The data directory contains a number of examples of adjustments that illustrate features of data that can be adjusted by program GHOST.
Sub-directory HOACS is a generic data set containing a number of different types of data including terrestrial data, doppler data, VLBI data and auxiliary parameters. The auxiliary parameters illustrate the ability to solve for certain scale differences between different terrestrial data sets and the 3D data. The solution of the difference between different 3D data set coordinate systems can also be solved.
The positional data illustrates both position equations and position difference equations. Position equations are set up with the covariance is in relation to the coordinate system.
The coordinate difference equations are set up with the covariance in relation to the first station.
The data set is fictitious but can be used to illustrate a number of different types of data without having the complication of a lot of data to sort out.
Sub_directory SUDBURY is a network that illustrates the use of the program to integrate a city network to the framework. There are a number of phases that the adjustment requires. The data is adjusted first to ensure that the data fits within itself. Next the clean data is analyzed to ensure the assigned standard deviations are accurate and finally the data set is adjusted to fit the existing control.
The associated sub-directories illustrate different approaches to the integration problem.
Coordinate values from the 1986 July adjustment are used to fix the framework stations. From an analytical point of view this assumes the framework to be errorless which is untrue.
The possible distortion caused by this assumption makes this method of little use in an analysis of the network. In many cases the city network will have been measured with equivalent accuracy although the geometry may be inferior.
You may decide to maintain the framework values and as a result use this method.
Coordinate values are constrained to within the covariance of the framework adjustment. The resultant weight matrix will contain all the influence from the framework observations on the framework stations.
In order to define the Weighted station constraint, stations are chosen from an area wide enough (SELSTA) that any significant influence from the city network is contained within these stations.
This method will give results that are almost equivalent to a readjustment of the whole framework network including the city network. The two differences are that the framework normal equations remain linearized at the original adjusted values and the nature of least squares adjustments, when a network is under stress, that the correction will go to a area of weak geometry. This weak geometry may be in the framework portion of the network and thus unnecessary distortion may be in the framework.
This method should give the best results for analysis purposes.
This method allows the user to apply a realistic variance for the control station without fixing them to a specific value. (CNSTRT) If there is no other source of positional constraint then the coordinate value won't change. If you use a number of constaints with a realistic variance value, you can expect the coordinate values to change within their variance.
The advantage of this method is that it allows the framework values to assume some of the error. The disadvantage is that unnecessary distortion may be inserted into the framework network if you decide to retain the new values.
This method is similar to the above with the exception that the corrections to the initial value of the coordinate are constrained within the variance associated with equation. (CNSTRT) In subsequent passes the initial values used to form the observation equations havechanged however the constraint remains the same. If the varianceis such that a correction other than zero is applied, then the adjustment will slowly migrate to the values that would have been computed without the constraint.
This method is useful where there is minimum constraint or where one unknown is constrained to be within a certain value of another unknown. For example two different tellurometer data sets with different auxiliary parameters but not suficient data to determine either one may be constrained to solve together.
This method could be used where the framework values are not available but where it is important to include some covariance for the framework stations.
One could assign approiate values to a coordinate value in a position equation, standard deviations to a set of inverse distances and azimuths and use these fictitious values to compute a weighted station adjustment. This weighted station adjustment could then be used to constrain the network.
The advantage is that the weighted station coordinates will always be constrained to the same values. The disadvantage is that you may imply constraint that is not realistic.
Sub-directories of SUDBURY [.SUDBLK...] illustrate the use of the Helmert Block adjustment approach. The initial (parent) sub-directory contains coordinate data for the junctions between sibling data sets. The sibling coordinate data is contained in two sub_directories of the parent. Each sub_directory at the lowest level contains both the coordinate data and the observations for a portion of the network.
Each sibling data set at the lowest level is reduced to a set of partially reduced normal equations. These are then transferred to the parent sub-directory and appended to the similar partially reduced normal equatioms from other sibling blocks. These become the pseudo observations which are further reduced to their junctions and appended to similiar partially reduced normal equations from siblings of the same parent. The highest or initial parent is then fully adjusted. This provides a solution for the highest level coordinates. If this solution is satisfactory the partial solution vectoris passed to each of the siblings in turn and further solved.
To iterate the process the user would reduce the normal equations at the lowest level having formed the normal equations at the new coordinate values. The process up and down the tree would be repeated until a satisfactory solution was found.
This is intended as an illustration of the Helmert Block approch that can easily be seen on a diagram. The amount of overhead used in loading and unloading programs and data makes it unrealistic for this size of network, but the results can quickly be obtained and seen.
Sub_directory CALGARY provides for a more realistic size of a data set. The data set contains examples of standard triangulation, city networks and triangulation with scale control. The initial sub-directory contains the complete data set. Sub-directories contain examples of a helmert block adjustment and a weighted station adjustment.
For the Helmert Block adjustment the network is subdivided into blocks for an illustration of the blocking process.
There are two subdirectories containing an illustration of weighted station adjustment. The network is adjusted without the addition of a spur network to locate a reservoir. These observations are saved in a separte subdirectory. The adjustment is completed and a weighted station adjustment is abstracted from the covariance matrix to include stations within a window. The resultant matrix is then copied to the other subdirectory with the other saved observations. The weighted station matrix is then appended to the saved observations and the data is then adjusted.
Sub-directory GIMLI contains an example of a set of GPS position difference observations. The covariance information has been derived from the GPS reduction programs.
Sub-directory DOPTST contains an example of an adjustment of doppler data where the data is in the form of TAPE9 position equations as computed by program GEODOP. These are then converted to position difference equations in the adjustment process.
Subdirectory REGINA contains an example of a vertical adjustment of a bench mark stability test done in that city.