Geoida Help

Editing an Extract file

A Geoida Standard Extract file is a formatted text file and can be edited within the Geoida View File option, in any external text editor or word processor (such as Windows WordPAD) that can save in text format, or simply by clicking the Edit button in the Process Extracted Observations option after the Extract file required for processing has been selected.

Description line - The first line in the file contains the description of the traverse data or data file from which it was derived and MUST ALWAYS be present.

Columns - The columns in which the various data values occur are fixed and must not be changed; however if necessary, the position of a decimal point may be adjusted provided that the actual data value does not extend outside its allocated columns. Horizontal angle, direction or bearing observations are written to the Extract file to two decimal places of a second.

Directions - Azimuth (bearing) or horizontal angle observations occupy only one line in the Extract file. However, a block of direction observations at a setup begins with a backsight or reference observation where the horizontal angle value may be 0.000000 or some other value (the distance and VC fields may or may not be blank) - this record must NOT be separated from the associated observations that follow, or be deleted - to do so would convert the remaining observations to angles which may or may not then be correct depending on the value of the record deleted. For a definition of the convention used to distinguish different observation types, see 'Observation Types' under Sample Extract file.

Flags - A '*' flag next to a distance value indicates that this is an unbalanced one-way distance observation to which curvature-refraction correction will be applied during subsequent processing if required - it should not be removed, or the correction will not be applied making it a less reliable observation, or the user may not be able to identify observations to which a greater standard deviation should be applied for least-squares adjustment.

A '+' flag next to a distance value indicates that this is a duplicate distance/VC whose original values were meaned with an earlier observation - the values recorded will be the same as those for the record with which they were meaned. The duplicated values may be necessary to permit a complete traverse reduction (adjustment mode None or Bowditch), but for a least-squares Network adjustment are not necessary and will be omitted. It is suggested that the '+' should NOT be removed or, in a least-squares adjustment, the values will be considered valid and will be used a second time thereby wrongly adding an extra redundant observation over the particular line.

Blank values - There may occasionally be observations within the file that have blank angle, distance or VC values. This may be due to total station observations for which no distance component was measured, may be additional 'observations' inserted by Geoida for backsight records, or may be observations entered in the Traverse option which is able to record single-component horizontal angles, distances or vertical height differences for least-squares network adjustment. The presence of blank values is normal in some situations and should not affect a least-squares adjustment but could prevent a traverse being completed where a missing value (horizontal angle or distance) is critical to the computation.

Traverse - In a file of original or derived total station data (e-Data option), the observations will often have been made in a non-consecutive and disjointed order that would not permit the traverse to be computed by normal means without rearrangement of the observations into the correct sequence. The Standard Extract file provides a means of editing the sequence of observations into the correct order for traverse reduction and adjustment using the None or Bowditch adjustment options. The editing required might be limited to simply moving observation records into the required continuous sequence for correct processing, but sometimes it may also be necessary to reverse the direction of horizontal angle observations when observed in a direction opposite to the general traverse route direction. For traverse reduction, the arrangement of the observations in the Extract file should generally be in the simplest and most logical continuous sequence that will permit reduction; the relative location of any side-shots from a traverse is not important. Where the data consists of multiple traverses, the major 'backbone' traverse(s) should occur at the top of the file to be adjusted first, working down to the least important supplementary spur traverses at the end.

Least-squares adjustment - When meaning of observations is enabled (Mean Control Observations check-box is crossed), and Geoida is executing 'Stage 3' of the meaning of observations process (see the section Meaning, sorting and correction of control, traverse and network observations in the Extract file topic), it will continue to mean together all same-point observations made at consecutive setups that have the same instrument station and backsight, as long as the instrument station and backsight point numbers do not change. If blocks of observations at setups with different instrument station and/or RO come between blocks of observations with identical setups, the observations at the earlier setup are NOT meaned with those at the later setup. This is to allow that, even though the setups appear to be identical and perhaps could be meaned, the earlier and later sets may have been made on different occasions and under different conditions, so it would not be technically correct to assume that they should be compounded and meaned.

This feature may be used to advantage prior to a least squares network adjustment, to retain the separation between multiple observations of different reliability or precision, in order to gain the full statistical advantage of their individual contributions to the result of the adjustment, as follows ...

Step 1. Create a first Standard Extract file from the appropriate data reduction option but with meaning disabled, i.e. Mean Control Observations UN-crossed

Step 2. Edit the Extract file, and group together blocks of observations according to their type, reliability and precision so that they may be meaned. If it is required to keep different groups of observations made at the same instrument station and with the same RO separated so that they are not meaned together, simply move another block of records made at a DIFFERENT setup IN BETWEEN the two groups

Step 3. Now process the edited Extract file with the Process Extracted Observations option with the Mean Control Observations check-box now CROSSED - this will cause the observations to be meaned according to their groupings, a new Standard Extract file to be created, and processing will be completed.

Standard deviations - The standard deviations of individual observations initially are set as per the a-priori values defined in the option used to create the Extract file, but these may also be modified as required to account for different precisions, variations in observing conditions, or to place constraints upon certain observations.

Merging Standard Extract files - If it is necessary to merge two or more Standard Extract files together into one file, at the point of merger between one file and the next, delete (or comment-out with ";") the line beginning End at the end of the first file, and the first line of the next file which contains the data description. Do not remove the description line from the very top of the combined file.

Caution - After combining and/or editing one or more Extract files, be careful that the subsequent use of the Traverse or e-Data options does not over-write your file. To avoid the danger of this occurring, the simplest safeguard is to modify the Extract description line, for example, by adding the word "EDITED", since Geoida will not over-write an Extract file which has a description line different from the standard description it would otherwise write for the particular file and options selected. Alternatively, name the file with an extension that is not likely to be used by Geoida - it is recommended that the extension should still begin with '&' to allow easy identification as a Standard Extract file, and that other non-numeric characters be added. For example, jobname.&AA would be a suitable name - refer to Extract file for details of the Extract naming convention.

As a result of any editing that may be done on the Extract file, the edited file must retain the form of a normal Standard Extract file as shown in Sample Extract file.

Processing of the combined or re-arranged data may then be continued by the Process Extracted Observations option - if the Mean Control Observations check-box is NOT crossed, the observations will be processed as read from the Extract file; however, if it IS crossed, the observations will again be sorted and meaned and written to a new Extract file, and the new data will be processed.