever.  It is only when high
accuracy astrometry is used, such as is obtainable by CCD  cameras on good
telecopes, that the datum becomes a significant factor.  Nevertheless, I would
suggest that anyone who posts their geographical position of observation to
high levels of precision (i.e. 10 to 20 metres or so) might usefully mention
which spheroid and datum those co-ordinates refer to.

Cheers,       Chris Olsson
57° 02' 30.9" N   3° 10' 25.9" W   Ht=314m
wrt WGS84!

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