The following are extracts from ICSM SP1.
Class is a function of the planned and achieved precision of a survey network and is dependent upon the following components:
- the network design
- the survey practices adopted
- the equipment and instruments used
- the reduction techniques employed
all of which are usually proven by the results of a successful, minimally constrained least squares network adjustment computed on the ellipsoid associated with the datum on which the observations were acquired.
Order is a function of the Class of a survey, the conformity of the new survey data with an existing network coordinate set and the precision of any transformation process required to convert results from one datum to another. Stations in horizontal control surveys are assigned an Order commensurate with the Class of the survey and the conformity of the survey data with the existing coordinate set.
The Order assigned to the stations in a new survey network following constraint of that network to the existing coordinate set may be:
- not higher than the Order of existing stations constraining that network
- not higher than the Class assigned to that survey
Interpretation of SCIMS class for horizontal coordinates
The following table refers to Class codes as they apply to coordinates in the SCIMS database and should not be used for any other purpose.
|3A||Special high precision surveys|
|2A||High precision national geodetic surveys|
|A||National and State geodetic surveys|
|B||State survey control networks|
|C||Cadastral control surveys|
|D||Cadastral and other surveys|
|E||Approximate and lower order surveys|
|U||Unknown or unreliable|
Interpretation of SCIMS class for heights
It is accepted that some heighting techniques such as differential leveling propagate errors in proportion to the square root of the distance. Other techniques such as GPS and trigonometric leveling propagate errors mainly in proportion to the distance. This is particularly apparent on distances greater than 1 km. Therefore different types of class and order are assigned according to the heighting technique used. Refer to ICSM SP1 for details of observation techniques.
The following table refers to Class codes as they apply to heights in the SCIMS database and should not be used for any other purpose.
|L2A||Precise levelling - Forward and backrun misclose < 2√d|
|LA||1st Order levelling - Forward and backrun misclose < 4√d|
|LB||2nd Order levelling - Forward and backrun misclose < 8√d|
|LC||3rd Order levelling - Forward and backrun misclose < 12√d|
|LD||Levelling - Forward and backrun misclose < 18√d|
|LE||Levelling - Forward and backrun misclose < 36√d|
|2A||Precise trigonometric or GPS heighting - Standard deviations of observations < 3 (d+0.2) mm|
|A||Trigonometric or GPS heighting for state survey control - Standard deviations of observations < 7.5 (d+0.2) mm|
|B||Trigonometric or GPS heighting for cadastral control - Standard deviations of observations < 15 (d+0.2) mm|
|C||Trigonometric or GPS heighting - Standard deviations of observations < 30 (d+0.2) mm|
|D||Trigonometric or GPS heighting - Standard deviations of observations < 50 (d+0.2) mm|
|E||Trigonometric or GPS heighting - Standard deviations of observations < 100 (d+0.2) mm|
|U||Unknown or unreliable|
("d" in the table above refers to distance in kilometres)