Surveying

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Site Recorder includes all the features required to undertake survey work on marine and intertidal sites. You can add survey points and many different types of measurements then process them together to form a survey plan. The plan can then be used as a framework for detail recording tasks.

The program includes a survey quality adjustment tool that takes the measurements and uses them to calculate the optimum position for the points, a powerful feature not available in other GIS programs.

Along with the adjustment capability, Site Recorder 4 also includes a built-in expert that can find survey problems called the Analyser. The Analyser can track down many common survey problems and provides a report on the work so far in a friendly and easily understood format.

The procedure for adding survey points and processing 3D trilateration measurements is described in Technical Note 3

Survey Points

Survey work starts by planning the positions of the survey points to be used. This can be as simple as planning the location of a single tape baseline along the length of a site or as complex as a trilateration survey over a large area using many survey points.

Three types of survey point are used:

Primary control points are established in the planning phase and are the main reference points for the survey
Secondary points may be added later to solve line-of-sight problems or to reduce measurement lengths.
Detail points are the points on artefacts and features used to position these objects.

Primary control points are the most important survey points and at least three primary control points must remain after the work has been completed so the site can be relocated. Primary points should not be placed on the artefacts or structure of the site but should be fixed firmly and securely to the seabed as the artefacts may be moved or recovered. Some recent re-surveys of sites have reused points installed more than 20 years ago.

Secondary control points can be placed on rigid structure or artefacts that are unlikely to move. It is not as important to ensure that these points survive or that what they are attached to does not move. Enough measurements can be made to the secondary point from surrounding primary points to enable reconstruction of the secondary points’ original positions. Secondary control points should be left in place after excavation where possible.

Detail points are the actual points on artefacts or structure and are positioned relative to the control points. Small objects may only need one detail point to be positioned whereas larger objects, such as cannons and anchors, have two or more points so the orientation of the object can be determined.

Measurements

Site Recorder 4 supports a number of different measurement types and they are all made to a survey point or between two survey points.

Distance - A distance measurement between two survey points
Depth - The depth of a survey point
Offset - An offset measurement between a baseline and a survey point
Ties - Ties measurements between a baseline and a survey point (sometimes known as baseline trilateration)
Radial - A distance and direction measured from one survey point to another
Position - The position of a survey point taken from a GPS or acoustic positioning system (APS)

Adjustment

Once we have added some measurements the next step is to use them to work out accurate positions for each of the survey points. Using these measurements between a set of survey points we can calculate the position of the points and the accuracy to which we know those positions.

In three dimensions only three perfect distance measurements are needed to work out the position of a survey point. Unfortunately, measurements are never ‘perfect’ so we need to use a technique that copes with this problem. The method used here is for many measurements to be made to the new point from other known points but the difficulty is that a selection of three of the measurements would give a different position for the new point. Using a statistical technique to compute the most likely position can solve this additional problem.

The Adjust tool is used to calculate positions of the Survey Points using measurements made to the points. The adjustment takes the positions of the Survey Points and computes a better position for each point using the measurements given to it. In essence, the adjustment moves the points around until the measurements fit the positions as well as possible. The Tool also includes the quality measures for the positions that are needed for any true survey.

The Analyser Tool

The Analyser Tool has been designed to help get the best results from your survey and to help find any rogue or blunder survey measurements. The Analyser tells you a lot about the survey in the form of comments in three categories: statistics, points and measurements. The statistics information tells you about how good the survey is in general, the point information tells you about each survey point in turn while the measurement information gives details of each measurement.

The Analyser will tell you:

Whether the positions of the points are good enough
Which points are positioned correctly after an adjustment and which are not
Which points need more measurements and will suggest a measurement to make
Which points do not have depth measurements, as all points need them
Which points may be unstable and causing the adjustment to fail
Which measurements are good, which are rejected
Which measurements have been rejected if automatic measurement rejection is on
Which measurements should be rejected if automatic measurement rejection is switched off
Which rejected measurements are good enough to be used

The survey work is complete when there are no warnings being shown by the Analyser.

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