The Director of Surveys is pleased to announce our newest team member, Alexander Turner, who is taking over from Geoff Banham as the Project Leader of our Geomatics Unit. The following article has been prepared by Turner.
Alberta has an extensive network of survey control markers. There are some new markers being integrated into our database but most have enjoyed a long and storied life in prairie, forest, or municipality. They were integrated using the survey methods of the time that satisfied the budget available. As such, not every marker can be treated with the same confidence when planning or processing surveys. To address this, our team includes information on the confidence and condition of control markers in the ASCM information cards available through SPIN2.
As there have been a few inquiries about the quality of published coordinates on the ASCM cards lately, a summary of some key components is discussed below. A more in-depth review of what is included in our ASCM cards can be found in the Alberta Survey Control Products Manual available on the Geodetic Control Unit webpage (
https://www.alberta.ca/geodetic-control-unit).
Marker Condition
The sands of time affect all things, even our precious control markers. Although Alberta has long carried good standards for installation of ASCMs to ensure stability, they are not impervious to being disturbed by various activities and so the condition of each marker is included in the ASCM card based on the most recent ASCM condition report submitted by land surveyors. The condition is reported as “GOOD,” “DESTROYED,” or “ANOMOLOUS.”
The condition “ANOMOLOUS” is used when the condition of the ASCM has changed (e.g., brass cap is missing – the ASCM has been disturbed). The published data of these anomalous ASCMs should not be trusted for any kind of integration, site calibration, or control but the marker cards are made available as they may be useful in retracing historical cadastral surveys.
Horizontal
The horizontal positions are adjusted relative to the ASCM network. So rather than providing the uncertainty of the published coordinates, it is more useful to provide a horizontal Class and Order to specify confidence relative to the surrounding ASCMs.
For details on the Class, please refer to the Alberta Survey Control Products Manual available on the Geodetic Control Unit webpage.
The Order is not a statement of the accuracy of the published coordinates but rather how well these coordinates agree with the closest ASCMs.
In practice, if a series of Order 1 ASCMs are used for a site calibration, you can expect minimal disagreements when checking to nearby ASCMs or nearby coordinates integrated with higher-order ASCMs. If a series of Order 4 ASCMs are used in a site calibration, you can expect large disagreements when checking to nearby ASCMs as well as any coordinates integrated with a different site calibration.
Vertical
The vertical coordinates of the ASCMs were established in reference to the federal vertical network that was the realization of CGVD28, thus the published elevations are also in CGVD28. A vertical Order is assigned to each ASCM but, unlike the horizontal, the Order refers to the method used to establish the elevation.
A complete list of vertical survey methods is given in Appendix 1 of the Alberta Survey Control Products Manual available on the Geodetic Control Unit webpage. Each of these methods have a different uncertainty related to them and it must be accounted for if they are to be used as vertical control.
In practice, if a 3D site calibration is going to be done using ASCMs, only elevations that were surveyed with spirit-levelling should be used in the calibration. A hierarchical consideration should be applied to vertical site calibration because the precision of the elevations is not published. The methodology for spirit-levelling integration is rigorous, therefore these published elevations can be accepted, and a site polynomial solution used to fit the calibration to the control.
Published elevations integrated with GNSS or trig levelling have relative precisions similar in magnitude to measured elevations. Therefore, a single vertical shift should be applied, based on a best fit of available ASCMs. This is because the variation between published and measured elevations is explained by the uncertainty and not a scalable error.
Elevations integrated through other means have a significantly greater uncertainty than the three vertical survey methods discussed above. Therefore, these published elevations should only be used if more precise elevations are not available, and if they are used great caution should be exercised. A single best fit shift should be applied in this case.
Any questions on this, or other geodetic related topics can be submitted to Alexander Turner (Geodetic Unit Lead, Director of Surveys office) at
Alexander.Turner@gov.ab.ca.
Posted on October 24, 2024