VERTICAL CONTROL STANDARDS |
In the case of heighting, the situation is a little more complicated. Some
heighting techniques (for example, spirit levelling) propagate errors in
proportion to the square root of the distance, while other techniques (for
example, GPS and trigonometric levelling) propagate errors as a function
of distance.
The Australian S& S propose the allocation of CLASS to any vertical survey on the basis of (ICSM, 1994):
| " whether the standard deviation of each height value (from a minimally constrained network adjustment) is less than or equal to the maximum allowable value (r) defined according to: |
 |
(10.2-2) |
where
r is the maximum allowable error in millimetres, c is an empirically derived factor, given below for each CLASS of survey, and d is the distance to any station in kilometres (> 1km). "
Values of c are assigned according to:
| CLASS | c (one sigma) |
|---|---|
| A | 4 |
| B | 8 |
| C | 12 |
| D | 50 |
| E | 100 |
| F | 250 |
The U.S. standards for conventional vertical surveys are unchanged (and are based on the usual square-root-distance classification), but the GPS standards recognise that the use of GPS heights can vary depending on whether:
Orthometric height
differences are required, then the accuracy of the GPS derived orthometric
heights are a function of BOTH the accuracy of the GPS ellipsoidal heights
and the accuracy of relative geoid heights.
GPS ellipsoid height
differences are being measured for the purpose of monitoring the change
in height between stations, then it is not necessary to have any accurate
information on the shape of the geoid.
The U.S. accuracy standards for GPS orthometric heighting are given in Table
below (FGCC, 1988).
U.S. orthometric height difference accuracy standards (95% C.I.).
| Minimum eleation difference accuracy standard |
Minimum geoid height difference accuracy standard |
|---|
| ORDER-CLASS | ppm | 1:e | ppm | 1:e |
|---|---|---|---|---|
| AA | 2 |
1:500000 |
2 |
1:500000 |
| A | 2 |
1:500000 |
2 |
1:500000 |
| B | 5 |
1:200000 |
5 |
1:200000 |
| 1 | 15 |
1:67000 |
10 |
1:100000 |
| 2-I | 20 |
1:50000 |
10 |
1:100000 |
| 2-II | 50 |
1:20000 |
20 |
1:50000 |
| 3-I | 100 |
1:10000 |
40 |
1:25000 |
Note that at the higher orders, the error is dominated by the accuracy of
the relative geoid height values, whereas, for the lower orders the major
source of error is in the ellipsoid height differences. "GPS levelling"
is discussed in section 11.3.10.
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© Chris Rizos, SNAP-UNSW, 1999